Construction of electronic cigarettes

File:Eleaf Istick 50w E Cigarette - Box Mod - Vaporizer - Vape Mod (16721884182).jpg

An e-cigarette is a handheld battery-powered vaporizer that simulates smoking, but without tobacco combustion. Once the user inhales, the airflow activates the flow sensor, and then the heating element atomizes the liquid solution.{{cite journal|vauthors=Rahman MA, Hann N, Wilson A, Worrall-Carter L |title=Electronic cigarettes: patterns of use, health effects, use in smoking cessation and regulatory issues|journal=Tob Induc Dis|volume=12|issue=1|pages=21|year=2014|doi=10.1186/1617-9625-12-21|pmc=4350653|pmid=25745382 |doi-access=free }} The different kinds of trigger sensor or sensors used are acoustic, pressure, touch, capacitive, optical, Hall Effect or electromagnetic field. Most devices have a manual push-button switch to turn them on or off.{{cite journal|last1=Kaisar|first1=Mohammad Abul|last2=Prasad|first2=Shikha|last3=Liles|first3=Tylor|last4=Cucullo|first4=Luca|title=A Decade of e-Cigarettes: Limited Research & Unresolved Safety Concerns|journal=Toxicology|year=2016|volume=365|pages=67–75|issn=0300-483X|doi=10.1016/j.tox.2016.07.020|pmid=27477296|pmc=4993660}} E-cigarettes do not turn on by trying to "light" the device with a flame.

The e-liquid reaches a temperature of roughly 100-250 °C within a chamber to create an aerosolized vapor. Variable voltage devices can raise the temperature.{{cite journal|last1=Bertholon|first1=J.F.|last2=Becquemin|first2=M.H.|last3=Annesi-Maesano|first3=I.|last4=Dautzenberg|first4=B.|title=Electronic Cigarettes: A Short Review|journal=Respiration|year=2013|issn=1423-0356|volume=86|issue=5|pages=433–8|doi=10.1159/000353253|pmid=24080743|doi-access=free}} A glycerin-only liquid vaporizes at a higher temperature than a propylene glycol-glycerin liquid. Rather than cigarette smoke, the user inhales an aerosol, commonly but inaccurately called vapor. E-cigarettes do not create vapor between puffs.{{cite web|url=http://www.apha.org/policies-and-advocacy/public-health-policy-statements/policy-database/2015/01/05/12/58/supporting-regulation-of-electronic-cigarettes|title=Supporting regulation of electronic cigarettes|publisher=American Public Health Association|location=US|date=18 November 2014}}

Vaping is different from tobacco smoking, but there are some similarities with their behavioral habits, including the hand-to-mouth action and a vapor that looks like cigarette smoke. E-cigarettes provide a flavor and feel similar to smoking. A noticeable difference between the traditional cigarette and the e-cigarette is sense of touch. A traditional cigarette is smooth and light but an e-cigarette is rigid, cold and slightly heavier. Since e-cigarettes are more complex than traditional cigarettes, a learning curve is needed to use them correctly.

Compared to traditional cigarettes, the general e-cigarette puff time is much longer, and requires a more forceful suction than a regular cigarette.{{cite journal|last1=Evans|first1=S. E.|last2=Hoffman|first2=A. C.|title=Electronic cigarettes: abuse liability, topography and subjective effects|journal=Tobacco Control|volume=23|issue=Supplement 2|year=2014|pages=ii23–ii29|issn=0964-4563|doi=10.1136/tobaccocontrol-2013-051489|pmc=3995256|pmid=24732159}} The volume of vapor created by e-cigarette devices in 2012 declined with vaping. Thus, to create the same volume of vapor increasing puff force is needed. Later-generation e-cigarettes with concentrated nicotine liquids may deliver nicotine at levels similar to traditional cigarettes. Many e-cigarette versions include a power control to adjust the volume of vapor created. The amount of vapor produced is controlled by the power from the battery, which has led some users to adjust their devices to increase battery power.{{cite journal|last1=Rowell|first1=Temperance R|last2=Tarran|first2=Robert|title=Will Chronic E-Cigarette Use Cause Lung Disease?|journal=American Journal of Physiology. Lung Cellular and Molecular Physiology|volume=309|issue=12|year=2015|pages=L1398–L1409|issn=1040-0605|doi=10.1152/ajplung.00272.2015|pmc=4683316|pmid=26408554}} Larger percentages of glycerin in e-liquid also increase vapor production.{{cite news|url=https://vaping360.com/best-e-liquids/vg-vs-pg/|title=PG vs VG: What They Are and How to Use Them|publisher=Vaping360|date=3 August 2018}}

File:Miniciggy size compared with ordinary cigarette.jpg

E-cigarettes come in many variations, such as cigarette-shaped, pen-shaped, and tank-shaped styles.{{cite journal|last=Grana|first=R|author2=Benowitz, N|author3=Glantz, SA |title=E-cigarettes: a scientific review.|journal=Circulation|date=13 May 2014|volume=129|issue=19|pages=1972–86|doi=10.1161/circulationaha.114.007667|pmc=4018182|pmid=24821826}} Some e-cigarettes look like traditional cigarettes, but others do not.{{cite journal|last1=Pepper|first1=J. K.|last2=Brewer|first2=N. T.|title=Electronic nicotine delivery system (electronic cigarette) awareness, use, reactions and beliefs: a systematic review|journal=Tobacco Control|volume=23|issue=5|year=2013|pages=375–384|issn=0964-4563|doi=10.1136/tobaccocontrol-2013-051122|pmc=4520227|pmid=24259045}} There are three main types of e-cigarettes: cigalikes, looking like cigarettes; eGos, bigger than cigalikes with refillable liquid tanks; and mods, assembled from basic parts or by altering existing products.

E-cigarette components include a mouthpiece, a cartridge (liquid storage area), a heating element/atomizer, a microprocessor, a battery, and some have a LED light on the end. E-cigarettes are sold in disposable or reusable variants.{{cite journal|last1=Ebbert|first1=Jon O.|last2=Agunwamba|first2=Amenah A.|last3=Rutten|first3=Lila J.|title=Counseling Patients on the Use of Electronic Cigarettes|journal=Mayo Clinic Proceedings|volume=90|issue=1|year=2015|pages=128–134|issn=0025-6196|doi=10.1016/j.mayocp.2014.11.004|pmid=25572196|doi-access=free}} Most versions are reusable, though some are disposable.{{cite journal|last1=Drope|first1=Jeffrey|last2=Cahn|first2=Zachary|last3=Kennedy|first3=Rosemary|last4=Liber|first4=Alex C.|last5=Stoklosa|first5=Michal|last6=Henson|first6=Rosemarie|last7=Douglas|first7=Clifford E.|last8=Drope|first8=Jacqui|title=Key issues surrounding the health impacts of electronic nicotine delivery systems (ENDS) and other sources of nicotine|journal=CA: A Cancer Journal for Clinicians|volume=67|issue=6|pages=449–471|year=2017|issn=0007-9235|doi=10.3322/caac.21413|pmid=28961314|doi-access=free}} They range in cost from under $10 to over $200.{{cite journal|vauthors=Couch ET, Chaffee BW, Gansky SA, Walsh MM|title=The changing tobacco landscape: What dental professionals need to know.|journal=J Am Dent Assoc|year=2016|volume=147|issue=7|pages=561–9|doi=10.1016/j.adaj.2016.01.008|pmc=4925234|pmid=26988178}} An entry-level reusable e-cigarette costs around $25.{{cite journal|last1=Liber|first1=Alex C|last2=Drope|first2=Jeffrey M|last3=Stoklosa|first3=Michal|title=Combustible cigarettes cost less to use than e-cigarettes: global evidence and tax policy implications|journal=Tobacco Control|volume=26|issue=2|year=2017|pages=158–163|issn=0964-4563|doi=10.1136/tobaccocontrol-2015-052874|pmid=27022059|s2cid=24577577}}

Disposable e-cigarettes are discarded once the liquid in the cartridge is used up, while rechargeable e-cigarettes may be used indefinitely.{{cite journal|last1=Franck|first1=C.|last2=Budlovsky|first2=T.|last3=Windle|first3=S. B.|last4=Filion|first4=K. B.|last5=Eisenberg|first5=M. J.|title=Electronic Cigarettes in North America: History, Use, and Implications for Smoking Cessation|journal=Circulation|volume=129|issue=19|year=2014|pages=1945–1952|issn=0009-7322|doi=10.1161/CIRCULATIONAHA.113.006416|pmid=24821825|doi-access=free}} Even with rechargeable cigarettes (pod-based systems), there is a risk of littering.{{Cite web|url=https://www.reuters.com/article/us-health-ecigarettes-waste/e-cigarette-policy-should-consider-environmental-effects-expert-says-idUSKCN1N403P|title=E-cigarette policy should consider environmental effects, expert says|first=Carolyn|last=Crist}}{{Cite web|url=https://www.inquirer.com/philly/health/juul-e-cigarettes-recycling-garbage-blu-njoy-20181030.html|title=E-cigarettes: the new hazardous waste|first=Mari A.|last=Schaefer|date=October 30, 2018|website=www.inquirer.com}} There are thoughts on how to prevent pods from ending up in the environment (i.e. a deposit-system for e-cigarette pods).{{Cite web|url=https://www.nationalgeographic.com/environment/2019/08/cigarettes-story-of-plastic/|archive-url=https://web.archive.org/web/20190810125852/https://www.nationalgeographic.com/environment/2019/08/cigarettes-story-of-plastic/|url-status=dead|archive-date=August 10, 2019|title=Cigarette butts are toxic plastic pollution. Should they be banned?|date=9 August 2019|website=Environment}} One piece devices are normally disposable.

E-cigarettes are typically designed as one, two, three or multiple pieces.{{cite web|last1=Garner|first1=Charles|last2=Stevens|first2=Robert|date=February 2014|title=A Brief Description of History, Operation and Regulation|url=http://www.coresta.org/Reports/ECIG-RefRep_Ecig-History-Operation-Regulation_Feb2014b.pdf|website=Coresta|url-status=dead|archive-url=https://web.archive.org/web/20160303041105/http://www.coresta.org/Reports/ECIG-RefRep_Ecig-History-Operation-Regulation_Feb2014b.pdf|archive-date=3 March 2016}} A disposable e-cigarette lasts to around 400 puffs.{{cite news|url=http://www.huffingtonpost.com/oscar-raymundo/how-to-get-started-with-e_b_6548388.html|title=How to Get Started with E-Cigarettes|author=Oscar Raymundo|work=HuffPost| date=27 January 2015}} Reusable e-cigarettes are refilled by hand or exchanged for pre-filled cartridges, and general cleaning is required.{{cite journal|last1=Orellana-Barrios|first1=Menfil A.|last2=Payne|first2=Drew|last3=Mulkey|first3=Zachary|last4=Nugent|first4=Kenneth|title=Electronic cigarettes-a narrative review for clinicians|journal=The American Journal of Medicine|volume=128|issue=7|pages=674–81|year=2015|issn=0002-9343|doi=10.1016/j.amjmed.2015.01.033|pmid=25731134|doi-access=free}} A wide range of disposable and reusable e-cigarettes exist.{{cite journal|last1=Brown|first1=C. J.|last2=Cheng|first2=J. M.|title=Electronic cigarettes: product characterisation and design considerations|journal=Tobacco Control|volume=23|issue=Supplement 2|year=2014|pages=ii4–ii10|issn=0964-4563|doi=10.1136/tobaccocontrol-2013-051476|pmc=3995271|pmid=24732162}} Disposable e-cigarettes are offered for a few dollars, and higher-priced reusable e-cigarettes involve an up-front investment for a starter kit. Some e-cigarettes have a LED at the tip to resemble the glow of burning tobacco. The LED may also indicate the battery status.{{cite journal|last1=Caponnetto|first1=Pasquale|last2=Campagna|first2=Davide|last3=Papale|first3=Gabriella|last4=Russo|first4=Cristina|last5=Polosa|first5=Riccardo|title=The emerging phenomenon of electronic cigarettes|journal=Expert Review of Respiratory Medicine|volume=6|issue=1|year=2012|pages=63–74|issn=1747-6348|doi=10.1586/ers.11.92|pmid=22283580|s2cid=207223131}} The LED is not generally used in personal vaporizers or mods.{{cite web|url=https://www.usfa.fema.gov/downloads/pdf/publications/electronic_cigarettes.pdf|title=Electronic Cigarette Fires and Explosions in the United States 2009 - 2016|pages=1–56|publisher=United States Fire Administration|date=July 2017}} {{PD-notice}}

