Carbon dioxide#Below 1%

{{See also|Molecular orbital diagram#Carbon dioxide}}

The symmetry of a carbon dioxide molecule is linear and centrosymmetric at its equilibrium geometry. The length of the carbon–oxygen bond in carbon dioxide is 116.3 pm, noticeably shorter than the roughly 140 pm length of a typical single C–O bond, and shorter than most other C–O multiply bonded functional groups such as carbonyls. Since it is centrosymmetric, the molecule has no electric dipole moment.

File:Co2 vibrations.svg of the {{CO2}} molecule. Upper left: symmetric stretching. Upper right: antisymmetric stretching. Lower line: degenerate pair of bending modes.]]

As a linear triatomic molecule, {{CO2}} has four vibrational modes as shown in the diagram. In the symmetric and the antisymmetric stretching modes, the atoms move along the axis of the molecule. There are two bending modes, which are degenerate, meaning that they have the same frequency and same energy, because of the symmetry of the molecule. When a molecule touches a surface or touches another molecule, the two bending modes can differ in frequency because the interaction is different for the two modes. Some of the vibrational modes are observed in the infrared (IR) spectrum: the antisymmetric stretching mode at wavenumber 2349 cm⁻¹ (wavelength 4.25 μm) and the degenerate pair of bending modes at 667 cm⁻¹ (wavelength 15.0 μm). The symmetric stretching mode does not create an electric dipole so is not observed in IR spectroscopy, but it is detected in Raman spectroscopy at 1388 cm⁻¹ (wavelength 7.20 μm), with a Fermi resonance doublet at 1285 cm⁻¹.{{cite book | vauthors = Atkins P, de Paula J | title = Physical Chemistry | edition = 8th | publisher = W.H. Freeman | date = 2006 | pages = 461, 464 | isbn = 978-0-7167-8759-4}}

In the gas phase, carbon dioxide molecules undergo significant vibrational motions and do not keep a fixed structure. However, in a Coulomb explosion imaging experiment, an instantaneous image of the molecular structure can be deduced. Such an experiment{{cite journal | vauthors = Siegmann B, Werner U, Lutz HO, Mann R | title = Complete Coulomb fragmentation of {{CO2}} in collisions with 5.9 MeV u⁻¹ Xe¹⁸⁺ and Xe⁴³⁺ | journal = J Phys B | volume = 35 | issue = 17 | page = 3755 | year = 2002 | doi = 10.1088/0953-4075/35/17/311 | bibcode = 2002JPhB...35.3755S | s2cid = 250782825}} has been performed for carbon dioxide. The result of this experiment, and the conclusion of theoretical calculations{{cite journal |vauthors = Jensen P, Spanner M, Bunker PR |title = The {{CO2}} molecule is never linear− |journal = J Mol Struct |volume = 1212 |page = 128087 |year = 2020 |doi = 10.1016/j.molstruc.2020.128087 |bibcode = 2020JMoSt121228087J |hdl = 2142/107329 |hdl-access = free }} based on an ab initio potential energy surface of the molecule, is that none of the molecules in the gas phase are ever exactly linear. This counter-intuitive result is trivially due to the fact that the nuclear motion volume element vanishes for linear geometries. This is so for all molecules except diatomic molecules.

{{See also|Carbonic acid}}

Carbon dioxide is soluble in water, in which it reversibly forms {{chem2|H2CO3}} (carbonic acid), which is a weak acid, because its ionization in water is incomplete.

:{{chem2|CO2 + H2O <-> H2CO3}}

The hydration equilibrium constant of carbonic acid is, at 25 °C:

:$K\_\backslash mathrm\{h\}\; =\; \backslash frac\{\backslash ce\{[H2CO3]\}\}\{\backslash ce\{[CO2\_\{(aq)\}]\}\}\; =\; 1.70\; \backslash times\; 10^\{-3\}$

Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as {{CO2}} molecules, not affecting the pH.

The relative concentrations of {{CO2}}, {{chem2|H2CO3}}, and the deprotonated forms {{chem2|HCO3-}} (bicarbonate) and {{chem2|CO3(2-)}}(carbonate) depend on the pH. As shown in a Bjerrum plot, in neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater. In very alkaline water (pH > 10.4), the predominant (>50%) form is carbonate. The oceans, being mildly alkaline with typical pH = 8.2–8.5, contain about 120 mg of bicarbonate per liter.

Being diprotic, carbonic acid has two acid dissociation constants, the first one for the dissociation into the bicarbonate (also called hydrogen carbonate) ion ({{chem2|HCO3-}}):

:{{chem2|H2CO3 <-> HCO3- + H+}}

:K_a1 = 2.5 × 10⁻⁴ mol/L; pK_a1 = 3.6 at 25 °C.{{Greenwood&Earnshaw2nd|pages=305–314|name-list-style=vanc}}

This is the true first acid dissociation constant, defined as

:$K\_\backslash mathrm\{a1\}\; =\; \backslash frac\{\backslash ce\{[HCO3-\; ][H+]\}\}\{\backslash ce\{[H2CO3]\}\}$

where the denominator includes only covalently bound {{chem2|H2CO3}} and does not include hydrated {{CO2}}(aq). The much smaller and often-quoted value near 4.16 × 10⁻⁷ (or pK_a1 = 6.38) is an apparent value calculated on the (incorrect) assumption that all dissolved {{CO2}} is present as carbonic acid, so that

:$K\_\backslash mathrm\{a1\}\{\backslash rm\{(apparent)\}\}=\backslash frac\{\backslash ce\{[HCO3-\; ][H+]\}\}\{\backslash ce\{[H2CO3]\; +\; [CO2\_\{(aq)\}]\}\}$

Since most of the dissolved {{CO2}} remains as {{CO2}} molecules, K_a1(apparent) has a much larger denominator and a much smaller value than the true K_a1.{{cite book | vauthors = Jolly WL | title = Modern Inorganic Chemistry | publisher = McGraw-Hill | date = 1984 | pages = 196 | isbn = 978-0-07-032760-3}}

The bicarbonate ion is an amphoteric species that can act as an acid or as a base, depending on pH of the solution. At high pH, it dissociates significantly into the carbonate ion ({{chem2|CO3(2-)}}):

:{{chem2|HCO3- <-> CO3(2-) + H+}}

:K_a2 = 4.69 × 10⁻¹¹ mol/L; pK_a2 = 10.329

In organisms, carbonic acid production is catalysed by the enzyme known as carbonic anhydrase.

In addition to altering its acidity, the presence of carbon dioxide in water also affects its electrical properties. File:Millipore co2.svg When carbon dioxide dissolves in desalinated water, the electrical conductivity increases significantly from below 1 μS/cm to nearly 30 μS/cm. When heated, the water begins to gradually lose the conductivity induced by the presence of $\backslash mathrm\{CO\_\{2\}\}$ , especially noticeable as temperatures exceed 30 °C.

The temperature dependence of the electrical conductivity of fully deionized water without {{CO2}} saturation is comparably low in relation to these data.

{{CO2}} is a potent electrophile having an electrophilic reactivity that is comparable to benzaldehyde or strongly electrophilic α,β-unsaturated carbonyl compounds. However, unlike electrophiles of similar reactivity, the reactions of nucleophiles with {{CO2}} are thermodynamically less favored and are often found to be highly reversible.{{cite journal | vauthors = Li Z, Mayer RJ, Ofial AR, Mayr H | title = From Carbodiimides to Carbon Dioxide: Quantification of the Electrophilic Reactivities of Heteroallenes | journal = Journal of the American Chemical Society | volume = 142 | issue = 18 | pages = 8383–8402 | date = May 2020 | pmid = 32338511 | doi = 10.1021/jacs.0c01960 | bibcode = 2020JAChS.142.8383L | s2cid = 216557447}} The reversible reaction of carbon dioxide with amines to make carbamates is used in {{CO2}} scrubbers and has been suggested as a possible starting point for carbon capture and storage by amine gas treating.

Only very strong nucleophiles, like the carbanions provided by Grignard reagents and organolithium compounds react with {{CO2}} to give carboxylates:

:{{chem2|MR + CO2 -> RCO2M}}

:where M = Li or MgBr and R = alkyl or aryl.

In metal carbon dioxide complexes, {{CO2}} serves as a ligand, which can facilitate the conversion of {{CO2}} to other chemicals.{{cite book | veditors = Aresta M | date = 2010 | title = Carbon Dioxide as a Chemical Feedstock | publisher = Wiley-VCH | location = Weinheim | isbn = 978-3-527-32475-0}}

The reduction of {{CO2}} to CO is ordinarily a difficult and slow reaction:

:{{chem2|CO2 + 2 e- + 2 H+ -> CO + H2O}}

The redox potential for this reaction near pH 7 is about −0.53 V versus the standard hydrogen electrode. The nickel-containing enzyme carbon monoxide dehydrogenase catalyses this process.{{cite journal | vauthors = Finn C, Schnittger S, Yellowlees LJ, Love JB | title = Molecular approaches to the electrochemical reduction of carbon dioxide | journal = Chemical Communications | volume = 48 | issue = 10 | pages = 1392–1399 | date = February 2012 | pmid = 22116300 | doi = 10.1039/c1cc15393e | url = https://www.pure.ed.ac.uk/ws/files/10852481/Molecular_approaches_to_the_electrochemical_reduction_of_carbon_dioxide.pdf | access-date = 6 December 2019 | url-status = live | hdl-access = free | archive-date = 19 April 2021 | archive-url = https://web.archive.org/web/20210419185431/https://www.pure.ed.ac.uk/ws/files/10852481/Molecular_approaches_to_the_electrochemical_reduction_of_carbon_dioxide.pdf | hdl = 20.500.11820/b530915d-451c-493c-8251-da2ea2f50912 | s2cid = 14356014}}

Photoautotrophs (i.e. plants and cyanobacteria) use the energy contained in sunlight to photosynthesize simple sugars from {{CO2}} absorbed from the air and water:

:{{chem2|n CO2 + n H2O -> (CH2O)_{n} + n O2}}

{{further|Carbon dioxide data}}

File:Dry Ice Pellets Subliming.jpg

Carbon dioxide is colorless. At low concentrations, the gas is odorless; however, at sufficiently high concentrations, it has a sharp, acidic odor.{{cite web |title=Carbon Dioxide |website=Air Products |url=http://www.airproducts.com/~/media/Files/PDF/company/product-summary-carbon-dioxide.pdf?la=en |access-date=28 April 2017 |archive-date=29 July 2020 |archive-url=https://web.archive.org/web/20200729131131/http://www.airproducts.com/~/media/Files/PDF/company/product-summary-carbon-dioxide.pdf?la=en |url-status=dead}} At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m³, about 1.53 times that of air.{{cite web |url=https://www.engineeringtoolbox.com/gas-density-d_158.html |title=Gases – Densities |publisher=Engineering Toolbox |access-date=21 November 2020 |archive-date=2 March 2006 |archive-url=https://web.archive.org/web/20060302054722/https://www.engineeringtoolbox.com/gas-density-d_158.html |url-status=live}}

Carbon dioxide has no liquid state at pressures below 0.51795(10) MPa (5.11177(99) atm). At a pressure of 1 atm (0.101325 MPa), the gas deposits directly to a solid at temperatures below 194.6855(30) K (−78.4645(30) °C) and the solid sublimes directly to a gas above this temperature. In its solid state, carbon dioxide is commonly called dry ice.