First-generation e-cigarettes usually simulated smoking implements, such as cigarettes or cigars, in their use and appearance.{{cite journal|last1=Farsalinos|first1=Konstantinos E.|last2=Spyrou|first2=Alketa|last3=Tsimopoulou|first3=Kalliroi|last4=Stefopoulos|first4=Christos|last5=Romagna|first5=Giorgio|last6=Voudris|first6=Vassilis|title=Nicotine absorption from electronic cigarette use: comparison between first and new-generation devices|journal=Scientific Reports|volume=4|pages=4133|year=2014|issn=2045-2322|doi=10.1038/srep04133|pmc=3935206|pmid=24569565|bibcode=2014NatSR...4E4133F}} Later-generation e-cigarettes often called mods, PVs (personal vaporizer) or APVs (advanced personal vaporizer) have an increased nicotine-dispersal performance, house higher capacity batteries, and come in various shapes such as metal tubes and boxes.{{cite journal|pmid=21571692|doi=10.1093/ntr/ntr088|journal=Nicotine & Tobacco Research|year=2011|volume=13|issue=9|title=Interviews with "vapers": implications for future research with electronic cigarettes|last2=Tower|author3-last=Sumner|first1=Amy|last1=McQueen|first2=Stephanie|first3=Walton|pages=860–7|doi-access=free}} They contain silver, steel, metals, ceramics, plastics, fibers, aluminum, rubber and spume, and lithium batteries.{{cite journal|last1=SA|first1=Meo|last2=SA|first2=Al Asiri|title=Effects of electronic cigarette smoking on human health|url=http://www.europeanreview.org/wp/wp-content/uploads/3315-3319.pdf|journal=Eur Rev Med Pharmacol Sci|volume=18|issue=21|year=2014|pages=3315–9|pmid=25487945}} A growing subclass of vapers called cloud-chasers configure their atomizers to produce large amounts of vapor by using low-resistance heating coils.{{cite news|url=http://vapenewsmagazine.com/february-2014/vapeview-the-cloud-chasers|title=The Cloud Chasers|author=Mary Plass|publisher=Vape News Magazine|date=29 January 2014}} This practice is known as cloud-chasing.{{cite news|url=http://www.huffingtonpost.com/2014/08/05/cloud-chasing-vaping-competition_n_5652528.html|title=This Man Is An Athlete In The Sport Of 'Cloud Chasing'|author=Dominique Mosbergen|work=HuffPost|date=5 August 2014}} Many e-cigarettes are made of standardized replaceable parts that are interchangeable between brands.{{cite news|url=http://www.clubic.com/materiel-informatique/article-704447-1-cigarette-electronique.html|archive-url=https://web.archive.org/web/20140530022757/http://www.clubic.com/materiel-informatique/article-704447-1-cigarette-electronique.html|url-status=live|archive-date=30 May 2014|title=A la découverte de la cigarette électronique|author=Jérôme Cartegini|publisher=Clubic|date=27 May 2014}} A wide array of component combinations exists.{{cite web|url=http://www.digitaltrends.com/features/inside-the-world-of-vapers-the-subculture-that-might-save-smokers-lives/|title=Inside the world of vapers, the subculture that might save smokers' lives|last=Couts |first=Andrew|publisher=Digital Trends|date=13 May 2013}} Many e-cigarettes are sold with a USB charger.{{cite news|url=https://www.theguardian.com/technology/2014/nov/21/e-cigarettes-malware-computers|author=Alex Hern|work=The Guardian|title=Now e-cigarettes can give you malware|date=21 November 2014}} E-cigarettes that resemble pens or USB memory sticks are also sold for those who may want to use the device unobtrusively.{{cite journal|last1=Schraufnagel|first1=Dean E.|last2=Blasi|first2=Francesco|last3=Drummond|first3=M. Bradley|last4=Lam|first4=David C. L.|last5=Latif|first5=Ehsan|last6=Rosen|first6=Mark J.|last7=Sansores|first7=Raul|last8=Van Zyl-Smit|first8=Richard|title=Electronic Cigarettes. A Position Statement of the Forum of International Respiratory Societies|journal=American Journal of Respiratory and Critical Care Medicine|volume=190|issue=6|year=2014|pages=611–618|issn=1073-449X|doi=10.1164/rccm.201407-1198PP|pmid=25006874|s2cid=43763340 |url=https://figshare.com/articles/journal_contribution/10758143 }}

The increasing numbers of new vaping products combined with unrelated functions attest to a clear trend toward customization of e-cigarettes. It seems that experienced users like to adopt the e-cigarette to their (inhalation) needs, leading to e-cigarettes with adjusted airflow inlet using atomizer heads with different sized air holes. This is applied in the most recently introduced models, which are activated by a pressure difference when the user inhales from the e-cigarette, avoiding pressing a button to heat the device. Other interesting new e-cigarette-like devices provide a combined function with other electronic products such as a Bluetooth e-cigarette, which combines vaping with listening to music or calling friends and another device can be used both as e-cigarette and mobile phone.

Smartphone applications were introduced that track the number of e-cigarette puffs taken, calculate cost savings and increased life expectancy, and have features such as auto-shut down and password protection safety. In line with this, Philip Morris International has filed a patent for an e-cigarette that is Wi-Fi connected, and thus would be able to connect to other devices. This device could potentially synchronize to a smartphone application that is intended to help people quit smoking, and carefully track their progress. A similar product is the Vaporcade Jupiter, a "cellular vaporizer," combining a smartphone with an e-cigarette. This allows the user to monitor the e-cigarette use, the e-liquid remaining, and the flavor used.

As the e-cigarette industry continues to evolve, new products are quickly developed and brought to market.{{cite journal|last1=Glasser|first1=A. M.|last2=Cobb|first2=C. O.|last3=Teplitskaya|first3=L.|last4=Ganz|first4=O.|last5=Katz|first5=L.|last6=Rose|first6=S. W.|last7=Feirman|first7=S.|last8=Villanti|first8=A. C.|title=Electronic nicotine delivery devices, and their impact on health and patterns of tobacco use: a systematic review protocol|journal=BMJ Open|volume=5|issue=4|year=2015|pages=e007688|issn=2044-6055|doi=10.1136/bmjopen-2015-007688|pmc=4420972|pmid=25926149}} The early devices looked like a traditional cigarette, often including a small light on the tip that lit when the user puffed. These early systems were generally inefficient at delivering nicotine, in part because the particle sizes of the aerosol were too large to penetrate deep into the lungs. Newer versions feature replaceable or refillable reservoirs and rechargeable batteries that generate smaller particles and more efficient nicotine delivery.{{cite journal|last1=Glantz|first1=Stanton A.|last2=Bareham|first2=David W.|title=E-Cigarettes: Use, Effects on Smoking, Risks, and Policy Implications|journal=Annual Review of Public Health|volume=39|issue=1|pages=215–235|date=January 2018|issn=0163-7525|doi=10.1146/annurev-publhealth-040617-013757|pmid=29323609|pmc=6251310}} {{CC-notice|cc=by4|url=http://www.annualreviews.org/doi/pdf/10.1146/annurev-publhealth-040617-013757|author(s)=Stanton A. Glantz and David W. Bareham}} Since e-cigarettes are not regulated in many countries, the device designs can change often.{{cite journal |last1=Ramôa |first1=C. P. |last2=Eissenberg |first2=T. |last3=Sahingur |first3=S. E. |year=2017 |title=Increasing popularity of waterpipe tobacco smoking and electronic cigarette use: Implications for oral healthcare |journal=Journal of Periodontal Research |volume=52 |issue=5 |pages=813–823 |doi=10.1111/jre.12458 |issn=0022-3484 |pmc=5585021 |pmid=28393367}} There is wide differences in the quality of e-cigarettes, such as the airflow rate, aerosol production, and leaking of e-liquid cartridges.{{cite journal|last1=McCausland|first1=Kahlia|last2=Maycock|first2=Bruce|last3=Jancey|first3=Jonine|title=The messages presented in online electronic cigarette promotions and discussions: a scoping review protocol|journal=BMJ Open|volume=7|issue=11|year=2017|pages=e018633|issn=2044-6055|doi=10.1136/bmjopen-2017-018633|pmc=5695349|pmid=29122804}}

File:Vaporizers, E-Cigs, and other Electronic Nicotine Delivery Systems.jpg

First-generation e-cigarettes started off as patents in periods of 1927–1936 and 1963–1998, but becoming commercially mainstream in 2003.{{cite web | url=https://casaa.org/education/vaping/historical-timeline-of-electronic-cigarettes/ | title=History of Vaping - Historical Timeline of Events }} They tend to look like tobacco cigarettes and so are called "cigalikes". The three parts of a cigalike e-cigarette initially were a cartridge, an atomizer, and a battery. A cigalike e-cigarette currently contains a cartomizer (cartridge atomizer), which is connected to a battery. Most cigalikes look like cigarettes but there is some variation in size.{{cite journal|last1=Bhatnagar|first1=A.|last2=Whitsel|first2=L. P.|last3=Ribisl|first3=K. M.|last4=Bullen|first4=C.|last5=Chaloupka|first5=F.|last6=Piano|first6=M. R.|last7=Robertson|first7=R. M.|last8=McAuley|first8=T.|last9=Goff|first9=D.|last10=Benowitz|first10=N.|title=Electronic Cigarettes: A Policy Statement From the American Heart Association|journal=Circulation|volume=130|issue=16|year=2014|pages=1418–1436|issn=0009-7322|doi=10.1161/CIR.0000000000000107|pmid=25156991|s2cid=16075813|url=https://cloudfront.escholarship.org/dist/prd/content/qt52p2317d/qt52p2317d.pdf?t=otlwup|pmc=7643636}}

They may be a single unit comprising a battery, coil and filling saturated with e-liquid in a single tube to be used and discarded after the battery or e-liquid is depleted. They may also be a reusable device with a battery and cartridge called a cartomizer.{{cite web|url=http://www.ncsct.co.uk/usr/pub/e-cigarette_briefing.pdf|title=Electronic cigarettes|author=Hayden McRobbie|pages=1–16|publisher=National Centre for Smoking Cessation and Training|year=2014|access-date=2015-05-18|archive-date=2022-04-23|archive-url=https://web.archive.org/web/20220423185420/https://www.ncsct.co.uk/usr/pub/e-cigarette_briefing.pdf|url-status=dead}} The cartomizer cartridge can be separated from the battery so the battery can be charged and the empty cartomizer replaced when the e-liquid runs out.

The battery section may contain an electronic airflow sensor triggered by drawing breath through the device. Other models use a power button that must be held during operation. An LED in the power button or on the end of the device may also show when the device is vaporizing.{{cite news|title=The skyrocketing popularity of e-cigarettes: A guide|url=http://theweek.com/article/index/232221/the-skyrocketing-popularity-of-e-cigarettes-a-guide|publisher=The Week|date=20 August 2012}}

Charging is commonly accomplished with a USB charger that attaches to the battery.{{cite news|url=http://www.engadget.com/2009/03/31/thankos-usb-powered-health-e-cigarettes-sound-healthy/|title=Thanko's USB-powered Health E-Cigarettes sound healthy|author=Tim Stevens|publisher=Engagdet|date=31 March 2009}} Some manufacturers also have a cigarette pack-shaped portable charging case (PCC), which contains a larger battery capable of recharging the individual e-cigarette batteries.{{cite news|url=https://www.engadget.com/2011/07/15/e-lite-electronic-cigarette-review-no-one-ever-said-healthy-was/|title=E-Lites electronic cigarette review|author=Terrence O'Brien|publisher=Engagdet|date=15 July 2011}} Reusable devices can come in a kit that contains a battery, a charger, and at least one cartridge. Varying nicotine concentrations are available and nicotine delivery to the user also varies based on different cartomizers, e-liquid mixtures, and power supplied by the battery.

These manufacturing differences affect the way e-cigarettes convert the liquid solution to an aerosol, and thus the levels of ingredients, that are delivered to the user and the surrounding air for any given liquid. First-generation e-cigarettes use lower voltages, around 3.7 V.{{cite journal|vauthors=Crotty Alexander LE, Vyas A, Schraufnagel DE, Malhotra A|title=Electronic cigarettes: the new face of nicotine delivery and addiction |journal=J Thorac Dis |volume=7|issue=8|pages=E248–51|year=2015|doi=10.3978/j.issn.2072-1439.2015.07.37 |pmc=4561260|pmid=26380791}}

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Second generation devices started in 2013, and tend to be used by people with more experience. They are larger overall and look less like tobacco cigarettes. They usually consist of two sections, basically a tank and a separate battery. Their batteries have higher capacity, and are not removable. Being rechargeable, they use a USB charger that attaches to the battery with a threaded connector. Certain batteries have a "passthrough" feature so they can be used even while they are charging.{{cite journal|last1=Farsalinos|first1=K. E.|last2=Polosa|first2=R.|title=Safety evaluation and risk assessment of electronic cigarettes as tobacco cigarette substitutes: a systematic review|journal=Therapeutic Advances in Drug Safety|volume=5|issue=2|year=2014|pages=67–86|issn=2042-0986|doi=10.1177/2042098614524430|pmc=4110871|pmid=25083263}}

Second-generation e-cigarettes commonly use a tank or a "clearomizer". Clearomizer tanks are meant to be refilled with e-liquid, while cartomizers are not. Because they're refillable and the battery is rechargeable, their cost of operation is lower. They can also use cartomizers, which are pre-filled only.

Some cheaper battery sections use a microphone that detects the turbulence of the air passing through to activate the device when the user inhales. Other batteries like the eGo style can use an integrated circuit, as well as a button for manual activation. The LED shows battery status. The power button can also switch off the battery so it is not activated accidentally.{{cite web|url=https://www.pcmag.com/article2/0,2817,2430142,00.asp|title=Joyetech eCom|website=PCMag|publisher=Ziff Davis|date=31 January 2014}} Second generation e-cigarettes may have lower voltages, around 3.7 V. Adjustable-voltage devices can be set between 3 V and 6 V.{{cite news|title=Vaping Basics – VAPE GEAR|url=http://spinfuel.com/guide-to-vaping/|publisher=Spinfuel eMagazine|author=Tom McBride|date=11 February 2013}}

File:Ecig gen3.jpg

The third-generation started in 2013, it included mechanical mods and variable voltage devices.{{cite news|author1=Daniel Culpan|title=E-cigarettes may only be harmful under 'extreme conditions'|url=https://www.wired.co.uk/news/archive/2015-05/21/e-cigarette-formaldehyde|publisher=Condé Nast|date=21 May 2015}}{{cite news|author1=Mark Benson|title=Are Third Generation Vaping Devices A Step Too Far?|url=http://spinfuel.com/vaping-a-step-too-far/|publisher=Spinfuel eMagazine|date=9 January 2015}} Battery sections are commonly called "mods," referencing their past when user modification was common. Mechanical mods do not contain integrated circuits. They are commonly cylindrical or box-shaped, and typical housing materials are wood, aluminium, stainless steel, or brass.{{cite news|title=Trading addictions: the inside story of the e-cig modding scene|url=https://www.engadget.com/2014/10/01/inside-story-e-cig-modding-uk/|publisher=Engadget|author=Michael Grothaus|date=1 October 2014}} A larger "box mod" can hold bigger and sometimes multiple batteries.

Mechanical mods and variable devices use larger batteries than those found in previous generations.{{cite news|title=What you need to know about vaporizers|url=https://www.engadget.com/2014/05/23/vaporizers-explainer/ |publisher=Engadget|author=Sean Cooper|date=23 May 2014}} Common battery sizes used are 18350, 18490, 18500 and 18650.{{cite web|url=http://spinfuel.com/understanding-milliamp-hours/|title=Understanding MilliAmp Hours|publisher=Spinfuel eMagazine|date=2 January 2014}} The battery is often removable, so it can be changed when depleted. The battery must be removed and charged externally.

Variable devices permit setting wattage, voltage, or both. These often have a USB connector for recharging; some can be used while charging, called a "passthrough" feature.{{cite news|title=The Vapologist will see you now: Inside New York's first e-cigarette bar|url=http://theweek.com/article/index/251049/the-vapologist-will-see-you-now-inside-new-yorks-first-e-cigarette-bar|publisher=The Week|date=11 October 2013}}

The power section may include additional options such as screen readout, support for a wide range of internal batteries, and compatibility with different types of atomizers. Third-generation devices can have rebuildable atomizers with different wicking materials. These rebuildables use handmade coils that can be installed in the atomizer to increase vapor production. Hardware in this generation is sometimes modified to increase power or flavor.{{cite news|author1=Eric Larson|title=Pimp My Vape: The Rise of E-Cigarette Hackers|url=http://mashable.com/2014/01/25/vaping-subculture/|publisher=Mashable|date=25 January 2014}}

The larger battery sections used also allow larger tanks to be attached that can hold more e-liquid. Recent devices can go up to 8 V, which can heat the e-liquid significantly more than earlier generations.

File:2017 Sigelei Evaya and Kayfun V4.jpg

A fourth-generation e-cigarette became available in the United States in 2014.{{cite journal|last1=Brandon|first1=T. H.|last2=Goniewicz|first2=M. L.|last3=Hanna|first3=N. H.|last4=Hatsukami|first4=D. K.|last5=Herbst|first5=R. S.|last6=Hobin|first6=J. A.|last7=Ostroff|first7=J. S.|last8=Shields|first8=P. G.|last9=Toll|first9=B. A.|last10=Tyne|first10=C. A.|last11=Viswanath|first11=K.|last12=Warren|first12=G. W.|title=Electronic Nicotine Delivery Systems: A Policy Statement from the American Association for Cancer Research and the American Society of Clinical Oncology|journal=Clinical Cancer Research|url=http://clincancerres.aacrjournals.org/content/early/2015/01/08/1078-0432.CCR-14-2544.full.pdf+html|year=2015|volume=21|issue=3|pages=514–525|issn=1078-0432|doi=10.1158/1078-0432.CCR-14-2544|pmid=25573384|doi-access=free}} Fourth-generation e-cigarettes can be made from stainless steel and pyrex glass, and contain very little plastics.{{cite web|url=https://gfn.net.co/downloads/2015/Plenary%203/Konstantinos%20Farsalinos.pdf|title=Electronic cigarette evolution from the first to fourth-generation and beyond|author1=Konstantinos Farsalinos|website=gfn.net.co|page=23|publisher=Global Forum on Nicotine|year=2015|archive-url=https://web.archive.org/web/20150708172614/http://gfn.net.co/downloads/2015/Plenary%203/Konstantinos%20Farsalinos.pdf|archive-date=2015-07-08|url-status=dead}} Included in the fourth-generation are sub ohm tanks and temperature control devices. The e-cigarette user can breathe in large puff volumes, which results in a significant usage of e-liquid per puff.{{cite journal|last1=Qasim|first1=Hanan|last2=Karim|first2=Zubair A.|last3=Rivera|first3=Jose O.|last4=Khasawneh|first4=Fadi T.|last5=Alshbool|first5=Fatima Z.|title=Impact of Electronic Cigarettes on the Cardiovascular System|journal=Journal of the American Heart Association|volume=6|issue=9|year=2017|pages=e006353|issn=2047-9980|doi=10.1161/JAHA.117.006353|pmid=28855171|pmc=5634286}} Usually used by experienced e-cigarettes users.{{cite journal|last1=Clapp|first1=Phillip W.|last2=Jaspers|first2=Ilona|title=Electronic Cigarettes: Their Constituents and Potential Links to Asthma|journal=Current Allergy and Asthma Reports|volume=17|issue=11|pages=79|year=2017|issn=1529-7322|doi=10.1007/s11882-017-0747-5|pmc=5995565|pmid=28983782}}

File:Dripping Atomizer crop.jpg)|alt=Inside view of an e-cigarette atomizer with the coil (heating element).]]

An atomizer consists of a small heating element that vaporizes e-liquid and a wicking material that draws liquid onto the coil. Along with a battery and e-liquid the atomizer is the main component of every personal vaporizer. When activated, the resistance wire coil heats up and vaporizes the liquid, which is then inhaled by the user.{{Cite patent|country=EP|number=2614731|status=application|pubdate=17 July 2013|fdate=12 January 2012|inventor=Yonghai Li, Zhongli Xu|title=An atomizer for electronic cigarette}}.

The electrical resistance of the coil, the voltage output of the device, the airflow of the atomizer and the efficiency of the wick all affect the vapor coming from the atomizer.{{cite news|author1=Joseph C. Martin, III|title=The World of the [RDA] Coil|url=http://spinfuel.com/guide-to-vaping-rda-building/|publisher=Spinfuel eMagazine|date=2 September 2015}} They also affect the vapor quantity or volume yielded.

Atomizer coils made of kanthal usually have resistances that vary from 0.4Ω (ohms) to 2.8Ω. Coils of lower ohms have increased vapor production but could risk fire and dangerous battery failures if the user is not knowledgeable enough about electrical principles and how they relate to battery safety.{{cite web|url=http://www.hardingenergy.com/pdfs/5%20Lithium%20Ion.pdf|title=Harding Battery Handbook For|publisher=Harding Energy, Inc.|url-status=dead|archive-url=https://web.archive.org/web/20151227093854/http://hardingenergy.com/pdfs/5%20Lithium%20Ion.pdf|archive-date=2015-12-27}}

Wicking materials vary from one atomizer to another. "Rebuildable" or "do it yourself" atomizers can use silica, cotton, rayon, porous ceramic, hemp, bamboo yarn, oxidized stainless steel mesh and even wire rope cables as wicking materials.{{cite web|url=http://www.twincities.com/health/ci_24192074/e-cigarette-stores-pop-up-twin-cities-so|last=Ngonngo|first=Nancy|title=As e-cigarette stores pop up in Twin Cities, so do the questions|publisher=Pioneer Press|date=28 September 2013}}

File:Carto-45mm.jpg

A "cartomizer" (a portmanteau of cartridge and atomizer.{{cite news|title=Logic Premium Electronic Cigarettes|url=https://www.pcmag.com/article2/0,2817,2422418,00.asp|date=30 July 2013|publisher=PC Magazine}}) or "carto" consists of an atomizer surrounded by a liquid-soaked poly-foam that acts as an e-liquid holder. They can have up to 3 coils and each coil will increase vapor production. The cartomizer is usually discarded when the e-liquid starts to taste burnt, which usually happens when the e-cigarette is activated with a dry coil or when the cartomizer gets consistently flooded (gurgling) because of sedimentation of the wick. Most cartomizers are refillable even if not advertised as such.

Cartomizers can be used on their own or in conjunction with a tank that allows more e-liquid capacity. The portmanteau word "cartotank" has been coined for this.{{cite web|url=https://lgaurejen.wordpress.com/|title=lgaurejen|date=17 February 2015}} When used in a tank, the cartomizer is inserted in a plastic, glass or metal tube and holes or slots have to be punched on the sides of the cartomizer so liquid can reach the coil.

File:E-cigarette.jpg to the coil that is located directly under the mouthpiece.|alt=eGo style e-cigarette with a top-coil clearomizer. Silica fibers are hanging down freely inside of the tank, drawing e-liquid by capillary action to the coil that is located directly under the mouthpiece.]]

File:ECig with RDA crop.jpg

The clearomizer was invented in 2009 that originated from the cartomizer design.{{cite web|author=Mike K|date=9 June 2015|title=What Does The Future Hold For Vaping Technology?|url=http://stevevape.com/what-does-the-future-hold-for-vaping-technology/|publisher=Steve K's Vaping World}} It contained the wicking material, an e-liquid chamber, and an atomizer coil within a single clear component. This allows the user to monitor the liquid level in the device. Clearomizers or "clearos", are like cartotanks, in that an atomizer is inserted into the tank.{{cite news|url=http://myjournalcourier.com/archive/8259/news-community-home_top-news-717182-smoking-going-electronic|title=Smoking going electronic|author=Greg Olson|work=Civistas Media|publisher=Journal-Courier|date=29 January 2014}} There are different wicking systems used inside clearomizers. Some rely on gravity to bring the e-liquid to the wick and coil assembly (bottom coil clearomizers for example) and others rely on capillary action or to some degree the user agitating the e-liquid while handling the clearomizer (top coil clearomizers). The coil and wicks are typically inside a prefabricated assembly or "head" that is replaceable by the user.{{cite news|url=http://vapenewsmagazine.com/april-2015/sub-%CF%89-tanks-and-the-end-of-non-hobbyist-building|title=Sub ohm tanks and the end of non hobbyist building|author1=Arvid Sollom|publisher=Vape Magazine|date=9 May 2015}}

Clearomizers are made with adjustable air flow control.{{cite book|title=VAPE Magazine March EU Special|publisher=Vape Magazine|page=50|url=http://issuu.com/mschramel/docs/vapei12-europe-march-lowrez|date=March 2015}} Tanks can be plastic or borosilicate glass. Some flavors of e-liquid have been known to damage plastic clearomizer tanks.{{cite web|url=http://ecigarettereviewed.com/e-liquid-and-tank-safety|author=Lindsay Fox|title=E-Liquid and Tank Safety|publisher=EcigaretteReviewed|date=24 March 2014}}

File:RDA Deck crop.jpg

A rebuildable atomizer (RBA) is an atomizer that allows the user to assemble or "build" the wick and coil themselves instead of replacing them with off-the-shelf atomizer "heads". They are generally considered advanced devices.{{cite news|title=The Rebuildable Atomizer – An Introduction And Overview|url=http://spinfuel.com/the-rebuildable-atomizer-an-introduction-and-overview/|publisher=Spinfuel eMagazine|date=7 January 2013}} They also allow the user to build atomizers at any desired electrical resistance.