File:Carbon dioxide pressure-temperature phase diagram.svg of carbon dioxide. Note that it is a log-lin chart.]]

Liquid carbon dioxide forms only at pressures above 0.51795(10) MPa (5.11177(99) atm); the triple point of carbon dioxide is 216.592(3) K (−56.558(3) °C) at 0.51795(10) MPa (5.11177(99) atm) (see phase diagram). The critical point is 304.128(15) K (30.978(15) °C) at 7.3773(30) MPa (72.808(30) atm). Another form of solid carbon dioxide observed at high pressure is an amorphous glass-like solid.{{cite journal | vauthors = Santoro M, Gorelli FA, Bini R, Ruocco G, Scandolo S, Crichton WA | title = Amorphous silica-like carbon dioxide | journal = Nature | volume = 441 | issue = 7095 | pages = 857–860 | date = June 2006 | pmid = 16778885 | doi = 10.1038/nature04879 | s2cid = 4363092 | bibcode = 2006Natur.441..857S}} This form of glass, called carbonia, is produced by supercooling heated {{CO2}} at extreme pressures (40–48 GPa, or about 400,000 atmospheres) in a diamond anvil. This discovery confirmed the theory that carbon dioxide could exist in a glass state similar to other members of its elemental family, like silicon dioxide (silica glass) and germanium dioxide. Unlike silica and germania glasses, however, carbonia glass is not stable at normal pressures and reverts to gas when pressure is released.

At temperatures and pressures above the critical point, carbon dioxide behaves as a supercritical fluid known as supercritical carbon dioxide.

{{clear}}

Table of thermal and physical properties of saturated liquid carbon dioxide:{{Cite book |last=Holman |first=Jack P. |title=Heat Transfer |publisher=McGraw-Hill Companies, Inc. |year=2002 |isbn=9780072406559 |edition=9th |location=New York, NY |pages=600–606 |language=English}}{{Cite book |last1=Incropera |last2=Dewitt |last3=Bergman |last4=Lavigne |first1=Frank P. |first2=David P. |first3=Theodore L. |first4=Adrienne S. |title=Fundamentals of Heat and Mass Transfer |publisher=John Wiley and Sons, Inc. |year=2007 |isbn=9780471457282 |edition=6th |location=Hoboken, NJ |pages=941–950 |language=English}}

Table of thermal and physical properties of carbon dioxide ({{CO2}}) at atmospheric pressure:

Carbon dioxide is an end product of cellular respiration in organisms that obtain energy by breaking down sugars, fats and amino acids with oxygen as part of their metabolism. This includes all plants, algae and animals and aerobic fungi and bacteria. In vertebrates, the carbon dioxide travels in the blood from the body's tissues to the skin (e.g., amphibians) or the gills (e.g., fish), from where it dissolves in the water, or to the lungs from where it is exhaled. During active photosynthesis, plants can absorb more carbon dioxide from the atmosphere than they release in respiration.

File:Calvin-cycle4.svg and carbon fixation]]

Carbon fixation is a biochemical process by which atmospheric carbon dioxide is incorporated by plants, algae and cyanobacteria into energy-rich organic molecules such as glucose, thus creating their own food by photosynthesis. Photosynthesis uses carbon dioxide and water to produce sugars from which other organic compounds can be constructed, and oxygen is produced as a by-product.

Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly abbreviated to RuBisCO, is the enzyme involved in the first major step of carbon fixation, the production of two molecules of 3-phosphoglycerate from {{CO2}} and ribulose bisphosphate, as shown in the diagram at left.

RuBisCO is thought to be the single most abundant protein on Earth.{{cite journal |vauthors=Dhingra A, Portis AR, Daniell H |date=April 2004 |title=Enhanced translation of a chloroplast-expressed RbcS gene restores small subunit levels and photosynthesis in nuclear RbcS antisense plants |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=101 |issue=16 |pages=6315–6320 |bibcode=2004PNAS..101.6315D |doi=10.1073/pnas.0400981101 |pmc=395966 |pmid=15067115 |quote=(Rubisco) is the most prevalent enzyme on this planet, accounting for 30–50% of total soluble protein in the chloroplast |doi-access=free}}

Phototrophs use the products of their photosynthesis as internal food sources and as raw material for the biosynthesis of more complex organic molecules, such as polysaccharides, nucleic acids, and proteins. These are used for their own growth, and also as the basis of the food chains and webs that feed other organisms, including animals such as ourselves. Some important phototrophs, the coccolithophores synthesise hard calcium carbonate scales.{{Cite book |title=Evolution of primary producers in the sea |vauthors=Falkowski P, Knoll AH |date=1 January 2007 |publisher=Elsevier, Academic Press |isbn=978-0-12-370518-1 |oclc=845654016}} A globally significant species of coccolithophore is Emiliania huxleyi whose calcite scales have formed the basis of many sedimentary rocks such as limestone, where what was previously atmospheric carbon can remain fixed for geological timescales.File:Auto-and heterotrophs.png (green), which can be respired (red) to water and {{CO2}}.]]

Plants can grow as much as 50% faster in concentrations of 1,000 ppm {{CO2}} when compared with ambient conditions, though this assumes no change in climate and no limitation on other nutrients.{{cite web |date=December 2002 |title=Carbon Dioxide In Greenhouses |url=http://www.omafra.gov.on.ca/english/crops/facts/00-077.htm |url-status=live |archive-url=https://web.archive.org/web/20190429202513/http://www.omafra.gov.on.ca/english/crops/facts/00-077.htm |archive-date=29 April 2019 |access-date=12 June 2007 |vauthors=Blom TJ, Straver WA, Ingratta FJ, Khosla S, Brown W}} Elevated {{CO2}} levels cause increased growth reflected in the harvestable yield of crops, with wheat, rice and soybean all showing increases in yield of 12–14% under elevated {{CO2}} in FACE experiments.{{cite journal |vauthors=Ainsworth EA |year=2008 |title=Rice production in a changing climate: a meta-analysis of responses to elevated carbon dioxide and elevated ozone concentration |url=http://www.plant-biotech.dk/Meetings/PBD_Symposium_Plant%20Stress_litterature/LisaAinsworth_pdf2.pdf |journal=Global Change Biology |volume=14 |issue=7 |pages=1642–1650 |bibcode=2008GCBio..14.1642A |doi=10.1111/j.1365-2486.2008.01594.x |archive-url=https://web.archive.org/web/20110719130608/http://www.plant-biotech.dk/Meetings/PBD_Symposium_Plant%20Stress_litterature/LisaAinsworth_pdf2.pdf |archive-date=19 July 2011 |s2cid=19200429}}{{cite journal |vauthors=Long SP, Ainsworth EA, Leakey AD, Nösberger J, Ort DR |date=June 2006 |title=Food for thought: lower-than-expected crop yield stimulation with rising {{CO2}} concentrations |url=http://www.as.wvu.edu/biology/bio463/Long%20et%20al%202006%20Lower%20yield%20than%20expected%20under%20increased%20CO2.pdf |url-status=live |journal=Science |volume=312 |issue=5782 |pages=1918–1921 |bibcode=2006Sci...312.1918L |citeseerx=10.1.1.542.5784 |doi=10.1126/science.1114722 |pmid=16809532 |archive-url=https://web.archive.org/web/20161020165354/http://www.as.wvu.edu/biology/bio463/Long%20et%20al%202006%20Lower%20yield%20than%20expected%20under%20increased%20CO2.pdf |archive-date=20 October 2016 |access-date=27 October 2017 |s2cid=2232629}}

Increased atmospheric {{CO2}} concentrations result in fewer stomata developing on plants{{cite journal |vauthors=Woodward F, Kelly C |year=1995 |title=The influence of {{CO2}} concentration on stomatal density |journal=New Phytologist |volume=131 |issue=3 |pages=311–327 |doi=10.1111/j.1469-8137.1995.tb03067.x |doi-access=free|bibcode=1995NewPh.131..311W }} which leads to reduced water usage and increased water-use efficiency.{{cite journal |vauthors=Drake BG, Gonzalez-Meler MA, Long SP |date=June 1997 |title=More Efficient Plants: A Consequence of Rising Atmospheric {{CO2}}? |journal=Annual Review of Plant Physiology and Plant Molecular Biology |volume=48 |issue=1 |pages=609–639 |doi=10.1146/annurev.arplant.48.1.609 |pmid=15012276 |s2cid=33415877}} Studies using FACE have shown that {{CO2}} enrichment leads to decreased concentrations of micronutrients in crop plants.{{cite journal |vauthors=Loladze I |year=2002 |title=Rising atmospheric {{CO2}} and human nutrition: toward globally imbalanced plant stoichiometry? |journal=Trends in Ecology & Evolution |volume=17 |issue=10 |pages=457–461 |doi=10.1016/S0169-5347(02)02587-9 |s2cid=16074723}} This may have knock-on effects on other parts of ecosystems as herbivores will need to eat more food to gain the same amount of protein.{{cite journal |vauthors=Coviella CE, Trumble JT |year=1999 |title=Effects of Elevated Atmospheric Carbon Dioxide on Insect-Plant Interactions |journal=Conservation Biology |volume=13 |issue=4 |pages=700–712 |doi=10.1046/j.1523-1739.1999.98267.x |jstor=2641685 |bibcode=1999ConBi..13..700C |s2cid=52262618}}

The concentration of secondary metabolites such as phenylpropanoids and flavonoids can also be altered in plants exposed to high concentrations of {{CO2}}.{{Cite journal |vauthors=Davey MP, Harmens H, Ashenden TW, Edwards R, Baxter R |year=2007 |title=Species-specific effects of elevated {{CO2}} on resource allocation in Plantago maritima and Armeria maritima |journal=Biochemical Systematics and Ecology |volume=35 |issue=3 |pages=121–129 |doi=10.1016/j.bse.2006.09.004}}{{cite journal |vauthors=Davey MP, Bryant DN, Cummins I, Ashenden TW, Gates P, Baxter R, Edwards R |date=August 2004 |title=Effects of elevated {{CO2}} on the vasculature and phenolic secondary metabolism of Plantago maritima |journal=Phytochemistry |volume=65 |issue=15 |pages=2197–2204 |doi=10.1016/j.phytochem.2004.06.016 |pmid=15587703|bibcode=2004PChem..65.2197D}}