These atomizers are divided into two main categories; rebuildable tank atomizers (RTAs) and rebuildable dripping atomizers (RDAs).{{cite web|url=http://vapenewsmagazine.com/november-2013/3-steps-to-rebuilding-atomizers |title=3 steps to rebuilding atomizers |author=Joshua Workman |publisher=Vapenews Magazine |date=15 December 2013 |archive-url=https://web.archive.org/web/20160305053228/http://vapenewsmagazine.com/november-2013/3-steps-to-rebuilding-atomizers |archive-date=March 5, 2016}}

RTAs have a tank to hold liquid that is absorbed by the wick. They can hold up to 4 ml of e-liquid. The tank can be either plastic, glass, or metal. One form of tank atomizers was the Genesis style atomizers.{{cite news|author1=Erick Potter |title=How to prepare a stainless steel wick and wrap a coil for a Genesis style rebuildable atomizer |url=http://vapenewsmagazine.com/february-2014/how-to-prepare-a-stainless-steel-wick-and-wrap-a-coil-for-a-genesis-style-rebuildable-atomizer |publisher=Vape Magazine |date=16 January 2014 |archive-url=https://web.archive.org/web/20140208002435/http://vapenewsmagazine.com/february-2014/how-to-prepare-a-stainless-steel-wick-and-wrap-a-coil-for-a-genesis-style-rebuildable-atomizer |archive-date=February 8, 2014}} They can use ceramic wicks, stainless steel mesh or rope for wicking material. The steel wick must be oxidized to prevent arcing of the coil. Another type is the Sub ohm tank.{{cite news|author1=Julia Hartley-Barnes|title=Vaping with Julia "Sub Ohm Tanks"|url=http://spinfuel.com/sub-ohm-tanks-julia/|publisher=Spinfuel eMagazine|date=17 September 2015}} These tanks have rebuildable or RBA kits. They can also use coil heads of 0.2 ohm, 0.4 ohm, and 0.5 ohm. These coil heads can have stainless steel coils.{{cite news|author1=John Manzione|title=Aspire Triton Full Review|url=http://spinfuel.com/aspire-triton-full-review/|date=27 July 2015}}

RDAs are atomizers where the e-liquid is dripped directly onto the coil and wick.{{cite news|title=Guide To Dripping e Liquid|url=http://spinfuel.com/guide-to-dripping/|author=Jason Little|publisher=Spinfuel eMagazine|date=13 July 2015}} The common nicotine strength of e-liquids used in RDA's is 3 mg and 6 mg. Liquids used in RDA's tend to have more vegetable glycerin. A fully saturated wick can give you as many as 10–20 puffs.{{Cite web|url=https://vapingcheap.com/rba-rda-guide-rebuildable-atomizers/|title=Rebuildable Atomizers: What does RDA stand for?|date=5 October 2019}} They typically consist only of an atomizer "building deck", commonly with three posts with holes drilled in them, which can accept one or more coils. The user needs to manually keep the atomizer wet by dripping liquid on the bare wick and coil assembly, hence their name.

Kanthal wire is used in RDA's, RBA's, RTA's, in addition to clearomizers, tanks, and cartomizers. Nickel wire or titanium wire can be used for temperature control.

The origins of a squonk mod bottom-feeding system go as far back as 2009. A member of the E-Cigarette Forum (ECF) named "Carlos49" was largely credited with developing the first squonker available in the marketplace. Squonk mods differ from other mod boxes with their construction. Squonk mods have a 510 connection that have been modified with the use of an e-liquid bottle placed inside the mod. The user squeezes an e-liquid bottle through an opening in the device to send e-liquid through a tube into the attached atomizer. Extra liquid goes back into the bottle when it is unsqueezed.{{cite news|url=http://vaping360.com/squonk-mods/|title=Best Squonk Mods 2017 - Ultimate Guide to Squonking|publisher=Vaping360|date=2 November 2017}}

File:Juul vaping device with pods (cropped).jpg

Pod mods heat up a liquid containing nicotine, flavors, and other ingredients that creates an aerosol. Pod mods are lightweight, portable,{{cite journal|last1=Goniewicz|first1=Maciej Lukasz|last2=Boykan|first2=Rachel|last3=Messina|first3=Catherine R|last4=Eliscu|first4=Alison|last5=Tolentino|first5=Jonatan|title=High exposure to nicotine among adolescents who use Juul and other vape pod systems ('pods')|journal=Tobacco Control|volume=28|issue=6|year=2018|pages=tobaccocontrol–2018–054565|issn=0964-4563|doi=10.1136/tobaccocontrol-2018-054565|pmc=6453732|pmid=30194085}} small, and reusable. Pod mods do not require pushing a button.{{cite news|url=https://torontosun.com/sponsored/sponsored-news/vaping-technology-101-the-latest-trends-in-a-growing-industry|title=Vaping technology 101: The latest trends in a growing industry|author=Rachel Becker|newspaper=Toronto Sun|date=6 May 2019}} A pod mod does not require much of a learning curve. With the majority of pod mods, users can just open their new package, put a pod into the device, and begin vaping. They are charged using a USB port. There are numerous pod mods in the marketplace{{cite journal|last1=Spindle|first1=Tory R.|last2=Eissenberg|first2=Thomas|title=Pod Mod Electronic Cigarettes—An Emerging Threat to Public Health|journal=JAMA Network Open|volume=1|issue=6|year=2018|pages=e183518|issn=2574-3805|doi=10.1001/jamanetworkopen.2018.3518|pmid=30646245|pmc=7058175}} and there are many kinds of pod mods.

The three categories for the different kinds of pod mods are an open system, a closed system, or those that use both.{{cite journal|last1=Galstyan|first1=Ellen|last2=Galimov|first2=Artur|last3=Sussman|first3=Steve|title=Commentary: The Emergence of Pod Mods at Vape Shops|journal=Evaluation & the Health Professions|volume=42|issue=1|year=2018|pages=118–124|issn=0163-2787|doi=10.1177/0163278718812976|pmc=6637958|pmid=30477337|url=https://escholarship.org/uc/item/7p31f4wt}} Pod mods come in varying colors and flavors.{{cite news|url=https://www.theverge.com/2018/11/21/18105969/juul-vaping-nicotine-salts-electronic-cigarettes-myblu-vuse-markten|title=Juul, the vape device teens are getting hooked on, explained|author=Julia Belluz|work=Vox|date=1 May 2018}} Many devices rely on replaceable liquid pods that may contain propylene glycol, glycerin, benzoic acid, nicotine, and artificial flavors.{{cite journal|last1=Bonilla|first1=Alex|last2=Blair|first2=Alexander J.|last3=Alamro|first3=Suliman M.|last4=Ward|first4=Rebecca A.|last5=Feldman|first5=Michael B.|last6=Dutko|first6=Richard A.|last7=Karagounis|first7=Theodora K.|last8=Johnson|first8=Adam L.|last9=Folch|first9=Erik E.|last10=Vyas|first10=Jatin M.|title=Recurrent spontaneous pneumothoraces and vaping in an 18-year-old man: a case report and review of the literature|journal=Journal of Medical Case Reports|volume=13|issue=1|pages=283|year=2019|issn=1752-1947|doi=10.1186/s13256-019-2215-4|pmc=6732835|pmid=31495337 |doi-access=free }} {{CC-notice|cc=by4|url=https://jmedicalcasereports.biomedcentral.com/articles/10.1186/s13256-019-2215-4|author(s)=Alex Bonilla, Alexander J. Blair, Suliman M. Alamro, Rebecca A. Ward, Michael B. Feldman, Richard A. Dutko, Theodora K. Karagounis, Adam L. Johnson, Erik E. Folch, and Jatin M. Vyas}} Some pod mods can be refillable, with flavors such as cotton candy, donut cream, and gummy bear. Pod mods that contain tetrahydrocannabinol (THC), the primary psychoactive chemical of cannabis, are being sold.

Pod mods can look like USB flash drives, cell phones, credit card holders, and highlighters.{{cite web|url=https://www.cdph.ca.gov/Programs/CCDPHP/Pages/Vaping-Health-Advisory.aspx|title=Vaping Related Lung Illness: A Summary of the Public Health Risks and Recommendations for the Public|publisher=California Tobacco Control Program|agency=California Department of Public Health|pages=1–5|date=26 September 2019}} {{PD-notice}} Because pod mods are small and generate less aerosol, it makes it easy to hide them. There are pod mods that can be concealed in the palm of a person's hand.{{cite news|url=https://www.sciencenews.org/article/teens-use-juul-e-cigarettes-much-more-often-other-vaping-products|title=Teens use Juul e-cigarettes much more often than other vaping products|last=Cunningham|first=Aimee|work=Science News|date=23 October 2018}} Later-generation pod mods are small like a Sharpie pen. Pod mods cost about half as much as larger e-cigarettes.

The latest generation of e-cigarettes, "pod products", such as Juul, have the highest nicotine content (59 mg/mL), in protonated salt, rather than the free-base nicotine form found in earlier generations, which makes it easier for less experienced users to inhale. Pod mods deliver higher levels of nicotine than regular e-cigarettes.{{cite news|url=https://www.thefix.com/fda-investigate-whether-vaping-causes-seizures|title=FDA To Investigate Whether Vaping Causes Seizures|last=Weedston|first=Lindsey|work=The Fix|date=8 April 2019}} One nicotine pod, in terms of nicotine, is roughly equivalent to one pack of regular cigarettes.{{cite web|url=https://www.canada.ca/en/public-health/news/2019/03/statement-from-the-council-of-chief-medical-officers-of-health-on-the-increasing-rates-of-youth-vaping-in-canada.html|title=Statement from the Council of Chief Medical Officers of Health on the increasing rates of youth vaping in Canada|publisher=Public Health Agency of Canada|date=11 April 2019}} The labels on products state pods contain 59 mg/mL of nicotine, but the levels can be considerably greater such as 75 mg/mL of nicotine. Some pod mods contained greater levels of nicotine than Juul which were as high as 6.5%.{{cite journal|last1=Jackler|first1=Robert K|last2=Ramamurthi|first2=Divya|s2cid=73433596|title=Nicotine arms race: JUUL and the high-nicotine product market|journal=Tobacco Control|volume=28|issue=6|year=2019|pages=tobaccocontrol–2018–054796|issn=0964-4563|doi=10.1136/tobaccocontrol-2018-054796|pmid=30733312}} In June 2015, Juul introduced a pod mod device.{{cite journal|last1=McKelvey|first1=Karma|last2=Baiocchi|first2=Mike|last3=Halpern-Felsher|first3=Bonnie|title=Adolescents' and Young Adults' Use and Perceptions of Pod-Based Electronic Cigarettes|journal=JAMA Network Open|volume=1|issue=6|year=2018|pages=e183535|issn=2574-3805|doi=10.1001/jamanetworkopen.2018.3535|pmc=6324423|pmid=30646249}} British American Tobacco told to The Verge in 2018 that "They've been incorporated in our Vuse e-liquid in the US since 2012."{{cite news|url=https://www.theverge.com/2018/11/21/18105969/juul-vaping-nicotine-salts-electronic-cigarettes-myblu-vuse-markten|title=Juul's nicotine salts are dominating the market — and other companies want in|author=Rachel Becker|work=The Verge|date=21 November 2018}}

Research on nicotine salts is limited. Tests show that the pod mods Juul, Bo, Phix, and Sourin contain nicotine salts in a solution with propylene glycol and glycerin. A nicotine base and a weak acid such as benzoic acid or levulinic acid is used to form a nicotine salt.{{cite journal|last1=Voos|first1=Natalie|last2=Goniewicz|first2=Maciej L.|last3=Eissenberg|first3=Thomas|title=What is the nicotine delivery profile of electronic cigarettes?|journal=Expert Opinion on Drug Delivery|volume=16|issue=11|year=2019|pages=1193–1203|issn=1742-5247|doi=10.1080/17425247.2019.1665647|pmc=6814574|pmid=31495244}} Benzoic acid is the most used acid to create a nicotine salt. A free-base nicotine solution with an acid reduces the pH, which makes it possible to provide higher levels of nicotine without irritating the throat.{{cite journal|last1=Jenssen|first1=Brian P.|last2=Wilson|first2=Karen M.|title=What is new in electronic-cigarettes research?|journal=Current Opinion in Pediatrics|volume=31|issue=2|year=2019|pages=262–266|issn=1040-8703|doi=10.1097/MOP.0000000000000741|pmc=6644064|pmid=30762705}} Nicotine salts are thought to amplify the level and rate of nicotine delivery to the user.