Plants also emit {{CO2}} during respiration, and so the majority of plants and algae, which use C3 photosynthesis, are only net absorbers during the day. Though a growing forest will absorb many tons of {{CO2}} each year, a mature forest will produce as much {{CO2}} from respiration and decomposition of dead specimens (e.g., fallen branches) as is used in photosynthesis in growing plants.{{cite web |title=Global Environment Division Greenhouse Gas Assessment Handbook – A Practical Guidance Document for the Assessment of Project-level Greenhouse Gas Emissions |url=http://www-wds.worldbank.org/external/default/WDSContentServer/WDSP/IB/2002/09/07/000094946_02081604154234/Rendered/INDEX/multi0page.txt |url-status=dead |archive-url=https://web.archive.org/web/20160603011630/http://www-wds.worldbank.org/external/default/WDSContentServer/WDSP/IB/2002/09/07/000094946_02081604154234/Rendered/INDEX/multi0page.txt |archive-date=3 June 2016 |access-date=10 November 2007 |publisher=World Bank}} Contrary to the long-standing view that they are carbon neutral, mature forests can continue to accumulate carbon{{cite journal |display-authors=6 |vauthors=Luyssaert S, Schulze ED, Börner A, Knohl A, Hessenmöller D, Law BE, Ciais P, Grace J |date=September 2008 |title=Old-growth forests as global carbon sinks |url=https://hal-cea.archives-ouvertes.fr/cea-00910763/file/Luyssaert2008.pdf |journal=Nature |volume=455 |issue=7210 |pages=213–215 |bibcode=2008Natur.455..213L |doi=10.1038/nature07276 |pmid=18784722 |s2cid=4424430}} and remain valuable carbon sinks, helping to maintain the carbon balance of Earth's atmosphere. Additionally, and crucially to life on earth, photosynthesis by phytoplankton consumes dissolved {{CO2}} in the upper ocean and thereby promotes the absorption of {{CO2}} from the atmosphere.{{cite journal |display-authors=6 |vauthors=Falkowski P, Scholes RJ, Boyle E, Canadell J, Canfield D, Elser J, Gruber N, Hibbard K, Högberg P, Linder S, Mackenzie FT, Moore B, Pedersen T, Rosenthal Y, Seitzinger S, Smetacek V, Steffen W |date=October 2000 |title=The global carbon cycle: a test of our knowledge of earth as a system |journal=Science |volume=290 |issue=5490 |pages=291–296 |bibcode=2000Sci...290..291F |doi=10.1126/science.290.5490.291 |pmid=11030643 |s2cid=1779934}}

{{See also|Carbon dioxide poisoning}}

File:Main symptoms of carbon dioxide toxicity.svg in air{{cite web |title=Toxicity of Carbon Dioxide Gas Exposure, {{CO2}} Poisoning Symptoms, Carbon Dioxide Exposure Limits, and Links to Toxic Gas Testing Procedures |url=http://www.inspect-ny.com/hazmat/CO2gashaz.htm |archive-url=https://web.archive.org/web/20090928073740/http://www.inspect-ny.com/hazmat/CO2gashaz.htm |archive-date=28 September 2009 |work=InspectAPedia |vauthors=Friedman D}}]]

Carbon dioxide content in fresh air (averaged between sea-level and 10 kPa level, i.e., about {{cvt|30|km}} altitude) varies between 0.036% (360 ppm) and 0.041% (412 ppm), depending on the location.{{cite web |title=CarbonTracker CT2011_oi (Graphical map of {{CO2}}) |url=http://www.esrl.noaa.gov/gmd/ccgg/carbontracker/ |url-status=live |archive-url=https://web.archive.org/web/20210213080315/https://www.esrl.noaa.gov/gmd/ccgg/carbontracker/ |archive-date=13 February 2021 |access-date=20 April 2007 |work=esrl.noaa.gov}}

In humans, exposure to {{CO2}} at concentrations greater than 5% causes the development of hypercapnia and respiratory acidosis.{{Cite journal |last1=Permentier |first1=Kris |last2=Vercammen |first2=Steven |last3=Soetaert |first3=Sylvia |last4=Schellemans |first4=Christian |date=2017-04-04 |title=Carbon dioxide poisoning: a literature review of an often forgotten cause of intoxication in the emergency department |journal=International Journal of Emergency Medicine |volume=10 |issue=1 |page=14 |doi=10.1186/s12245-017-0142-y |issn=1865-1372 |pmc=5380556 |pmid=28378268 |doi-access=free}}50x50px Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License Concentrations of 7% to 10% (70,000 to 100,000 ppm) may cause suffocation, even in the presence of sufficient oxygen, manifesting as dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour.{{cite news |title=Carbon Dioxide as a Fire Suppressant: Examining the Risks |publisher=U.S. Environmental Protection Agency |url=http://www.epa.gov/ozone/snap/fire/co2/co2report.html |archive-url=https://web.archive.org/web/20151002093443/http://www.epa.gov/ozone/snap/fire/co2/co2report.html |archive-date=2 October 2015}} Concentrations of more than 10% may cause convulsions, coma, and death. {{CO2}} levels of more than 30% act rapidly leading to loss of consciousness in seconds.

Because it is heavier than air, in locations where the gas seeps from the ground (due to sub-surface volcanic or geothermal activity) in relatively high concentrations, without the dispersing effects of wind, it can collect in sheltered/pocketed locations below average ground level, causing animals located therein to be suffocated. Carrion feeders attracted to the carcasses are then also killed. Children have been killed in the same way near the city of Goma by {{CO2}} emissions from the nearby volcano Mount Nyiragongo.{{cite web |date=1 November 2005 |title=Volcano Under the City |url=https://www.pbs.org/wgbh/nova/transcripts/3215_volcanoc.html |archive-url=https://web.archive.org/web/20110405155241/http://www.pbs.org/wgbh/nova/transcripts/3215_volcanoc.html |archive-date=5 April 2011 |work=A NOVA Production by Bonne Pioche and Greenspace for WGBH/Boston |publisher=Public Broadcasting System}}. The Swahili term for this phenomenon is {{lang|sw|mazuku}}.

File:Apollo13 apparatus.jpg astronauts, who had to adapt cartridges from the command module to supply the carbon dioxide scrubber in the Apollo Lunar Module, which they used as a lifeboat.]]

Adaptation to increased concentrations of {{CO2}} occurs in humans, including modified breathing and kidney bicarbonate production, in order to balance the effects of blood acidification (acidosis). Several studies suggested that 2.0 percent inspired concentrations could be used for closed air spaces (e.g. a submarine) since the adaptation is physiological and reversible, as deterioration in performance or in normal physical activity does not happen at this level of exposure for five days.{{cite report |url=http://archive.rubicon-foundation.org/6045 |title=Carbon Dioxide Tolerance Studies |id=SAM-TR-67-77 |access-date=2 May 2008 |archive-url=https://web.archive.org/web/20080509072828/http://archive.rubicon-foundation.org/6045 |archive-date=9 May 2008 |url-status=usurped |vauthors=Glatte Jr HA, Motsay GJ, Welch BE |year=1967 |series=Brooks AFB, TX School of Aerospace Medicine Technical Report}}{{cite report |url=http://archive.rubicon-foundation.org/3861 |title=Carbon Dioxide Tolerance and Toxicity |publisher=Environmental Biomedical Stress Data Center, Institute for Environmental Medicine, University of Pennsylvania Medical Center |id=No. 2-71 |access-date=2 May 2008 |archive-url=https://web.archive.org/web/20110724044527/http://archive.rubicon-foundation.org/3861 |archive-date=24 July 2011 |url-status=usurped |vauthors=Lambertsen CJ |year=1971 |series=IFEM Report}} Yet, other studies show a decrease in cognitive function even at much lower levels.{{cite journal |vauthors=Satish U, Mendell MJ, Shekhar K, Hotchi T, Sullivan D, Streufert S, Fisk WJ |date=December 2012 |title=Is {{CO2}} an indoor pollutant? Direct effects of low-to-moderate {{CO2}} concentrations on human decision-making performance |url=http://ehp.niehs.nih.gov/wp-content/uploads/2012/09/ehp.1104789.pdf |url-status=dead |journal=Environmental Health Perspectives |volume=120 |issue=12 |pages=1671–1677 |doi=10.1289/ehp.1104789 |pmc=3548274 |pmid=23008272 |archive-url=https://web.archive.org/web/20160305212909/http://ehp.niehs.nih.gov/wp-content/uploads/2012/09/ehp.1104789.pdf |archive-date=5 March 2016 |access-date=11 December 2014}}{{cite journal |author-link=Joseph G. Allen |vauthors=Allen JG, MacNaughton P, Satish U, Santanam S, Vallarino J, Spengler JD |date=June 2016 |title=Associations of Cognitive Function Scores with Carbon Dioxide, Ventilation, and Volatile Organic Compound Exposures in Office Workers: A Controlled Exposure Study of Green and Conventional Office Environments |journal=Environmental Health Perspectives |volume=124 |issue=6 |pages=805–812 |doi=10.1289/ehp.1510037 |pmc=4892924 |pmid=26502459|bibcode=2016EnvHP.124..805A }} Also, with ongoing respiratory acidosis, adaptation or compensatory mechanisms will be unable to reverse the condition.

There are few studies of the health effects of long-term continuous {{CO2}} exposure on humans and animals at levels below 1%. Occupational {{CO2}} exposure limits have been set in the United States at 0.5% (5000 ppm) for an eight-hour period.{{cite web |title=Exposure Limits for Carbon Dioxide Gas – {{CO2}} Limits |url=http://www.inspectapedia.com/hazmat/CO2_Exposure_Limits.htm |url-status=live |archive-url=https://web.archive.org/web/20180916235612/https://inspectapedia.com/hazmat/CO2_Exposure_Limits.htm |archive-date=16 September 2018 |access-date=19 October 2014 |publisher=InspectAPedia.com}} At this {{CO2}} concentration, International Space Station crew experienced headaches, lethargy, mental slowness, emotional irritation, and sleep disruption.{{cite report |url=http://ston.jsc.nasa.gov/collections/trs/_techrep/TP-2010-216126.pdf |title=In-Flight Carbon Dioxide Exposures and Related Symptoms: Associations, Susceptibility and Operational Implications |id=TP–2010–216126 |access-date=26 August 2014 |archive-url=https://web.archive.org/web/20110627061502/http://ston.jsc.nasa.gov/collections/TRS/_techrep/TP-2010-216126.pdf |archive-date=27 June 2011 |url-status=dead |vauthors=Law J, Watkins S, Alexander D |year=2010 |series=NASA Technical Report}} Studies in animals at 0.5% {{CO2}} have demonstrated kidney calcification and bone loss after eight weeks of exposure.{{cite journal |vauthors=Schaefer KE, Douglas WH, Messier AA, Shea ML, Gohman PA |year=1979 |title=Effect of prolonged exposure to 0.5% {{CO2}} on kidney calcification and ultrastructure of lungs |url=http://handle.dtic.mil/100.2/ADA075625 |url-status=dead |journal=Undersea Biomedical Research |volume=6 |issue=Suppl |pages=S155–S161 |pmid=505623 |archive-url=https://web.archive.org/web/20141019131035/http://handle.dtic.mil/100.2/ADA075625 |archive-date=19 October 2014 |access-date=19 October 2014}} A study of humans exposed in 2.5 hour sessions demonstrated significant negative effects on cognitive abilities at concentrations as low as 0.1% (1000{{nbsp}}ppm) {{CO2}} likely due to {{CO2}} induced increases in cerebral blood flow. Another study observed a decline in basic activity level and information usage at 1000 ppm, when compared to 500 ppm.