The speed of nicotine salts uptake into the body is close to the speed of nicotine uptake from traditional cigarettes.{{cite news|url=https://www.aafp.org/patient-care/public-health/tobacco-nicotine/tools/e-cigs.html|title=JUUL: An Electronic Cigarette You Should Know About|publisher=American Academy of Family Physicians|year=2019|access-date=2019-09-30|archive-date=2019-09-30|archive-url=https://web.archive.org/web/20190930174401/https://www.aafp.org/patient-care/public-health/tobacco-nicotine/tools/e-cigs.html|url-status=dead}} Nicotine salts are less harsh and less bitter, and as a consequence e-liquids that contain nicotine salts are more tolerable even with high nicotine concentrations. Traditional cigarettes provide high levels of nicotine, but with the bad taste of smoking. Pod mods, however, can provide high levels of nicotine without the negative smoking experience.

Variable devices are variable wattage, variable voltage or both. Variable power and/or variable voltage have an electronic chip allowing the user to adjust the power applied to the heating element. The amount of power applied to the coil affects the heat produced, thus changing the vapor output. Greater heat from the coil increases vapor production. Variable power devices monitor the coil's resistance and automatically adjust the voltage to apply the user-specified level of power to the coil.{{cite news|url=http://spinfuel.com/taking-the-mystery-out-of-variable-wattage/|title=Taking The Mystery Out Of Variable Wattage|publisher=Spinfuel eMagazine|author=Tom McBride|date=28 February 2013}} Recent devices can go up to 8 V.

They are often rectangular but can also be cylindrical. They usually have a screen to show information such as voltage, power, and resistance of the coil.{{cite news|last1=Beach|first1=Dania|title=Vapor Corp. Launches New Store-in-Store VaporX(R) Retail Concept at Tobacco Plus Convenience Expo in Las Vegas|url=https://www.wsj.com/article/PR-CO-20140129-910446.html|archive-url=https://web.archive.org/web/20140222010101/http://online.wsj.com/article/PR-CO-20140129-910446.html|work=The Wall Street Journal|date=29 January 2014|url-status=dead|archive-date=22 February 2014}} To adjust the settings, the user presses buttons or rotates a dial to turn the power up or down. Some of these devices include additional settings through their menu system such as: atomizer resistance meter, remaining battery voltage, puff counter, and power-off or lock.{{cite web|url=http://realelectriccigarettereviews.com/joyetech-evic-review/|title=JoyeTech eVic Review|publisher=Real Electric Cigarettes Reviews|archive-url=https://web.archive.org/web/20150224064933/http://realelectriccigarettereviews.com/joyetech-evic-review/|archive-date=2015-02-24|url-status=dead}} The power source is the biggest component of an e-cigarette,{{cite journal|last1=Rom|first1=Oren|last2=Pecorelli|first2=Alessandra|last3=Valacchi|first3=Giuseppe|last4=Reznick|first4=Abraham Z.|title=Are E-cigarettes a safe and good alternative to cigarette smoking?|journal=Annals of the New York Academy of Sciences|volume=1340|issue=1|year=2014|pages=65–74|issn=0077-8923|doi=10.1111/nyas.12609|pmid=25557889|bibcode=2015NYASA1340...65R|s2cid=26187171}} which is frequently a rechargeable lithium-ion battery.

Smaller devices contain smaller batteries and are easier to carry but typically require more repeated recharging. Some e-cigarettes use a long lasting rechargeable battery, a non-rechargeable battery or a replaceable battery that is either rechargeable or non-rechargeable for power. Some companies offer portable chargeable cases to recharge e-cigarettes. Nickel-cadmium (NiCad), nickel metal-hydride (NiMh), lithium ion (Li-ion), alkaline and lithium polymer (Li-poly), and lithium manganese (LiMn) batteries have been used for the e-cigarettes power source.

File:Innokin SVD.jpg

Temperature control devices allow the user to set the temperature. There is a predictable change to the resistance of a coil when it is heated.{{cite web|url=http://spinfuel.com/temperature-coefficients-coil-wires/|title=Temperature Coefficients and Coil Wires|author1=Staff|date=8 December 2015}} The resistance changes are different for different types of wires, and must have a high temperature coefficient of resistance. Temperature control is done by detecting that resistance change to estimate the temperature and adjusting the voltage to the coil to match that estimate.{{cite news|url=http://spinfuel.com/temperature-control-vaping-the-decision-is-yours/|title=Temperature Control Vaping: The Decision Is Yours|author1=Tom McBride|date=8 December 2015}}

Nickel, titanium, NiFe alloys, and certain grades of stainless steel are common materials used for wire in temperature control. The most common wire used, kanthal, cannot be used because it has a stable resistance regardless of the coil temperature. Nickel was the first wire used because it has the highest coefficient of the common metals.

File:King Mod And Kaifun Lite.jpg

The temperature can be adjusted in Celsius or Fahrenheit.{{cite news|author1=Spinfuel Staff|title=HCigar VT40 Evolv DNA40 Mod|url=http://spinfuel.com/hcigar-vt40-evolv-dna40-mod/|publisher=Spinfuel eMagazine|date=3 August 2015}} The Evolv's DNA40 and YiHi's SX350J are control boards used in temperature control devices.{{cite news|author1=Tim Hanlon|title=Temperature-controlled e-cigs: The next giant leap in harm reduction of nicotine use?|url=http://www.gizmag.com/temperature-control-electronic-cigarettes/36095/|publisher=Gizmag|date=15 February 2015}} Temperature control can stop dry wicks from burning, or e-liquid overheating.

Mechanical PVs or mechanical "mods", often called "mechs", are devices without integrated circuits, electronic battery protection, or voltage regulation. They are activated by a switch. They rely on the natural voltage output of the battery and the metal that the mod is made of often is used as part of the circuit itself.{{cite web|url=http://onvaping.com/battery-safety-and-ohms-law/|title=Battery Safety and Ohm's Law|publisher=onVaping|author=Dale Amann|date=10 February 2014}}

The term "mod" was originally used instead of "modification". Users would modify existing hardwares to get better performance, and as an alternative to the e-cigarettes that looked like traditional cigarettes. Users would also modify other unrelated items like flashlights as battery compartments to power atomizers. The word mod is often used to describe most personal vaporizers.

Mechanical PVs have no power regulation and are unprotected. Because of this ensuring that the battery does not over-discharge and that the resistance of the atomizer requires electric current within the safety limits of the battery is the responsibility of the user.

E-cigarette liquid, E-Cig liquid, e-liquid, juice, vapor juice, vape juice, smoke juice, vaping fluid,{{cite news |last=M. Rosenberg |first=Joyce |date=10 October 2019 |title=Vaping fallout: Small stores suffer as vapers turn away |url=https://www.apnews.com/3fea9cf981794cd680dde081003fa926 |work=Associated Press}} vaping juice,{{cite news |author=Julie Stalmer |date=31 May 2018 |title=Vaping's Wild West |url=https://www.sandiegoreader.com/news/2018/may/31/stringers-vapings-wild-west/ |work=San Diego Reader}} e-juice,{{cite web |date=26 January 2018 |title=Experts Clear the Air on E-Cigarettes |url=https://www.mailman.columbia.edu/public-health-now/news/experts-clear-air-e-cigarettes |publisher=Columbia University Mailman School of Public Health}} e-fluid or vape oil{{cite news |author=Jarrett Lyons |date=5 October 2017 |title=Vaping is better for you than smoking cigarettes, says new study |url=https://www.salon.com/2017/10/05/vaping-vs-cigarettes/ |work=Salon}} is the mixture used in vapor products including e-cigarettes. E-liquids vary across and within brands.

File:Various E-liquid Bottles crop.jpg

Various e-liquids offer different nicotine strengths and flavors.{{cite journal|last1=Bekki|first1=Kanae|last2=Uchiyama|first2=Shigehisa|last3=Ohta|first3=Kazushi|last4=Inaba|first4=Yohei|last5=Nakagome|first5=Hideki|last6=Kunugita|first6=Naoki|title=Carbonyl Compounds Generated from Electronic Cigarettes|journal=International Journal of Environmental Research and Public Health|volume=11|issue=11|year=2014|pages=11192–11200|issn=1660-4601|doi=10.3390/ijerph111111192|pmid=25353061|pmc=4245608|doi-access=free}}{{#tag:ref|The liquid composition of each brand of e-cigarettes may differ, making it difficult to generalize about the potential toxic properties of these devices.{{cite journal|last1=Nansseu|first1=Jobert Richie N.|last2=Bigna|first2=Jean Joel R.|title=Electronic Cigarettes for Curbing the Tobacco-Induced Burden of Noncommunicable Diseases: Evidence Revisited with Emphasis on Challenges in Sub-Saharan Africa|journal=Pulmonary Medicine|volume=2016|year=2016|pages=1–9|issn=2090-1836|doi=10.1155/2016/4894352|pmc=5220510|pmid=28116156|doi-access=free}} {{CC-notice|cc=by4|url=https://www.hindawi.com/journals/pm/2016/4894352/|author(s)=Jobert Richie N. Nansseu and Jean Joel R. Bigna}}|group=notes}}

Common ingredients are propylene glycol, glycerin, flavorings, and nicotine.{{cite journal|last1=Caponnetto|first1=P.|last2=Russo|first2=C.|last3=Bruno|first3=C.M.|last4=Alamo|first4=A.|last5=Amaradio|first5=M.D.|last6=Polosa|first6=R.|title=Electronic cigarette: a possible substitute for cigarette dependence|journal=Monaldi Archives for Chest Disease|volume=79|issue=1|pages=12–19|date=March 2013|issn=1122-0643|doi=10.4081/monaldi.2013.104|pmid=23741941|doi-access=free}} Propylene glycol and glycerin typically comprise 95% , with the remaining 5% combining flavorings, nicotine, and other additives.{{cite journal|last1=Jimenez Ruiz|first1=CA|last2=Solano Reina|first2=S|last3=de Granda Orive|first3=JI|last4=Signes-Costa Minaya|first4=J|last5=de Higes Martinez|first5=E|last6=Riesco Miranda|first6=JA|last7=Altet Gómez|first7=N|last8=Lorza Blasco|first8=JJ|last9=Barrueco Ferrero|first9=M|last10=de Lucas Ramos|first10=P|title=The electronic cigarette. Official statement of the Spanish Society of Pneumology and Thoracic Surgery (SEPAR) on the efficacy, safety and regulation of electronic cigarettes.|journal=Archivos de Bronconeumologia|date=August 2014|volume=50|issue=8|pages=362–7|doi=10.1016/j.arbres.2014.02.006|pmid=24684764}}{{cite journal|last1=Schick|first1=Suzaynn F.|last2=Blount|first2=Benjamin C|last3=Jacob|first3=Peyton|last4=Saliba|first4=Najat A|author-link4=Najat A. Saliba |last5=Bernert|first5=John T|last6=El Hellani|first6=Ahmad|last7=Jatlow|first7=Peter|last8=Pappas|first8=R Steve|last9=Wang|first9=Lanqing|last10=Foulds|first10=Jonathan|last11=Ghosh|first11=Arunava|last12=Hecht|first12=Stephen S|last13=Gomez|first13=John C|last14=Martin|first14=Jessica R|last15=Mesaros|first15=Clementina|last16=Srivastava|first16=Sanjay|last17=St. Helen|first17=Gideon|last18=Tarran|first18=Robert|last19=Lorkiewicz|first19=Pawel K|last20=Blair|first20=Ian A|last21=Kimmel|first21=Heather L|last22=Doerschuk|first22=Claire M.|last23=Benowitz|first23=Neal L|last24=Bhatnagar|first24=Aruni|title=Biomarkers of Exposure to New and Emerging Tobacco and Nicotine Delivery Products|journal=American Journal of Physiology. Lung Cellular and Molecular Physiology|volume=313|issue=3|year=2017|pages=L425–L452|issn=1040-0605|doi=10.1152/ajplung.00343.2016|pmid=28522563|pmc=5626373}} Flavorings may contain menthol, sugars, esters, and pyrazines. Flavor components include diacetyl, acetoin, and 2,3-pentanedione (buttery flavor), eucalyptol, camphor and cyclohexanone (minty flavor), cinnamaldehyde (cinnamon flavor), benzaldehyde (cherry or almond flavor), cresol (leathery or medicinal flavor), butyraldehyde (chocolate flavor), and isoamyl acetate (banana flavor), methyl salicylate, pulegone, ethyl salicylate, eugenol, diphenyl ether, and coumarin. A 2017 review added 1,3-butanediol, 1,3-propanediol, ethylene and diethylene glycol, safrole, ethyl vanillin, and α-thujone to the list of ingreidents.{{cite journal |last1=Naskar |first1=Subrata |last2=Jakati |first2=PraveenKumar |year=2017 |title="Vaping:" Emergence of a new paraphernalia |journal=Indian Journal of Psychological Medicine |volume=39 |issue=5 |pages=566–572 |doi=10.4103/IJPSYM.IJPSYM_142_17 |issn=0253-7176 |pmc=5688881 |pmid=29200550 |doi-access=free}}

Caffeinated e-liquids typically contain considerably less caffeine than dietary products.{{sfn|Stratton|2018|p=Other Toxicants, Caffeine; 197}} E-liquids are available with vitamins or cannabis flavors. E-cigarettes (mods) are available that can vaporize herbs, oils, or fruits. Dual-function devices handle both concentrates and e-liquids using multiple cartridges.{{cite journal|last1=Staal|first1=Yvonne CM|last2=van de Nobelen|first2=Suzanne|last3=Havermans|first3=Anne|last4=Talhout|first4=Reinskje|title=New Tobacco and Tobacco-Related Products: Early Detection of Product Development, Marketing Strategies, and Consumer Interest|journal=JMIR Public Health and Surveillance|volume=4|issue=2|year=2018|pages=e55|issn=2369-2960|doi=10.2196/publichealth.7359|pmid=29807884|pmc=5996176 |doi-access=free }} {{CC-notice|cc=by4|url=http://publichealth.jmir.org/2018/2/e55/|author(s)=Yvonne CM Staal, Suzanne van de Nobelen, Anne Havermans, and Reinskje Talhout}}

More than 90% of e-liquids contain nicotine.{{cite web|url=http://roar.uel.ac.uk/1875/1/2013_Dawkins_e-cig_survey.pdf|title=Vaping' profiles and preferences: an online survey of electronic cigarette users|author1=Lynne Dawkins |author2=John Turner |author3=Amanda Roberts |author4=Kirstie Soar |publisher=School of Psychology-University of East London|year=2013}} Some are unflavored. Water replaces propylene glycol in some products.{{cite journal|last1=Oh|first1=Anne Y.|last2=Kacker|first2=Ashutosh|title=Do electronic cigarettes impart a lower potential disease burden than conventional tobacco cigarettes?: Review on e-cigarette vapor versus tobacco smoke|journal=The Laryngoscope|date=December 2014|volume=124|issue=12|pages=2702–2706|doi=10.1002/lary.24750|pmid=25302452|s2cid=10560264|doi-access=free}} E-liquids may contain THC or other cannabinoids.{{cite journal|last1=Giroud|first1=Christian|last2=de Cesare|first2=Mariangela|last3=Berthet|first3=Aurélie|last4=Varlet|first4=Vincent|last5=Concha-Lozano|first5=Nicolas|last6=Favrat|first6=Bernard|title=E-Cigarettes: A Review of New Trends in Cannabis Use|journal=International Journal of Environmental Research and Public Health|volume=12|issue=8|year=2015|pages=9988–10008|issn=1660-4601|doi=10.3390/ijerph120809988|pmc=4555324|pmid=26308021|doi-access=free}} Some e-liquids contain a little alcohol.{{cite book |author1=Xiaolong Zheng |url=https://books.google.com/books?id=cM5yCwAAQBAJ&pg=PA279 |title=Smart Health: International Conference, ICSH 2015, Phoenix, AZ, USA, November 17-18, 2015. Revised Selected Papers |author2=Daniel Dajun Zeng |author3=Hsinchun Chen |author4=Scott J. Leischow |date=22 January 2016 |publisher=Springer |isbn=978-3-319-29175-8 |pages=279–}}

E-liquids are not required to use only US Pharmacopeia grade nicotine, a tobacco plant, or tobacco dust extract, or a synthetic nicotine.{{cite journal |last1=Chang |first1=H. |title=Research gaps related to the environmental impacts of electronic cigarettes |journal=Tobacco Control |volume=23 |issue=Supplement 2 |year=2014 |pages=ii54–ii58 |issn=0964-4563 |doi=10.1136/tobaccocontrol-2013-051480 |pmc=3995274 |pmid=24732165}} Nicotine levels vary.{{cite journal|last1=Burstyn|first1=I|title=Peering through the mist: systematic review of what the chemistry of contaminants in electronic cigarettes tells us about health risks.|journal=BMC Public Health|date=9 January 2014|volume=14|pages=18|doi=10.1186/1471-2458-14-18|pmc=3937158|pmid=24406205|doi-access=free}} A user survey reported that 97% of respondents used nicotine e-liquids.{{cite journal|last1=Tomashefski|first1=Amy|title=The perceived effects of electronic cigarettes on health by adult users: A state of the science systematic literature review|journal=Journal of the American Association of Nurse Practitioners|volume=28|issue=9|year=2016|pages=510–5|issn=2327-6886|doi=10.1002/2327-6924.12358|pmid=26997487|s2cid=42900184}}

A 2016 study reported that measurable amounts of arsenic, nickel and other metals were present in some e-liquids.{{cite journal|last1=Chun|first1=Lauren F|last2=Moazed|first2=Farzad|last3=Calfee|first3=Carolyn S|author-link3=Carolyn Calfee|last4=Matthay|first4=Michael A.|last5=Gotts|first5=Jeffrey Earl|title=Pulmonary Toxicity of E-cigarettes|journal=American Journal of Physiology. Lung Cellular and Molecular Physiology|volume=313|issue=2|year=2017|pages=L193–L206|issn=1040-0605|doi=10.1152/ajplung.00071.2017|pmc=5582932|pmid=28522559}} Over 80 chemicals, including formaldehyde and metallic nanoparticles were reported in e-liquids (2019).{{cite journal|last1=Thirión-Romero|first1=Ireri|last2=Pérez-Padilla|first2=Rogelio|last3=Zabert|first3=Gustavo|last4=Barrientos-Gutiérrez|first4=Inti|s2cid=73511138|title=Respiratory Impact of Electronic Cigarettes and Low-Risk Tobacco|journal=Revista de investigación Clínica|volume=71|issue=1|pages=17–27|year=2019|issn=0034-8376|doi=10.24875/RIC.18002616|pmid=30810544|doi-access=free}} E-liquid can contain toxicants and impurities.{{cite journal|last1=Weaver|first1=Michael|last2=Breland|first2=Alison|last3=Spindle|first3=Tory|last4=Eissenberg|first4=Thomas|title=Electronic Cigarettes|journal=Journal of Addiction Medicine|volume=8|issue=4|year=2014|pages=234–240|issn=1932-0620|doi=10.1097/ADM.0000000000000043|pmc=4123220|pmid=25089953}} A 2013 study reported as high as five times the permitted levels of impurities. E-liquids contained low levels of some of tobacco smoke toxicants and small concentrations of carcinogens.File:Empty-510-cartridge.jpg

In 2009, FDA reported tobacco-specific nitrosamines (TSNAs), diethylene glycol, cotinine, anabasine, myosmine, and beta-nicotyrine in e-liquids. TSNAs N-nitrosonornicotine (NNN), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), N-nitrosoanabasine, and nitrosoanatabine were detected in five e-liquid samples from two companies at levels comparable to other nicotine replacement products.{{cite journal|vauthors=Jerry JM, Collins GB, Streem D|title=E-cigarettes: Safe to recommend to patients?|journal=Cleve Clin J Med|volume=82 |issue=8|pages=521–6|year=2015|doi=10.3949/ccjm.82a.14054|pmid=26270431|doi-access=free}}{{cite journal|last1=Naik|first1=Pooja|last2=Cucullo|first2=Luca|title=Pathobiology of tobacco smoking and neurovascular disorders: untied strings and alternative products|journal=Fluids and Barriers of the CNS|volume=12|issue=1|pages=25|year=2015|issn=2045-8118|doi=10.1186/s12987-015-0022-x|pmc=4628383|pmid=26520792 |doi-access=free }} TSNAs present in tobacco smoke were reported in trace amounts.{{cite journal|last1=Breland|first1=Alison B.|last2=Spindle|first2=Tory|last3=Weaver|first3=Michael|last4=Eissenberg|first4=Thomas|title=Science and Electronic Cigarettes|journal=Journal of Addiction Medicine|volume=8|issue=4|year=2014|pages=223–233|issn=1932-0620|doi=10.1097/ADM.0000000000000049|pmc=4122311|pmid=25089952}} A 2013 review of other e-liquids did not detect diethylene glycol.{{cite journal|last1=Hajek|first1=P|last2=Etter|first2=JF|last3=Benowitz|first3=N|last4=Eissenberg|first4=T|last5=McRobbie|first5=H|title=Electronic cigarettes: review of use, content, safety, effects on smokers and potential for harm and benefit.|url=http://europepmc.org/backend/ptpmcrender.fcgi?accid=PMC4487785&blobtype=pdf|journal=Addiction|date=31 July 2014|volume=109|issue=11|pages=1801–10|doi=10.1111/add.12659|pmc=4487785|pmid=25078252}} The majority of the e-liquids analyzed contained NNN from 0.34 to 60.08 μg/L and NNK from 0.22 to 9.84 μg/L. The FDA warned several e-cigarette companies for selling e-cartridges and refill solutions containing active pharmaceutical ingredients such as rimonabant (Zimulti) for weight loss purposes and reducing smoking, and tadalafil (for erictile dysfunction). FDA analyses reported the presence of am,ino-tadalafil rather than tadalafil, and rimonabant and one of its oxidative products.{{cite journal|title=Electronic cigarettes and vaping: a new challenge in clinical medicine and public health. A literature review.|first1=Dominic L.|last1=Palazzolo|journal=Frontiers in Public Health|volume=1|issue=56|pages=56|date=November 2013|doi=10.3389/fpubh.2013.00056|pmc=3859972|pmid=24350225|doi-access=free}} {{CC-notice|cc=by3|url=http://journal.frontiersin.org/article/10.3389/fpubh.2013.00056/full|author(s)=Dominic L. Palazzolo}}

E-liquids often contain unknown and/or undisclosed substances.{{cite journal |last1=Jenssen |first1=Brian P. |last2=Wilson |first2=Karen M. |title=Tobacco Control and Treatment for the Pediatric Clinician: Practice, Policy, and Research Updates |journal=Academic Pediatrics |volume=17 |issue=3 |pages=233–242 |year=2017 |issn=1876-2859 |doi=10.1016/j.acap.2016.12.010 |pmid=28069410}}

Poor quality control may allow nicotine and TSNAs to be present in e-liquids labelled 'no nicotine'. Some e-liquid were reported to contain low levels of anthracene, phenanthrene, 1-methyl phenanthrene and pyren.{{cite journal|last1=Orr|first1=Michael S|title=Electronic cigarettes in the USA: a summary of available toxicology data and suggestions for the future: Table 1|journal=Tobacco Control|volume=23|issue=suppl 2|year=2014|pages=ii18–ii22|issn=0964-4563|doi=10.1136/tobaccocontrol-2013-051474|pmid=24732158|pmc=3995288}} A 2015 study reported hydrocarbons, terpenic compounds and aldehydes, particularly formaldehyde and acrolein in some e-liquids.{{cite journal|last1=Varlet|first1=Vincent|last2=Farsalinos|first2=Konstantinos|last3=Augsburger|first3=Marc|last4=Thomas|first4=Aurélien|last5=Etter|first5=Jean-François|title=Toxicity Assessment of Refill Liquids for Electronic Cigarettes|journal=International Journal of Environmental Research and Public Health|volume=12|issue=5|year=2015|pages=4796–4815|issn=1660-4601|doi=10.3390/ijerph120504796|pmid=25941845|pmc=4454939|doi-access=free}}

A 2014 study reported that e-liquids from a specific manufacturer contained greater amounts of ethylene glycol than glycerin or propylene glycol, possibly the result of improper manufacturing methods.{{cite journal|last1=Dinakar|first1=Chitra|last2=Longo|first2=Dan L.|last3=O'Connor|first3=George T.|title=The Health Effects of Electronic Cigarettes|journal=New England Journal of Medicine|volume=375|issue=14|year=2016|pages=1372–1381|issn=0028-4793|doi=10.1056/NEJMra1502466|pmid=27705269}} A 2015 study reported finding solvents such as 1,3-butadiene, cyclohexane, and acetone in e-liquids. A 2016 study reported that some e-liquids contained tobacco alkaloids such as nornicotine, anabasine, anatabine, and TSNAs, such as N-nitrosonornicotine (NNN), 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone (NNK), nitrates, and phenol. A 2014 study reported small quantities of volatile organic compounds (VOCs) such as benzene, toluene, xylene, and styrene. Diethyl phthalate and diethylhexyl phthalate have been found in e-liquids.{{sfn|Stratton|2018|p=Other Toxicants, Phthalates; 196}} Some e-liquids contain tin "whiskers," microscopic crystals that originate from tin in solder joints.