However a review of the literature found that a reliable subset of studies on the phenomenon of carbon dioxide induced cognitive impairment to only show a small effect on high-level decision making (for concentrations below 5000 ppm). Most of the studies were confounded by inadequate study designs, environmental comfort, uncertainties in exposure doses and differing cognitive assessments used.{{cite journal |vauthors=Du B, Tandoc MC, Mack ML, Siegel JA |date=November 2020 |title=Indoor {{CO2}} concentrations and cognitive function: A critical review |journal=Indoor Air |volume=30 |issue=6 |pages=1067–1082 |doi=10.1111/ina.12706 |pmid=32557862 |bibcode=2020InAir..30.1067D |s2cid=219915861|doi-access=free}} Similarly a study on the effects of the concentration of {{CO2}} in motorcycle helmets has been criticized for having dubious methodology in not noting the self-reports of motorcycle riders and taking measurements using mannequins. Further when normal motorcycle conditions were achieved (such as highway or city speeds) or the visor was raised the concentration of {{CO2}} declined to safe levels (0.2%).{{Cite web |date=4 June 2019 |title=Ask the doc: Does my helmet make me stupid? - RevZilla |url=https://www.revzilla.com/common-tread/ask-the-doc-does-my-helmet-make-me-stupid |url-status=live |archive-url=https://web.archive.org/web/20210522081133/https://www.revzilla.com/common-tread/ask-the-doc-does-my-helmet-make-me-stupid |archive-date=22 May 2021 |access-date=2021-05-22 |website=www.revzilla.com |vauthors=Kaplan L}}{{cite journal |vauthors=Brühwiler PA, Stämpfli R, Huber R, Camenzind M |date=September 2005 |title={{CO2}} and {{O2|nolink=no}} concentrations in integral motorcycle helmets |journal=Applied Ergonomics |volume=36 |issue=5 |pages=625–633 |doi=10.1016/j.apergo.2005.01.018 |pmid=15893291}}

File:CO2Mini monitor TFA Dostmann.jpg that measures {{CO2}} concentration using a nondispersive infrared sensor]]

Poor ventilation is one of the main causes of excessive {{CO2}} concentrations in closed spaces, leading to poor indoor air quality. Carbon dioxide differential above outdoor concentrations at steady state conditions (when the occupancy and ventilation system operation are sufficiently long that {{CO2}} concentration has stabilized) are sometimes used to estimate ventilation rates per person.{{Cite web |title=Standard Guide for Using Indoor Carbon Dioxide Concentrations to Evaluate Indoor Air Quality and Ventilation |url=https://www.astm.org/d6245-98.html |access-date=2024-06-12 |website=www.astm.org |language=en}} Higher {{CO2}} concentrations are associated with occupant health, comfort and performance degradation.{{cite journal |vauthors=Allen JG, MacNaughton P, Satish U, Santanam S, Vallarino J, Spengler JD |date=June 2016 |title=Associations of Cognitive Function Scores with Carbon Dioxide, Ventilation, and Volatile Organic Compound Exposures in Office Workers: A Controlled Exposure Study of Green and Conventional Office Environments |journal=Environmental Health Perspectives |volume=124 |issue=6 |pages=805–812 |doi=10.1289/ehp.1510037 |pmc=4892924 |pmid=26502459|bibcode=2016EnvHP.124..805A }}{{Cite web |date=26 October 2015 |title=Exclusive: Elevated {{CO2}} Levels Directly Affect Human Cognition, New Harvard Study Shows |url=https://thinkprogress.org/exclusive-elevated-co2-levels-directly-affect-human-cognition-new-harvard-study-shows-2748e7378941/ |url-status=live |archive-url=https://web.archive.org/web/20191009092140/https://thinkprogress.org/exclusive-elevated-co2-levels-directly-affect-human-cognition-new-harvard-study-shows-2748e7378941/ |archive-date=9 October 2019 |access-date=14 October 2019 |website=ThinkProgress |vauthors=Romm J}} ASHRAE Standard 62.1–2007 ventilation rates may result in indoor concentrations up to 2,100 ppm above ambient outdoor conditions. Thus if the outdoor concentration is 400 ppm, indoor concentrations may reach 2,500 ppm with ventilation rates that meet this industry consensus standard. Concentrations in poorly ventilated spaces can be found even higher than this (range of 3,000 or 4,000 ppm).

Miners, who are particularly vulnerable to gas exposure due to insufficient ventilation, referred to mixtures of carbon dioxide and nitrogen as "blackdamp", "choke damp" or "stythe". Before more effective technologies were developed, miners would frequently monitor for dangerous levels of blackdamp and other gases in mine shafts by bringing a caged canary with them as they worked. The canary is more sensitive to asphyxiant gases than humans, and as it became unconscious would stop singing and fall off its perch. The Davy lamp could also detect high levels of blackdamp (which sinks, and collects near the floor) by burning less brightly, while methane, another suffocating gas and explosion risk, would make the lamp burn more brightly.

In February 2020, three people died from suffocation at a party in Moscow when dry ice (frozen {{CO2}}) was added to a swimming pool to cool it down.{{cite web |date=29 February 2020 |title=Three die in dry-ice incident at Moscow pool party |url=https://www.bbc.co.uk/news/world-europe-51680049 |archive-url=https://web.archive.org/web/20200229151448/https://www.bbc.co.uk/news/world-europe-51680049 |archive-date=29 February 2020 |work=BBC News |quote=The victims were connected to Instagram influencer Yekaterina Didenko.}} A similar accident occurred in 2018 when a woman died from {{CO2}} fumes emanating from the large amount of dry ice she was transporting in her car.{{Cite web |date=2 August 2018 |title=A Woman Died from Dry Ice Fumes. Here's How It Can Happen |url=https://www.livescience.com/63241-dry-ice-death.html |url-status=live |archive-url=https://web.archive.org/web/20210522082215/https://www.livescience.com/63241-dry-ice-death.html |archive-date=22 May 2021 |access-date=2021-05-22 |website=Live Science |language=en |vauthors=Rettner R}}

{{clear}}

Humans spend more and more time in a confined atmosphere (around 80-90% of the time in a building or vehicle). According to the French Agency for Food, Environmental and Occupational Health & Safety (ANSES) and various actors in France, the {{CO2}} rate in the indoor air of buildings (linked to human or animal occupancy and the presence of combustion installations), weighted by air renewal, is "usually between about 350 and 2,500 ppm".{{Cite report |url=https://www.anses.fr/en/system/files/AIR2012sa0093Ra.pdf |title=Concentrations de CO2 dans l'air intérieur et effets sur la santé |date=July 2013 |publisher=ANSES |pages=294 |language=fr}}

In homes, schools, nurseries and offices, there are no systematic relationships between the levels of {{CO2}} and other pollutants, and indoor {{CO2}} is statistically not a good predictor of pollutants linked to outdoor road (or air, etc.) traffic.{{Cite journal |last1=Chatzidiakou |first1=Lia |last2=Mumovic |first2=Dejan |last3=Summerfield |first3=Alex |date=March 2015 |title=Is CO 2 a good proxy for indoor air quality in classrooms? Part 1: The interrelationships between thermal conditions, CO 2 levels, ventilation rates and selected indoor pollutants |url=http://journals.sagepub.com/doi/10.1177/0143624414566244 |journal=Building Services Engineering Research and Technology |language=en |volume=36 |issue=2 |pages=129–161 |doi=10.1177/0143624414566244 |s2cid=111182451 |issn=0143-6244}} {{CO2}} is the parameter that changes the fastest (with hygrometry and oxygen levels when humans or animals are gathered in a closed or poorly ventilated room). In poor countries, many open hearths are sources of {{CO2}} and CO emitted directly into the living environment.{{Cite journal |last1=Cetin |first1=Mehmet |last2=Sevik |first2=Hakan |date=2016 |title=INDOOR QUALITY ANALYSIS OF CO2 FOR KASTAMONU UNIVERSITY |url=http://www.universitypublications.net/proceedings/0903/pdf/H6V141.pdf |journal=Conference of the International Journal of Arts & Sciences |volume=9 |issue=3 |pages=71}}

Local concentrations of carbon dioxide can reach high values near strong sources, especially those that are isolated by surrounding terrain. At the Bossoleto hot spring near Rapolano Terme in Tuscany, Italy, situated in a bowl-shaped depression about {{cvt|100|m}} in diameter, concentrations of {{CO2}} rise to above 75% overnight, sufficient to kill insects and small animals. After sunrise the gas is dispersed by convection.{{Cite book |title=Plant responses to elevated {{CO2}}: Evidence from natural springs |vauthors=van Gardingen PR, Grace J, Jeffree CE, Byari SH, Miglietta F, Raschi A, Bettarini I |publisher=Cambridge University Press |year=1997 |isbn=978-0-521-58203-2 |veditors=Raschi A, Miglietta F, Tognetti R, van Gardingen PR |location=Cambridge |pages=69–86 |chapter=Long-term effects of enhanced {{CO2}} concentrations on leaf gas exchange: research opportunities using {{CO2}} springs}} High concentrations of {{CO2}} produced by disturbance of deep lake water saturated with {{CO2}} are thought to have caused 37 fatalities at Lake Monoun, Cameroon in 1984 and 1700 casualties at Lake Nyos, Cameroon in 1986.{{Cite book |title=Plant responses to elevated {{CO2}}: Evidence from natural springs |vauthors=Martini M |publisher=Cambridge University Press |year=1997 |isbn=978-0-521-58203-2 |veditors=Raschi A, Miglietta F, Tognetti R, van Gardingen PR |location=Cambridge |pages=69–86 |chapter={{CO2}} emissions in volcanic areas: case histories and hazards}}

The body produces approximately {{convert|2.3|lb|kg}} of carbon dioxide per day per person,{{cite web |title=How much carbon dioxide do humans contribute through breathing? |url=http://www.epa.gov/climatechange/fq/emissions.html |archive-url=https://web.archive.org/web/20110202140715/http://www.epa.gov/climatechange/fq/emissions.html |archive-date=2 February 2011 |access-date=30 April 2009 |work=EPA.gov}} containing {{convert|0.63|lb|g}} of carbon. {{anchor|partial pressure}} In humans, this carbon dioxide is carried through the venous system and is breathed out through the lungs, resulting in lower concentrations in the arteries. The carbon dioxide content of the blood is often given as the partial pressure, which is the pressure which carbon dioxide would have had if it alone occupied the volume.{{cite book |url=https://archive.org/details/chemistry00henr |title=Chemistry |vauthors=Henrickson C |publisher=Cliffs Notes |year=2005 |isbn=978-0-7645-7419-1}} In humans, the blood carbon dioxide contents are shown in the adjacent table.