∗A 2013 analysis tested a total of 42 bottles of e-liquids.{{cite journal|last1=Varlet|first1=Vincent|last2=Farsalinos|first2=Konstantinos|last3=Augsburger|first3=Marc|last4=Thomas|first4=Aurélien|last5=Etter|first5=Jean-François|title=Aldehydes (in μg/g) in 42 bottles of e-liquids, 2013.|journal=International Journal of Environmental Research and Public Health|volume=12|issue=5|pages=4796–4815|year=2015|doi=10.3390/ijerph120504796|pmc=4454939|pmid=25941845|doi-access=free}}

The e-liquid is sold in bottles, pre-filled disposable cartridges, or as a kit for consumers to make their own e-liquids.{{cite web | last=Nguyen | first=Terry | title=There's a community of DIY vapers who mix their own flavors | website=Vox | date=2019-09-27 | url=https://www.vox.com/the-goods/2019/9/27/20887283/diy-vapers-mix-own-flavors | access-date=2021-12-31}} E-liquids are made with various tobacco, fruit, and other flavors, as well as variable nicotine concentrations (including nicotine-free versions). The standard notation "mg/ml" is often used on labels to denote nicotine concentration, and is sometimes shortened to "mg".{{cite web|url=http://www.healthnz.co.nz/Ruyan_ecigarette.htm|author=Murray Laugesen|title=The Ruyan e-cigarette; Technical Information Sheet|publisher=Health New Zealand|date=17 October 2007|access-date=18 May 2015|archive-date=24 April 2008|archive-url=https://web.archive.org/web/20080424083531/http://www.healthnz.co.nz/Ruyan_ecigarette.htm|url-status=dead}} Some flavors are created to resemble the flavors used in traditional cigarettes such as tobacco and menthol-tobacco.{{cite journal|last1=DeVito|first1=Elise E.|last2=Krishnan-Sarin|first2=Suchitra|title=E-cigarettes: Impact of E-Liquid Components and Device Characteristics on Nicotine Exposure|journal=Current Neuropharmacology|volume=16|issue=4|year=2018|pages=438–459|issn=1570-159X|doi=10.2174/1570159X15666171016164430|pmc=6018193|pmid=29046158}} Adults in general also preferred sweet flavors (though smokers like tobacco flavor the most) and disliked flavors that elicit bitterness or harshness. Young adults overall preferred sweet, menthol, and cherry flavors, while non-smokers in particular preferred coffee and menthol flavors.{{cite journal|last1=Cormet-Boyaka|first1=Estelle|last2=Zare|first2=Samane|last3=Nemati|first3=Mehdi|last4=Zheng|first4=Yuqing|title=A systematic review of consumer preference for e-cigarette attributes: Flavor, nicotine strength, and type|journal=PLOS ONE|volume=13|issue=3|year=2018|pages=e0194145|issn=1932-6203|doi=10.1371/journal.pone.0194145|pmid=29543907|pmc=5854347|bibcode=2018PLoSO..1394145Z|doi-access=free}} {{CC-notice|cc=by4|url=http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0194145|author(s)=Samane Zare, Mehdi Nemati, and Yuqing Zheng}}

In surveys of regular e-cigarette users, the most popular e-liquids had a nicotine content of 18 mg/ml, and the preferred flavors were largely tobacco, mint and fruit. Men tend to favor flavors with tobacco, while women tend to favor chocolate or sweet flavors.{{cite web |year=2013 |title=Electronic Cigarettes – An Overview |url=http://www.dkfz.de/en/presse/download/RS-Vol19-E-Cigarettes-EN.pdf |publisher=German Cancer Research Center |pages=3, 18}} The most favorite flavors among regular e-cigarette users reported in a 2017 UK survey were fruit, tobacco, and menthol/mint.{{sfn|McNeill|2018|p=95}} The survey also found 2.6% regular e-cigarette users used no flavors.{{sfn|McNeill|2018|p=95}} A 2013 study examined 33 countries and found that only 1% of the adult smokers exclusively used non-nicotine e-cigarettes. A cartridge may contain 0 to 20 mg of nicotine.

Refill liquids are often sold in the size range from 15 to 30 ml.{{cite journal|last1=Hildick-Smith|first1=Gordon J.|last2=Pesko|first2=Michael F.|last3=Shearer|first3=Lee|last4=Hughes|first4=Jenna M.|last5=Chang|first5=Jane|last6=Loughlin|first6=Gerald M.|last7=Ipp|first7=Lisa S.|title=A Practitioner's Guide to Electronic Cigarettes in the Adolescent Population|journal=Journal of Adolescent Health|year=2015|volume=57|issue=6|pages=574–9|issn=1054-139X|doi=10.1016/j.jadohealth.2015.07.020|pmid=26422289|doi-access=free}} E-liquids are frequently sold in dropper bottles.{{cite journal|last1=Zainol Abidin|first1=Najihah|last2=Zainal Abidin|first2=Emilia|last3=Zulkifli|first3=Aziemah|last4=Karuppiah|first4=Karmegam|last5=Syed Ismail|first5=Sharifah Norkhadijah|last6=Amer Nordin|first6=Amer Siddiq|title=Electronic cigarettes and indoor air quality: a review of studies using human volunteers|journal=Reviews on Environmental Health|issue=3|pages=235–244|year=2017|volume=32|issn=2191-0308|doi=10.1515/reveh-2016-0059|pmid=28107173|s2cid=6885414|url=http://psasir.upm.edu.my/id/eprint/61699/1/Electronic%20cigarettes%20and%20indoor%20air%20quality.pdf}} One cartridge may typically last as long as one pack of cigarettes.{{cite journal|last1=Odum|first1=L. E.|last2=O'Dell|first2=K. A.|last3=Schepers|first3=J. S.|title=Electronic Cigarettes: Do They Have a Role in Smoking Cessation?|journal=Journal of Pharmacy Practice|volume=25|issue=6|year=2012|pages=611–614|issn=0897-1900|doi=10.1177/0897190012451909|pmid=22797832|s2cid=12140044}} A refill bottle can contain up to 100 mg/ml of nicotine,{{cite journal|last1=Cervellin|first1=Gianfranco|last2=Borghi|first2=Loris|last3=Mattiuzzi|first3=Camilla|last4=Meschi|first4=Tiziana|last5=Favaloro|first5=Emmanuel|last6=Lippi|first6=Giuseppe|title=E-Cigarettes and Cardiovascular Risk: Beyond Science and Mysticism|journal=Seminars in Thrombosis and Hemostasis|volume=40|issue=1|year=2013|pages=060–065|issn=0094-6176|doi=10.1055/s-0033-1363468|pmid=24343348|doi-access=free}} which is meant to be diluted before use.{{cite journal|last1=Chatham-Stephens|first1=Kevin|last2=Law|first2=Royal|last3=Taylor|first3=Ethel|last4=Kieszak|first4=Stephanie|last5=Melstrom|first5=Paul|last6=Bunnell|first6=Rebecca|last7=Wang|first7=Baoguang|last8=Day|first8=Hannah|last9=Apelberg|first9=Benjamin|last10=Cantrell|first10=Lee|last11=Foster|first11=Howell|last12=Schier|first12=Joshua G.|title=Exposure Calls to U. S. Poison Centers Involving Electronic Cigarettes and Conventional Cigarettes—September 2010–December 2014|journal=Journal of Medical Toxicology|volume=12|issue=4|pages=350–357|date=June 2016|issn=1556-9039|doi=10.1007/s13181-016-0563-7|pmc=5135675|pmid=27352081}} Some users, probably due to financial reasons and the willingness to experiment, are opting to make homemade e-liquids.{{cite journal|last1=Jankowski|first1=Mateusz|last2=Brożek|first2=Grzegorz|last3=Lawson|first3=Joshua|last4=Skoczyński|first4=Szymon|last5=Zejda|first5=Jan|title=E-smoking: Emerging public health problem?|journal=International Journal of Occupational Medicine and Environmental Health|volume=30|issue=3|pages=329–344|year=2017|issn=1232-1087|doi=10.13075/ijomeh.1896.01046|pmid=28481369|doi-access=free}} A small percentage of liquids without flavoring is also sold.{{cite journal|last1=Tierney|first1=Peyton A|last2=Karpinski|first2=Clarissa D|last3=Brown|first3=Jessica E|last4=Luo|first4=Wentai|last5=Pankow|first5=James F|title=Flavour chemicals in electronic cigarette fluids|journal=Tobacco Control|volume=25|issue=e1|year=2016|pages=e10–e15|issn=0964-4563|doi=10.1136/tobaccocontrol-2014-052175|pmid=25877377|pmc=4853541}} The flavorings may be natural or artificial. Certified organic e-liquid is also sold.{{cite news|author1=Dan Nosowitz|title=America's First Certified Organic E-Cigarette Vaping Liquid Is Here|url=http://modernfarmer.com/2015/06/americas-first-certified-organic-e-cigarette-vaping-liquid-is-here/|publisher=Modern Farmer|date=5 June 2015}} About 8,000 flavors existed in 2014.{{cite web|url=https://www.who.int/nmh/events/2014/backgrounder-e-cigarettes/en/|archive-url=https://web.archive.org/web/20140827152721/http://www.who.int/nmh/events/2014/backgrounder-e-cigarettes/en/|url-status=dead|archive-date=August 27, 2014|title=Backgrounder on WHO report on regulation of e-cigarettes and similar products|date=26 August 2014}} More than 15,500 flavors existed in 2018.{{cite journal|last1=Henry|first1=Travis S.|last2=Kligerman|first2=Seth J.|last3=Raptis|first3=Constantine A.|last4=Mann|first4=Howard|last5=Sechrist|first5=Jacob W.|last6=Kanne|first6=Jeffrey P.|title=Imaging Findings of Vaping-Associated Lung Injury|journal=American Journal of Roentgenology|volume=214|issue=3|year=2020|pages=498–505|issn=0361-803X|doi=10.2214/AJR.19.22251|pmid=31593518|s2cid=203985885}}

A user does not normally consume a whole cartridge in a single session. Most e-liquids are produced by a few manufacturers in China, the US, and Europe. An e-cigarette user will usually obtain 300 to 500 puffs per ml of e-liquid. A 2017 survey found that 62.2% of everyday e-cigarette users stated using lower than 4 ml daily and 1.5% used higher than 10 ml daily.{{sfn|McNeill|2018|p=92}} 18.1% of everyday e-cigarette users were not aware of the amount of e-liquid they use.{{sfn|McNeill|2018|p=92}}

E-liquids are manufactured by many producers, both in the US and across the world. First tier manufacturers use lab suits, gloves, hair covers, inside of certified clean rooms with air filtration similar to pharmaceutical-grade production areas.{{cite web|url=http://www.richmond.com/business/local/article_9f65201a-94c0-51b1-9663-f9fbb5c522af.html|title=Avail Vapor offers glimpse into the 'art and science' of e-liquids|author=John Reid Blackwell|publisher=Richmond Times-Dispatch|date=7 June 2015}}

E-liquid manufacturing requirements under the US Food and Drug Administration (FDA) rules include report user fee information, pay user fees, register their establishment and submit list of products, including labeling and advertisements, submit health documents, submit ingredient listing, include required warning statements on packages and advertisements, submit quantities of harmful and potentially harmful constituents, and submit a modified risk tobacco product application. The revision to the EU Tobacco Products Directive has some standards for e-liquids.