{{CO2}} is carried in blood in three different ways. Exact percentages vary between arterial and venous blood.

• Majority (about 70% to 80%) is converted to bicarbonate ions {{chem2|HCO3-}} by the enzyme carbonic anhydrase in the red blood cells,{{cite web |title=Carbon dioxide |url=http://www.solarnavigator.net/solar_cola/carbon_dioxide.htm |url-status=dead |archive-url=https://web.archive.org/web/20080914125551/http://www.solarnavigator.net/solar_cola/carbon_dioxide.htm |archive-date=14 September 2008 |access-date=12 October 2007 |publisher=solarnavigator.net}} by the reaction:

:{{chem2|CO2 + H2O → H2CO3 -> H+ + HCO3-}}

• 5–10% is dissolved in blood plasma
• 5–10% is bound to hemoglobin as carbamino compounds

Hemoglobin, the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbon dioxide. However, the {{CO2}} bound to hemoglobin does not bind to the same site as oxygen. Instead, it combines with the N-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, the binding of {{CO2}} decreases the amount of oxygen that is bound for a given partial pressure of oxygen. This is known as the Haldane Effect, and is important in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure of {{CO2}} or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the Bohr effect.

Carbon dioxide is one of the mediators of local autoregulation of blood supply. If its concentration is high, the capillaries expand to allow a greater blood flow to that tissue.{{cite journal |last1=Battisti-Charbonney |first1=A. |last2=Fisher |first2=J. |last3=Duffin |first3=J. |date=15 Jun 2011 |title=The cerebrovascular response to carbon dioxide in humans |journal=J. Physiol. |volume=589 |issue=12 |pages=3039–3048 |doi=10.1113/jphysiol.2011.206052 |pmc=3139085 |pmid=21521758}}

Bicarbonate ions are crucial for regulating blood pH. A person's breathing rate influences the level of {{CO2}} in their blood. Breathing that is too slow or shallow causes respiratory acidosis, while breathing that is too rapid leads to hyperventilation, which can cause respiratory alkalosis.{{cite journal |last1=Patel |first1=S. |last2=Miao |first2=J.H. |last3=Yetiskul |first3=E. |last4=Anokhin |first4=A. |last5=Majmunder |first5=S.H. |year=2022 |title=Physiology, Carbon Dioxide Retention |url=https://www.ncbi.nlm.nih.gov/books/NBK482456/ |publisher=National Center for Biotechnology Information, NIH |pmid=29494063 |access-date=20 August 2022 |website=National Library of Medicine}}

Although the body requires oxygen for metabolism, low oxygen levels normally do not stimulate breathing. Rather, breathing is stimulated by higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (such as pure nitrogen) can lead to loss of consciousness without ever experiencing air hunger. This is especially perilous for high-altitude fighter pilots. It is also why flight attendants instruct passengers, in case of loss of cabin pressure, to apply the oxygen mask to themselves first before helping others; otherwise, one risks losing consciousness.

The respiratory centers try to maintain an arterial {{CO2}} pressure of 40 mmHg. With intentional hyperventilation, the {{CO2}} content of arterial blood may be lowered to 10–20 mmHg (the oxygen content of the blood is little affected), and the respiratory drive is diminished. This is why one can hold one's breath longer after hyperventilating than without hyperventilating. This carries the risk that unconsciousness may result before the need to breathe becomes overwhelming, which is why hyperventilation is particularly dangerous before free diving.{{cite journal |last1=Wilmshurst |first1=Peter |date=1998 |title=ABC of oxygen |journal=BMJ |volume=317 |issue=7164 |pages=996–999 |doi=10.1136/bmj.317.7164.996 |pmc=1114047 |pmid=9765173}}

{{Further|Carbon cycle}}

{{excerpt|Carbon dioxide in Earth's atmosphere}}

File:Global carbon budget components.png

{{Main|Carbon cycle|Ocean acidification}}

Carbon dioxide dissolves in the ocean to form carbonic acid ({{chem2|H2CO3}}), bicarbonate ({{chem2|HCO3-}}), and carbonate ({{chem2|CO3(2-)}}). There is about fifty times as much carbon dioxide dissolved in the oceans as exists in the atmosphere. The oceans act as an enormous carbon sink, and have taken up about a third of {{CO2}} emitted by human activity.{{cite web |date=29 November 2006 |title=How Long Can the Ocean Slow Global Warming? |url=http://www.whoi.edu/oceanus/viewArticle.do?id=17726 |url-status=live |archive-url=https://web.archive.org/web/20080104004633/http://www.whoi.edu/oceanus/viewArticle.do?id=17726 |archive-date=4 January 2008 |access-date=21 November 2007 |publisher=Oceanus |vauthors=Doney SC, Levine NM}}

{{excerpt|ocean acidification|paragraphs=1-2|file=no}}

File:Pterapod shell dissolved in seawater adjusted to an ocean chemistry projected for the year 2100.jpg projected for the year 2100]]

{{excerpt|Ocean acidification#Decreased calcification in marine organisms|paragraphs=1-2|file=no}}

Carbon dioxide is also introduced into the oceans through hydrothermal vents. The Champagne hydrothermal vent, found at the Northwest Eifuku volcano in the Mariana Trench, produces almost pure liquid carbon dioxide, one of only two known sites in the world as of 2004, the other being in the Okinawa Trough.{{cite journal |display-authors=6 |vauthors=Lupton J, Lilley M, Butterfield D, Evans L, Embley R, Olson E, Proskurowski G, Resing J, Roe K, Greene R, Lebon G |year=2004 |title=Liquid Carbon Dioxide Venting at the Champagne Hydrothermal Site, NW Eifuku Volcano, Mariana Arc |journal=American Geophysical Union |volume=2004 |issue=Fall Meeting |at=V43F–08 |bibcode=2004AGUFM.V43F..08L}} The finding of a submarine lake of liquid carbon dioxide in the Okinawa Trough was reported in 2006.{{cite journal |display-authors=6 |vauthors=Inagaki F, Kuypers MM, Tsunogai U, Ishibashi J, Nakamura K, Treude T, Ohkubo S, Nakaseama M, Gena K, Chiba H, Hirayama H, Nunoura T, Takai K, Jørgensen BB, Horikoshi K, Boetius A |date=September 2006 |title=Microbial community in a sediment-hosted {{CO2}} lake of the southern Okinawa Trough hydrothermal system |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=103 |issue=38 |pages=14164–14169 |bibcode=2006PNAS..10314164I |doi=10.1073/pnas.0606083103 |pmc=1599929 |pmid=16959888 |doi-access=free}} Videos can be downloaded at {{cite web |title=Supporting Information |url=http://www.pnas.org/content/103/38/14164.full?tab=ds |archive-url=https://web.archive.org/web/20181019001732/http://www.pnas.org/content/103/38/14164.full?tab=ds |archive-date=19 October 2018}}

{{anchor|CO2 production}}The burning of fossil fuels for energy produces 36.8 billion tonnes of {{CO2}} per year as of 2023.{{Cite web |last=JV |title=Fossil CO2 emissions at record high in 2023 |url=https://globalcarbonbudget.org/fossil-co2-emissions-at-record-high-in-2023/ |access-date=2024-11-01 |website=Global Carbon Budget |language=en}} Nearly all of this goes into the atmosphere, where approximately half is subsequently absorbed into natural carbon sinks.{{Cite web |date=2024-04-09 |title=Climate Change: Atmospheric Carbon Dioxide {{!}} NOAA Climate.gov |url=https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide |access-date=2024-11-01 |website=www.climate.gov |language=en}} Less than 1% of {{CO2}} produced annually is put to commercial use.{{Rp|page=3}}

Carbon dioxide is a by-product of the fermentation of sugar in the brewing of beer, whisky and other alcoholic beverages and in the production of bioethanol. Yeast metabolizes sugar to produce {{CO2}} and ethanol, also known as alcohol, as follows:

:{{chem2|C6H12O6 -> 2 CO2 + 2 CH3CH2OH}}

All aerobic organisms produce {{CO2}} when they oxidize carbohydrates, fatty acids, and proteins. The large number of reactions involved are exceedingly complex and not described easily. Refer to cellular respiration, anaerobic respiration and photosynthesis. The equation for the respiration of glucose and other monosaccharides is:

:{{chem2|C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O}}

Anaerobic organisms decompose organic material producing methane and carbon dioxide together with traces of other compounds.{{cite web |date=11 January 2017 |title=Collecting and using biogas from landfills |url=http://www.eia.gov/Energyexplained/?page=biomass_biogas |url-status=live |archive-url=https://web.archive.org/web/20180711073415/https://www.eia.gov/Energyexplained/?page=biomass_biogas |archive-date=11 July 2018 |access-date=22 November 2015 |publisher=U.S. Energy Information Administration}} Regardless of the type of organic material, the production of gases follows well defined kinetic pattern. Carbon dioxide comprises about 40–45% of the gas that emanates from decomposition in landfills (termed "landfill gas"). Most of the remaining 50–55% is methane.{{cite web |date=January 2000 |title=Facts About Landfill Gas |url=http://www.dem.ri.gov/programs/benviron/waste/central/lfgfact.pdf |url-status=live |archive-url=https://web.archive.org/web/20150923213448/http://www.dem.ri.gov/programs/benviron/waste/central/lfgfact.pdf |archive-date=23 September 2015 |access-date=4 September 2015 |publisher=U.S. Environmental Protection Agency}}

The combustion of all carbon-based fuels, such as methane (natural gas), petroleum distillates (gasoline, diesel, kerosene, propane), coal, wood and generic organic matter produces carbon dioxide and, except in the case of pure carbon, water. As an example, the chemical reaction between methane and oxygen:

:{{chem2|CH4 + 2 O2 -> CO2 + 2 H2O}}

Iron is reduced from its oxides with coke in a blast furnace, producing pig iron and carbon dioxide:

{{Cite book | vauthors = Strassburger J | title = Blast Furnace Theory and Practice | publisher = American Institute of Mining, Metallurgical, and Petroleum Engineers | place = New York | year = 1969 | isbn = 978-0-677-10420-1}}

:{{chem2|Fe2O3 + 3 CO -> 3 CO2 + 2 Fe}}

Carbon dioxide is a byproduct of the industrial production of hydrogen by steam reforming and the water gas shift reaction in ammonia production. These processes begin with the reaction of water and natural gas (mainly methane).{{cite book |doi=10.1002/14356007.a05_165|chapter=Carbon Dioxide|title=Ullmann's Encyclopedia of Industrial Chemistry|year=2000| vauthors = Topham S |isbn=3527306730}}

It is produced by thermal decomposition of limestone, {{chem2|CaCO3}} by heating (calcining) at about {{convert|850|C}}, in the manufacture of quicklime (calcium oxide, CaO), a compound that has many industrial uses:

:{{chem2|CaCO3 -> CaO + CO2}}

Acids liberate {{CO2}} from most metal carbonates. Consequently, it may be obtained directly from natural carbon dioxide springs, where it is produced by the action of acidified water on limestone or dolomite. The reaction between hydrochloric acid and calcium carbonate (limestone or chalk) is shown below:

:{{chem2|CaCO3 + 2 HCl -> CaCl2 + H2CO3}}

The carbonic acid ({{chem2|H2CO3}}) then decomposes to water and {{CO2}}:

:{{chem2|H2CO3 -> CO2 + H2O}}

Such reactions are accompanied by foaming or bubbling, or both, as the gas is released. They have widespread uses in industry because they can be used to neutralize waste acid streams.

File:CO2 use in 2015 - IEA.png

Around 230 Mt of {{CO2}} are used each year,{{Cite web |title=CO2 Capture and Utilisation - Energy System |url=https://www.iea.org/energy-system/carbon-capture-utilisation-and-storage/co2-capture-and-utilisation |access-date=2024-10-30 |website=IEA |language=en-GB}} mostly in the fertiliser industry for urea production (130 million tonnes) and in the oil and gas industry for enhanced oil recovery (70 to 80 million tonnes).{{Rp|page=3}} Other commercial applications include food and beverage production, metal fabrication, cooling, fire suppression and stimulating plant growth in greenhouses.{{Rp|page=3}}

Technology exists to capture {{CO2}} from industrial flue gas or from the air. Research is ongoing on ways to use captured {{CO2}} in products and some of these processes have been deployed commercially.{{Cite journal |last1=Dziejarski |first1=Bartosz |last2=Krzyżyńska |first2=Renata |last3=Andersson |first3=Klas |date=June 2023 |title=Current status of carbon capture, utilization, and storage technologies in the global economy: A survey of technical assessment |journal=Fuel |volume=342 |pages=127776 |bibcode=2023Fuel..34227776D |doi=10.1016/j.fuel.2023.127776 |issn=0016-2361 |doi-access=free}}50x50px Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License However, the potential to use products is very small compared to the total volume of {{CO2}} that could foreseeably be captured.{{Cite web |title=CO2 Capture and Utilisation - Energy System |url=https://www.iea.org/energy-system/carbon-capture-utilisation-and-storage/co2-capture-and-utilisation |access-date=2024-07-18 |website=IEA |language=en-GB}}50x50px Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License The vast majority of captured {{CO2}} is considered a waste product and sequestered in underground geologic formations.{{Cite journal |last1=Sekera |first1=June |last2=Lichtenberger |first2=Andreas |date=6 October 2020 |title=Assessing Carbon Capture: Public Policy, Science, and Societal Need: A Review of the Literature on Industrial Carbon Removal |journal=Biophysical Economics and Sustainability |volume=5 |issue=3 |pages=14 |bibcode=2020BpES....5...14S |doi=10.1007/s41247-020-00080-5 |issn= |doi-access=free}}Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License

{{expand section|date=July 2014}}

{{See also|Sabatier reaction}}

In the chemical industry, carbon dioxide is mainly consumed as an ingredient in the production of urea, with a smaller fraction being used to produce methanol and a range of other products.{{cite web|url=https://www.ipcc.ch/pdf/special-reports/srccs/srccs_chapter7.pdf|title=IPCC Special Report on Carbon dioxide Capture and Storage|publisher=The Intergovernmental Panel on Climate Change|access-date=4 September 2015|archive-url=https://web.archive.org/web/20150924115331/http://www.ipcc.ch/pdf/special-reports/srccs/srccs_chapter7.pdf|archive-date=24 September 2015|url-status=dead}} Some carboxylic acid derivatives such as sodium salicylate are prepared using {{CO2}} by the Kolbe–Schmitt reaction.{{cite book | vauthors = Morrison RT, Boyd RN |title= Organic Chemistry |edition=4th |publisher=Allyn and Bacon |year=1983 |isbn=978-0-205-05838-9 |pages=[https://archive.org/details/organicchemistry04morr/page/976 976–977] |url=https://archive.org/details/organicchemistry04morr/page/976}}

Captured {{CO2}} could be to produce methanol or electrofuels. To be carbon-neutral, the {{CO2}} would need to come from bioenergy production or direct air capture.IEA (2020), [https://www.iea.org/reports/ccus-in-clean-energy-transitions CCUS in Clean Energy Transitions], IEA, Paris 50x50px Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License{{Rp|page=|pages=21–24}}

Carbon dioxide is used in enhanced oil recovery where it is injected into or adjacent to producing oil wells, usually under supercritical conditions, when it becomes miscible with the oil. This approach can increase original oil recovery by reducing residual oil saturation by 7–23% additional to primary extraction.{{cite book |url=http://www.globalccsinstitute.com/publications/accelerating-uptake-ccs-industrial-use-captured-carbon-dioxide |title=Accelerating the uptake of CCS: industrial use of captured carbon dioxide |date=20 December 2011 |website=Global CCS Institute |chapter=Appendix A: {{CO2}} for use in enhanced oil recovery (EOR) |access-date=2 January 2017 |chapter-url=http://hub.globalccsinstitute.com/publications/accelerating-uptake-ccs-industrial-use-captured-carbon-dioxide/appendix-co2-use |archive-url=https://web.archive.org/web/20170428013833/http://www.globalccsinstitute.com/publications/accelerating-uptake-ccs-industrial-use-captured-carbon-dioxide |archive-date=28 April 2017 |url-status=live}} It acts as both a pressurizing agent and, when dissolved into the underground crude oil, significantly reduces its viscosity, and changing surface chemistry enabling the oil to flow more rapidly through the reservoir to the removal well.{{cite journal |vauthors=Austell JM |year=2005 |title={{CO2}} for Enhanced Oil Recovery Needs – Enhanced Fiscal Incentives |url=http://www.touchoilandgas.com/enhanced-recovery-needs-enhanced-a423-1.html |journal=Exploration & Production: The Oil & Gas Review |archive-url=https://web.archive.org/web/20120207071349/http://www.touchoilandgas.com/enhanced-recovery-needs-enhanced-a423-1.html |archive-date=7 February 2012 |access-date=28 September 2007}}

Most {{CO2}} injected in {{CO2}}-EOR projects comes from naturally occurring underground {{CO2}} deposits.{{Cite web |date=2019-04-11 |title=Can CO2-EOR really provide carbon-negative oil? – Analysis |url=https://www.iea.org/commentaries/can-co2-eor-really-provide-carbon-negative-oil |access-date=2024-10-16 |website=IEA |language=en-GB}} Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License Some {{CO2}} used in EOR is captured from industrial facilities such as natural gas processing plants, using carbon capture technology and transported to the oilfield in pipelines.

Plants require carbon dioxide to conduct photosynthesis. The atmospheres of greenhouses may (if of large size, must) be enriched with additional {{CO2}} to sustain and increase the rate of plant growth.{{cite web |url=http://www.ext.colostate.edu/mg/gardennotes/141.html |title=Plant Growth Factors: Photosynthesis, Respiration, and Transpiration |website=CMG GardenNotes | vauthors = Whiting D, Roll M, Vickerman L |publisher=Colorado Master Gardener Program |date=August 2010 |access-date=10 October 2011 |archive-url=https://web.archive.org/web/20140902192633/http://www.ext.colostate.edu/mg/gardennotes/141.html |archive-date=2 September 2014}}{{cite book |chapter-url=http://www-formal.stanford.edu/jmc/nature/node21.html |chapter=Carbon dioxide |url=http://www-formal.stanford.edu/jmc/nature/nature.html |title=How Much Land Can Ten Billion People Spare for Nature? | vauthors = Waggoner PE |date=February 1994 |access-date=10 October 2011 |archive-date=12 October 2011 |archive-url=https://web.archive.org/web/20111012165809/http://www-formal.stanford.edu/jmc/nature/nature.html |url-status=live}} At very high concentrations (100 times atmospheric concentration, or greater), carbon dioxide can be toxic to animal life, so raising the concentration to 10,000 ppm (1%) or higher for several hours will eliminate pests such as whiteflies and spider mites in a greenhouse.{{cite journal | vauthors = Stafford N | title = Future crops: the other greenhouse effect | journal = Nature | volume = 448 | issue = 7153 | pages = 526–528 | date = August 2007 | pmid = 17671477 | doi = 10.1038/448526a | bibcode = 2007Natur.448..526S | s2cid = 9845813 | doi-access = free}} Some plants respond more favorably to rising carbon dioxide concentrations than others, which can lead to vegetation regime shifts like woody plant encroachment.{{Citation |last1=Archer |first1=Steven R. |title=Woody Plant Encroachment: Causes and Consequences |date=2017 |work=Rangeland Systems |pages=25–84 |editor-last=Briske |editor-first=David D. |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-319-46709-2_2 |isbn=978-3-319-46707-8 |last2=Andersen |first2=Erik M. |last3=Predick |first3=Katharine I. |last4=Schwinning |first4=Susanne |last5=Steidl |first5=Robert J. |last6=Woods |first6=Steven R.|doi-access=free }}

File:Soda bubbles macro.jpg

Carbon dioxide is a food additive used as a propellant and acidity regulator in the food industry. It is approved for usage in the EUUK Food Standards Agency: {{cite web |url=http://www.food.gov.uk/safereating/chemsafe/additivesbranch/enumberlist |title=Current EU approved additives and their E Numbers |access-date=27 October 2011 |archive-date=7 October 2010 |archive-url=https://web.archive.org/web/20101007124435/http://www.food.gov.uk/safereating/chemsafe/additivesbranch/enumberlist |url-status=live}} (listed as E number E290), US,US Food and Drug Administration: {{cite web |url=https://www.fda.gov/food/ingredientspackaginglabeling/foodadditivesingredients/ucm091048.htm |title=Food Additive Status List |website=Food and Drug Administration |access-date=13 June 2015 |archive-date=4 November 2017 |archive-url=https://web.archive.org/web/20171104061606/https://www.fda.gov/Food/IngredientsPackagingLabeling/FoodAdditivesIngredients/ucm091048.htm |url-status=live}} Australia and New ZealandAustralia New Zealand Food Standards Code{{cite web |url=http://www.comlaw.gov.au/Details/F2011C00827 |title=Standard 1.2.4 – Labelling of ingredients |date=8 September 2011 |access-date=27 October 2011 |archive-date=19 January 2012 |archive-url=https://web.archive.org/web/20120119082034/http://www.comlaw.gov.au/Details/F2011C00827 |url-status=live}} (listed by its INS number 290).

A candy called Pop Rocks is pressurized with carbon dioxide gas{{Cite book |url=https://books.google.com/books?id=0XeSJLflq90C&q=Pop+Rocks+is+pressurized+with+carbon+dioxide+gas&pg=PA7-IA3 |title=Futurific Leading Indicators Magazine |volume=1 |publisher=CRAES LLC |isbn=978-0-9847670-1-4 |access-date=9 November 2020 |archive-date=15 August 2021 |archive-url=https://web.archive.org/web/20210815224429/https://books.google.com/books?id=0XeSJLflq90C&q=Pop+Rocks+is+pressurized+with+carbon+dioxide+gas&pg=PA7-IA3 |url-status=live}} at about {{convert|4000|kPa|bar psi|abbr=on|lk=on}}. When placed in the mouth, it dissolves (just like other hard candy) and releases the gas bubbles with an audible pop.

Leavening agents cause dough to rise by producing carbon dioxide.{{Cite book |url=https://books.google.com/books?id=2bmaCgAAQBAJ&q=Leavening+agents+cause+dough+to+rise+by+producing+carbon+dioxide&pg=PT29 |title=Indian Breads: A Comprehensive Guide to Traditional and Innovative Indian Breads |vauthors=Vijay GP |date=25 September 2015 |publisher=Westland |isbn=978-93-85724-46-6}}{{Dead link|date=August 2023 |bot=InternetArchiveBot |fix-attempted=yes}} Baker's yeast produces carbon dioxide by fermentation of sugars within the dough, while chemical leaveners such as baking powder and baking soda release carbon dioxide when heated or if exposed to acids.

Carbon dioxide is used to produce carbonated soft drinks and soda water. Traditionally, the carbonation of beer and sparkling wine came about through natural fermentation, but many manufacturers carbonate these drinks with carbon dioxide recovered from the fermentation process. In the case of bottled and kegged beer, the most common method used is carbonation with recycled carbon dioxide. With the exception of British real ale, draught beer is usually transferred from kegs in a cold room or cellar to dispensing taps on the bar using pressurized carbon dioxide, sometimes mixed with nitrogen.

The taste of soda water (and related taste sensations in other carbonated beverages) is an effect of the dissolved carbon dioxide rather than the bursting bubbles of the gas. Carbonic anhydrase 4 converts carbon dioxide to carbonic acid leading to a sour taste, and also the dissolved carbon dioxide induces a somatosensory response.{{cite web |url= https://www.sciencedaily.com/releases/2009/10/091015141510.htm |title= Scientists Discover Protein Receptor For Carbonation Taste |website= ScienceDaily |date= 16 October 2009 |access-date= 29 March 2020 |archive-date= 29 March 2020 |archive-url= https://web.archive.org/web/20200329042900/https://www.sciencedaily.com/releases/2009/10/091015141510.htm |url-status= live}}

File:Dry ice used to preserve grapes after harvest.jpg

Carbon dioxide in the form of dry ice is often used during the cold soak phase in winemaking to cool clusters of grapes quickly after picking to help prevent spontaneous fermentation by wild yeast. The main advantage of using dry ice over water ice is that it cools the grapes without adding any additional water that might decrease the sugar concentration in the grape must, and thus the alcohol concentration in the finished wine. Carbon dioxide is also used to create a hypoxic environment for carbonic maceration, the process used to produce Beaujolais wine.

Carbon dioxide is sometimes used to top up wine bottles or other storage vessels such as barrels to prevent oxidation, though it has the problem that it can dissolve into the wine, making a previously still wine slightly fizzy. For this reason, other gases such as nitrogen or argon are preferred for this process by professional wine makers.

Carbon dioxide is often used to "stun" animals before slaughter.{{cite journal | vauthors = Coghlan A |title=A more humane way of slaughtering chickens might get EU approval |journal=New Scientist |date=3 February 2018 |url=https://www.newscientist.com/article/2159895-a-more-humane-way-of-slaughtering-chickens-might-get-eu-approval |access-date=24 June 2018 |archive-date=24 June 2018 |archive-url=https://web.archive.org/web/20180624204842/https://www.newscientist.com/article/2159895-a-more-humane-way-of-slaughtering-chickens-might-get-eu-approval/ |url-status=live}} "Stunning" may be a misnomer, as the animals are not knocked out immediately and may suffer distress.{{cite web |url=http://kb.rspca.org.au/What-is-CO2-stunning_118.html |archive-url=https://web.archive.org/web/20140409003755/http://kb.rspca.org.au/What-is-CO2-stunning_118.html |url-status=dead |archive-date=9 April 2014 |title=What is {{CO2}} stunning? |publisher=RSPCA}}{{cite journal | vauthors = Campbell A |title=Humane execution and the fear of the tumbril |journal=New Scientist |date=10 March 2018 |url=https://www.newscientist.com/letter/mg23731680-900-humane-execution-and-the-fear-of-the-tumbril-3 |access-date=24 June 2018 |archive-date=24 June 2018 |archive-url=https://web.archive.org/web/20180624204708/https://www.newscientist.com/letter/mg23731680-900-humane-execution-and-the-fear-of-the-tumbril-3/ |url-status=live}}

Carbon dioxide is one of the most commonly used compressed gases for pneumatic (pressurized gas) systems in portable pressure tools. Carbon dioxide is also used as an atmosphere for welding, although in the welding arc, it reacts to oxidize most metals. Use in the automotive industry is common despite significant evidence that welds made in carbon dioxide are more brittle than those made in more inert atmospheres.{{Cite book |last=International |first=Petrogav |url=https://books.google.com/books?id=ZS7JDwAAQBAJ |title=Production Course for Hiring on Offshore Oil and Gas Rigs |publisher=Petrogav International |pages=214 |language=en}} When used for MIG welding, {{CO2}} use is sometimes referred to as MAG welding, for Metal Active Gas, as {{CO2}} can react at these high temperatures. It tends to produce a hotter puddle than truly inert atmospheres, improving the flow characteristics. Although, this may be due to atmospheric reactions occurring at the puddle site. This is usually the opposite of the desired effect when welding, as it tends to embrittle the site, but may not be a problem for general mild steel welding, where ultimate ductility is not a major concern.

Carbon dioxide is used in many consumer products that require pressurized gas because it is inexpensive and nonflammable, and because it undergoes a phase transition from gas to liquid at room temperature at an attainable pressure of approximately {{convert|60|bar|psi atm|abbr=on|lk=on}}, allowing far more carbon dioxide to fit in a given container than otherwise would. Life jackets often contain canisters of pressured carbon dioxide for quick inflation. Aluminium capsules of {{CO2}} are also sold as supplies of compressed gas for air guns, paintball markers/guns, inflating bicycle tires, and for making carbonated water. High concentrations of carbon dioxide can also be used to kill pests. Liquid carbon dioxide is used in supercritical drying of some food products and technological materials, in the preparation of specimens for scanning electron microscopy{{cite journal | vauthors = Nordestgaard BG, Rostgaard J | title = Critical-point drying versus freeze drying for scanning electron microscopy: a quantitative and qualitative study on isolated hepatocytes | journal = Journal of Microscopy | volume = 137 | issue = Pt 2 | pages = 189–207 | date = February 1985 | pmid = 3989858 | doi = 10.1111/j.1365-2818.1985.tb02577.x | s2cid = 32065173}} and in the decaffeination of coffee beans.

File:US Army 53023 Fire Prevention Week.jpg

Carbon dioxide can be used to extinguish flames by flooding the environment around the flame with the gas. It does not itself react to extinguish the flame, but starves the flame of oxygen by displacing it. Some fire extinguishers, especially those designed for electrical fires, contain liquid carbon dioxide under pressure. Carbon dioxide extinguishers work well on small flammable liquid and electrical fires, but not on ordinary combustible fires, because they do not cool the burning substances significantly, and when the carbon dioxide disperses, they can catch fire upon exposure to atmospheric oxygen. They are mainly used in server rooms.{{Cite web |title=Types of Fire Extinguishers |url=https://www.firesafe.org.uk/types-use-and-colours-of-portable-fire-extinguishers/ |url-status=live |access-date=2021-06-28 |website=The Fire Safety Advice Centre |archive-date=28 June 2021 |archive-url=https://web.archive.org/web/20210628185630/https://www.firesafe.org.uk/types-use-and-colours-of-portable-fire-extinguishers/}}

Carbon dioxide has also been widely used as an extinguishing agent in fixed fire-protection systems for local application of specific hazards and total flooding of a protected space.National Fire Protection Association Code 12. International Maritime Organization standards recognize carbon dioxide systems for fire protection of ship holds and engine rooms. Carbon dioxide-based fire-protection systems have been linked to several deaths, because it can cause suffocation in sufficiently high concentrations. A review of {{CO2}} systems identified 51 incidents between 1975 and the date of the report (2000), causing 72 deaths and 145 injuries.Carbon Dioxide as a Fire Suppressant: Examining the Risks, US EPA. 2000.

{{See also|Supercritical carbon dioxide|Green chemistry}}

Liquid carbon dioxide is a good solvent for many lipophilic organic compounds and is used to decaffeinate coffee.{{cite book |url=https://books.google.com/books?id=5210HQIwxzsC&pg=PA185 |title=Modern drying technology |vauthors=Tsotsas E, Mujumdar AS |date=2011 |publisher=John Wiley & Sons |isbn=978-3-527-31558-1 |series=Vol. 3: Product quality and formulation |access-date=3 December 2019 |archive-url=https://web.archive.org/web/20200321173739/https://books.google.com/books?id=5210HQIwxzsC&pg=PA185 |archive-date=21 March 2020 |url-status=live}} Carbon dioxide has attracted attention in the pharmaceutical and other chemical processing industries as a less toxic alternative to more traditional solvents such as organochlorides. It is also used by some dry cleaners for this reason. It is used in the preparation of some aerogels because of the properties of supercritical carbon dioxide.