Standards for e-liquid manufacturing have been created by American E-liquid Manufacturing Standards Association (AEMSA), which is trade association dedicated to creating responsible and sustainable standards for the safe manufacturing of e-liquids used in vapor products.{{cite web|url=http://www.aemsa.org/welcome-to-the-aemsa/|title=About AEMSA|website=AEMSA|year=2015}} AEMSA has published a comprehensive list standards and best known methods, which are openly available for use by any manufacturer of e-Liquids. The AEMSA standards cover nicotine, ingredients, sanitary manufacturing rooms, safety packaging, age restrictions, and labeling.{{cite book|title=E-Liquid Manufacturing Standards|url=http://www.aemsa.org/wp-content/uploads/2015/09/AEMSA-Standards-v2.3.pdf|location=US|publisher=American E-Liquid Manufacturing Standards Association (AEMSA)|date=4 September 2015|pages=1–13}} AEMSA guidelines recommend that the nicotine levels in e-liquids be within the amount of ±10% from the levels stated on the label.{{cite journal|last1=Zulkifli|first1=Aziemah|last2=Abidin|first2=Emilia Zainal|last3=Abidin|first3=Najihah Zainol|last4=Amer Nordin|first4=Amer Siddiq|last5=Praveena|first5=Sarva Mangala|last6=Syed Ismail|first6=Sharifah Norkhadijah|last7=Rasdi|first7=Irniza|last8=Karuppiah|first8=Karmegam|last9=Rahman|first9=Anita Abd|title=Electronic cigarettes: a systematic review of available studies on health risk assessment|journal=Reviews on Environmental Health|issue=1|pages=43–52|year=2016|volume=33|issn=2191-0308|doi=10.1515/reveh-2015-0075|pmid=27101543|s2cid=3702954|url=http://psasir.upm.edu.my/id/eprint/72548/1/Electronic%20cigarettes.pdf}}

Effective August 8, 2016, under the FDA rules, a company that mixes or prepares e-liquids is regulated as a tobacco product manufacturer. Under the same regulation, a company that sells e-liquids is regulated as a tobacco retailer.{{cite web|url=https://www.fda.gov/TobaccoProducts/GuidanceComplianceRegulatoryInformation/ucm514606.htm|title=Pipe, Cigar, and Vape Shops that Are Regulated as Both Retailers and Manufacturers|publisher=United States Food and Drug Administration|date=8 August 2016}} Companies who import or try to sell for import into the US must conform to the Federal Food, Drug, and Cosmetic Act.{{cite web|url=https://www.fda.gov/TobaccoProducts/Labeling/ProductsIngredientsComponents/ucm456610.htm|title=Vaporizers, E-Cigarettes, and other Electronic Nicotine Delivery Systems (ENDS)|publisher=United States Food and Drug Administration|date=7 August 2016}} The 2016 FDA ruling did not incorporate regulation concerning flavoring of e-cigarettes.{{cite journal|last1=Biyani|first1=Sneh|last2=Derkay|first2=Craig S.|title=E-cigarettes: An update on considerations for the otolaryngologist|journal=International Journal of Pediatric Otorhinolaryngology|volume=94|year=2017|pages=14–16|issn=0165-5876|doi=10.1016/j.ijporl.2016.12.027|pmid=28167004}} Industry standards have been created and published by the American E-liquid Manufacturing Standards Association (AEMSA). The FDA authority to regulate e-liquids was announced in May 2016.{{cite news|url=https://www.cnn.com/2016/05/05/health/fda-e-cigarettes-regulation/index.html|title=FDA to extend tobacco regulations to e-cigarettes, other products|author=Jen Christensen|publisher=CNN|date=5 May 2016}}{{cite web|url=https://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm499234.htm|title=FDA takes significant steps to protect Americans from dangers of tobacco through new regulation|publisher=United States Food and Drug Administration|date=5 May 2016}} The FDA has sought to regulate e-liquid in 2014{{cite web|url=https://www.fda.gov/TobaccoProducts/Labeling/ucm388395.htm|archive-url=https://web.archive.org/web/20140426203356/https://www.fda.gov/TobaccoProducts/Labeling/ucm388395.htm|url-status=dead|archive-date=26 April 2014|title=Products, Guidance & Regulations - Deeming – Extending Authorities to Additional Tobacco Products|publisher=United States Food and Drug Administration|date=25 April 2014}} through use of the Family Smoking Prevention and Tobacco Control Act,{{cite news|url=http://www.rstreet.org/op-ed/fda-regulation-defining-e-cigarettes-as-tobacco-products/|title=FDA regulation: Defining e-cigarettes as tobacco products|author=Brad Rodu|publisher=R Street Institute|date=30 April 2014}} passed into law in June 2009.{{cite web|url=https://www.fda.gov/TobaccoProducts/GuidanceComplianceRegulatoryInformation/ucm246129.htm|title=Compliance, Enforcement & Training - Tobacco Control Act|publisher=United States Food and Drug Administration|date=17 January 2018}} In April 2014, the FDA issued its "Deeming" proposals for public comment, which would cover e-liquids manufacturing.{{cite web|url=https://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm394667.htm|title=FDA proposes to extend its tobacco authority to additional tobacco products, including e-cigarettes|date=24 April 2014|publisher=United States Food and Drug Administration}}

Manufacturers of e-liquid in the UK are required to inform the Government regarding the content in each liquid.{{cite news|url=http://metro.co.uk/2016/05/20/e-cigarette-laws-have-changed-heres-what-you-need-to-know-5893552/|title=E-cigarette laws have changed – here's what you need to know|author=Ashitha Nagesh|work=Metro (British newspaper)|date=20 May 2016}} The EU Tobacco Products Directive requires e-liquids to be tested 6 months before they are sold.{{cite news|url=http://www.gloucestershirelive.co.uk/why-are-there-so-many-vaping-shops-in-gloucester/story-29975013-detail/story.html#cxtWCkSEfcUihBWc.99|title=Why are there so many vaping shops in Gloucester and how is it sustainable?|author=Matt Discombe|publisher=Gloucestershire Live|date=11 December 2016|archive-url=https://web.archive.org/web/20161212130757/http://www.gloucestershirelive.co.uk/why-are-there-so-many-vaping-shops-in-gloucester/story-29975013-detail/story.html#cxtWCkSEfcUihBWc.99|archive-date=12 December 2016|url-status=dead}}

The Tobacco Products Directive in the EU limits the sale of e-liquid. It can only be sold in 10 ml bottles, which need to have a child-proof closure. They have to be pre-registered to the Medicines and Healthcare products Regulatory Agency before sale. There is also a limit on the nicotine content, meaning the nicotine strength of any e-liquid cannot exceed 20 mg/ml (2.0%).{{cite web|url=https://www.gov.uk/guidance/e-cigarettes-regulations-for-consumer-products|title=E-cigarettes: regulations for consumer products|publisher=GOV.UK|date=19 January 2018}} Refill liquids in the EU with more than 20 mg/ml of nicotine may be sold with prior authorization from the pharmaceutical regulation.

As of January 2020, the Food and Drug Administration put new regulations on the flavor of e-liquids. They ban companies from manufacturing any juices or pre-filled pods that contained fruity or minty flavors. This restriction also banned stores from selling any flavors of e-liquid that are fruity or minty that could have been imported from a different country.{{Cite journal | url=https://www.fda.gov/tobacco-products/products-ingredients-components/vaporizers-e-cigarettes-and-other-electronic-nicotine-delivery-systems-ends | title=Vaporizers, E-Cigarettes, and other Electronic Nicotine Delivery Systems (ENDS)| journal=FDA| date=13 April 2020}}

Smoking a traditional cigarette yields between 0.5 and 1.5 mg of nicotine,{{cite journal|last1=Schroeder|first1=M. J.|last2=Hoffman|first2=A. C.|title=Electronic cigarettes and nicotine clinical pharmacology|journal=Tobacco Control|volume=23|issue=Supplement 2|year=2014|pages=ii30–ii35|issn=0964-4563|doi=10.1136/tobaccocontrol-2013-051469|pmc=3995273|pmid=24732160}} but the nicotine content of the cigarette is only weakly correlated with the levels of nicotine in the smoker's bloodstream.{{cite journal|last1=Goniewicz|first1=Maciej L.|last2=Hajek|first2=Peter|last3=McRobbie|first3=Hayden|title=Nicotine content of electronic cigarettes, its release in vapour and its consistency across batches: regulatory implications|url=https://www.ocf.berkeley.edu/~dshuster/e-Cigarettes/Goniewicz_2013.pdf|journal=Addiction|volume=109|issue=3|year=2014|pages=500–507|issn=0965-2140|doi=10.1111/add.12410|pmid=24345184}} The amount of nicotine in the e-cigarette aerosol varies widely either from puff-to-puff or among devices of the same company. In practice e-cigarette users tend to reach lower blood nicotine concentrations than smokers, particularly when the users are inexperienced or using first-generation devices. Nicotine in cigarette smoke is absorbed into the bloodstream rapidly, and e-cigarette aerosol is relatively slow in this regard.

Vaping typically gives a lower amount of nicotine per puff than smoking cigarettes.{{cite journal|last1=Bullen|first1=Christopher|title=Electronic Cigarettes for Smoking Cessation|journal=Current Cardiology Reports|volume=16|pages=538|issue=11|year=2014|issn=1523-3782|doi=10.1007/s11886-014-0538-8|pmid=25303892|s2cid=2550483|url=http://qmro.qmul.ac.uk/xmlui/handle/123456789/16169}} E-liquids contain nicotine in a variety of different strengths.{{Cite journal|last1=Goniewicz|first1=Maciej L.|last2=Kuma|first2=Tomasz|last3=Gawron|first3=Michal|last4=Knysak|first4=Jakub|last5=Kosmider|first5=Leon|date=2013-01-01|title=Nicotine Levels in Electronic Cigarettes|url=https://academic.oup.com/ntr/article/15/1/158/1105400|journal=Nicotine & Tobacco Research|language=en|volume=15|issue=1|pages=158–166|doi=10.1093/ntr/nts103|pmid=22529223|issn=1462-2203|doi-access=free}} From no nicotine{{Cite journal|last1=Dawkins|first1=L. E.|last2=Cox|first2=S. A.|last3=Kosmider|first3=L.|last4=McRobbie|first4=H.|last5=Goniewicz|first5=M.|last6=Kimber|first6=C. F.|last7=Doig|first7=M.|date=2016-09-20|title=E-cigarette puffing patterns associated with high and low nicotine e-liquid strength: effects on toxicant and carcinogen exposure (study protocol)|journal=BMC Public Health|language=en|volume=16|page=999|doi=10.1186/s12889-016-3653-1|pmid=27650300|pmc=5028920|issn=1471-2458|doi-access=free}} to 36 mg/ml.{{cite journal|last1=Kaisar|first1=Mohammad Abul|last2=Prasad|first2=Shikha|last3=Liles|first3=Tylor|last4=Cucullo|first4=Luca|year=2016|title=A Decade of e-Cigarettes: Limited Research & Unresolved Safety Concerns|journal=Toxicology|volume=365|pages=67–75|doi=10.1016/j.tox.2016.07.020|issn=0300-483X|pmc=4993660|pmid=27477296}} On average a regular cigarette contains 6–28 mg of nicotine or the user will inhale about 1.1 to 1.8 mg of nicotine if just a portion is used.{{citation needed|date=July 2023}} On average an e-cigarette contains 0.5–15.4 mg of nicotine per 15 puffs.{{citation needed|date=July 2023}} In practice, the nicotine concentration in an e-liquid is not a reliable guide to the amount of nicotine that reaches the bloodstream.{{Sfn|McNeill|2015|p=69-70}}

{{reflist|group=notes}}

• {{cite web|last1=McNeill|first1=A|last2=Brose|first2=LS|last3=Calder|first3=R|last4=Bauld|first4=L|last5=Robson|first5=D|url=https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/684963/Evidence_review_of_e-cigarettes_and_heated_tobacco_products_2018.pdf|title=Evidence review of e-cigarettes and heated tobacco products 2018|location=UK|publisher=Public Health England|pages=1–243|date=February 2018|ref={{harvid|McNeill|2018}}}}
• {{cite book|last1=Stratton|first1=Kathleen|last2=Kwan|first2=Leslie Y.|last3=Eaton|first3=David L.|editor-first1=Kathleen |editor-first2=Leslie Y. |editor-first3=David L. |editor-last1=Stratton |editor-last2=Kwan |editor-last3=Eaton |url=https://www.nap.edu/resource/24952/012318ecigaretteHighlights.pdf|title=Public Health Consequences of E-Cigarettes|publisher=National Academies of Sciences, Engineering, and Medicine (National Academies Press)|pages=1–774|date=January 2018|doi=10.17226/24952|pmid=29894118|ref={{harvid|Stratton|2018}}|isbn=978-0-309-46834-3}}
• {{cite web|last1=McNeill|first1=A|last2=Brose|first2=LS|last3=Calder|first3=R|last4=Hitchman|first4=SC|last5=Hajek|first5=P|last6= McRobbie|first6=H|url=https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/454516/Ecigarettes_an_evidence_update_A_report_commissioned_by_Public_Health_England.pdf|title=E-cigarettes: an evidence update|location=UK|publisher=Public Health England|pages=1–113|date=August 2015|ref={{harvid|McNeill|2015}}}}
• Weingarten, A (March 2018). [https://www.eightvape.com/blogs/news/65586755-welcome-to-the-world-of-sub-ohm-vaping "Welcome to the World of Sub-Ohm Vaping"] USA: Industry Trades

{{reflist}}

{{Electronic cigarettes}}

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