{{see also|Refrigerant|Sustainable automotive air conditioning}}

File:Comparison carbon dioxide water phase diagrams.svg

Liquid and solid carbon dioxide are important refrigerants, especially in the food industry, where they are employed during the transportation and storage of ice cream and other frozen foods. Solid carbon dioxide is called "dry ice" and is used for small shipments where refrigeration equipment is not practical. Solid carbon dioxide is always below {{convert|-78.5|C|F}} at regular atmospheric pressure, regardless of the air temperature.

{{anchor|R744}} Liquid carbon dioxide (industry nomenclature R744 or R-744) was used as a refrigerant prior to the use of dichlorodifluoromethane (R12, a chlorofluorocarbon (CFC) compound).{{Cite web |last=Pearson |first=S. Forbes |title=Refrigerants Past, Present and Future |url=http://www.r744.com/files/pdf_597.pdf |url-status=dead |archive-url=https://web.archive.org/web/20180713171048/http://www.r744.com/files/pdf_597.pdf |archive-date=2018-07-13 |access-date=2021-03-30 |website=R744}} {{CO2}} might enjoy a renaissance because one of the main substitutes to CFCs, 1,1,1,2-tetrafluoroethane (R134a, a hydrofluorocarbon (HFC) compound) contributes to climate change more than {{CO2}} does. {{CO2}} physical properties are highly favorable for cooling, refrigeration, and heating purposes, having a high volumetric cooling capacity. Due to the need to operate at pressures of up to {{convert|130|bar|psi kPa}}, {{CO2}} systems require highly mechanically resistant reservoirs and components that have already been developed for mass production in many sectors. In automobile air conditioning, in more than 90% of all driving conditions for latitudes higher than 50°, {{CO2}} (R744) operates more efficiently than systems using HFCs (e.g., R134a). Its environmental advantages (GWP of 1, non-ozone depleting, non-toxic, non-flammable) could make it the future working fluid to replace current HFCs in cars, supermarkets, and heat pump water heaters, among others. Coca-Cola has fielded {{CO2}}-based beverage coolers and the U.S. Army is interested in {{CO2}} refrigeration and heating technology.{{cite web |url=http://www.coca-colacompany.com/cooling-equipment-pushing-forward-with-hfc-free |title=The Coca-Cola Company Announces Adoption of HFC-Free Insulation in Refrigeration Units to Combat Global Warming |access-date=11 October 2007 |date=5 June 2006 |publisher=The Coca-Cola Company |archive-date=1 November 2013 |archive-url=https://web.archive.org/web/20131101195654/http://www.coca-colacompany.com/cooling-equipment-pushing-forward-with-hfc-free |url-status=live}}{{cite news|title = Modine reinforces its {{CO2}} research efforts|url = http://www.r744.com/news/news_ida145.php|archive-url = https://web.archive.org/web/20080210194203/http://www.r744.com/news/news_ida145.php|url-status = dead|archive-date = 10 February 2008|date = 28 June 2007|publisher = R744.com}}

File:Carbon Dioxide Laser At The Laser Effects Test Facility.jpg]]

Carbon dioxide is the lasing medium in a carbon-dioxide laser, which is one of the earliest type of lasers.

Carbon dioxide can be used as a means of controlling the pH of swimming pools,{{Cite book |url=https://books.google.com/books?id=IWpWAAAAMAAJ&q=%C2%A0%C2%A0Carbon+dioxide+can+be+used+as+a+means+of+controlling+the+pH+of+swimming+pool |title=TCE, the Chemical Engineer |date=1990 |publisher=Institution of Chemical Engineers |access-date=2 June 2020 |archive-date=17 August 2021 |archive-url=https://web.archive.org/web/20210817030754/https://books.google.com/books?id=IWpWAAAAMAAJ&q=%C2%A0%C2%A0Carbon+dioxide+can+be+used+as+a+means+of+controlling+the+pH+of+swimming+pool |url-status=live}} by continuously adding gas to the water, thus keeping the pH from rising. Among the advantages of this is the avoidance of handling (more hazardous) acids. Similarly, it is also used in the maintaining reef aquaria, where it is commonly used in calcium reactors to temporarily lower the pH of water being passed over calcium carbonate in order to allow the calcium carbonate to dissolve into the water more freely, where it is used by some corals to build their skeleton.

Used as the primary coolant in the British advanced gas-cooled reactor for nuclear power generation.

Carbon dioxide induction is commonly used for the euthanasia of laboratory research animals. Methods to administer {{CO2}} include placing animals directly into a closed, prefilled chamber containing {{CO2}}, or exposure to a gradually increasing concentration of {{CO2}}. The American Veterinary Medical Association's 2020 guidelines for carbon dioxide induction state that a displacement rate of 30–70% of the chamber or cage volume per minute is optimal for the humane euthanasia of small rodents.{{cite web |url=https://www.avma.org/kb/policies/documents/euthanasia.pdf |title=AVMA guidelines for the euthanasia of animals: 2020 Edition |date=2020 |publisher=American Veterinary Medical Association |access-date=August 13, 2021 |archive-date=1 February 2014 |archive-url=https://web.archive.org/web/20140201174132/https://www.avma.org/KB/Policies/Documents/euthanasia.pdf |url-status=live}}{{Rp|5, 31}} Percentages of {{CO2}} vary for different species, based on identified optimal percentages to minimize distress.{{Rp|22}}

Carbon dioxide is also used in several related cleaning and surface-preparation techniques.

File:Carbon-dioxide-crystal-3D-vdW.png]]

Carbon dioxide was the first gas to be described as a discrete substance. In about 1640,{{cite journal |vauthors=Harris D |date=September 1910 |title=The Pioneer in the Hygiene of Ventilation |url=https://zenodo.org/record/2088803 |url-status=live |journal=The Lancet |volume=176 |issue=4542 |pages=906–908 |doi=10.1016/S0140-6736(00)52420-9 |archive-url=https://web.archive.org/web/20200317181844/https://zenodo.org/record/2088803 |archive-date=17 March 2020 |access-date=6 December 2019}} the Flemish chemist Jan Baptist van Helmont observed that when he burned charcoal in a closed vessel, the mass of the resulting ash was much less than that of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" (from Greek "chaos") or "wild spirit" (spiritus sylvestris).{{cite book |title=History of industrial gases |vauthors=Almqvist E |date=2003 |publisher=Springer |isbn=978-0-306-47277-0 |page=93}}

The properties of carbon dioxide were further studied in the 1750s by the Scottish physician Joseph Black. He found that limestone (calcium carbonate) could be heated or treated with acids to yield a gas he called "fixed air". He observed that the fixed air was denser than air and supported neither flame nor animal life. Black also found that when bubbled through limewater (a saturated aqueous solution of calcium hydroxide), it would precipitate calcium carbonate. He used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, English chemist Joseph Priestley published a paper entitled Impregnating Water with Fixed Air in which he described a process of dripping sulfuric acid (or oil of vitriol as Priestley knew it) on chalk in order to produce carbon dioxide, and forcing the gas to dissolve by agitating a bowl of water in contact with the gas.{{cite journal |author-link1=Joseph Priestley |vauthors=Priestley J, Hey W |year=1772 |title=Observations on Different Kinds of Air |url=http://web.lemoyne.edu/~GIUNTA/priestley.html |url-status=live |journal=Philosophical Transactions |volume=62 |pages=147–264 |doi=10.1098/rstl.1772.0021 |archive-url=https://web.archive.org/web/20100607170541/http://web.lemoyne.edu/%7Egiunta/priestley.html |archive-date=7 June 2010 |access-date=11 October 2007 |s2cid=186210131}}

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and Michael Faraday.{{cite journal |author-link=Humphry Davy |vauthors=Davy H |year=1823 |title=On the Application of Liquids Formed by the Condensation of Gases as Mechanical Agents |url=https://archive.org/details/jstor-107649 |journal=Philosophical Transactions |volume=113 |pages=199–205 |doi=10.1098/rstl.1823.0020 |jstor=107649 |doi-access=free}} The earliest description of solid carbon dioxide (dry ice) was given by the French inventor Adrien-Jean-Pierre Thilorier, who in 1835 opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid {{CO2}}.{{cite journal |vauthors=Thilorier AJ |year=1835 |title=Solidification de l'Acide carbonique |url=http://gallica.bnf.fr/ark:/12148/bpt6k29606/f194.item |url-status=live |journal=Comptes Rendus |volume=1 |pages=194–196 |archive-url=https://web.archive.org/web/20170902172202/http://gallica.bnf.fr/ark:/12148/bpt6k29606/f194.item |archive-date=2 September 2017 |access-date=1 September 2017}}{{cite journal |vauthors=Thilorier AJ |year=1836 |title=Solidification of carbonic acid |url=https://books.google.com/books?id=4GwqAAAAYAAJ&pg=PA446 |url-status=live |journal=The London and Edinburgh Philosophical Magazine |volume=8 |issue=48 |pages=446–447 |doi=10.1080/14786443608648911 |archive-url=https://web.archive.org/web/20160502065711/https://books.google.com/books?id=4GwqAAAAYAAJ&pg=PA446 |archive-date=2 May 2016 |access-date=15 November 2015}}

Carbon dioxide in combination with nitrogen was known from earlier times as Blackdamp, stythe or choke damp.{{efn|Sometimes spelt "choak-damp" in 19th Century texts.}} Along with the other types of damp it was encountered in mining operations and well sinking. Slow oxidation of coal and biological processes replaced the oxygen to create a suffocating mixture of nitrogen and carbon dioxide.{{cite journal | url=https://www.jstor.org/stable/115391 | jstor=115391 | title=Notes of an Enquiry into the Nature and Physiological Action of Black-Damp, as Met with in Podmore Colliery, Staffordshire, and Lilleshall Colliery, Shropshire | last1=Haldane | first1=John | journal=Proceedings of the Royal Society of London | date=1894 | volume=57 | pages=249–257 | bibcode=1894RSPS...57..249H}}

{{Portal|Chemistry}}

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• {{annotated link|Arterial blood gas test}}
• {{annotated link|Bosch reaction}}
• {{annotated link|Carbon dioxide removal}} (from the atmosphere)
• {{annotated link|Gilbert Plass}} (early work on {{CO2}} and climate change)
• {{annotated link|Greenhouse Gases Observing Satellite}}
• List of countries by carbon dioxide emissions
• List of least carbon efficient power stations
• {{annotated link|Meromictic lake}}
• NASA's {{annotated link|Orbiting Carbon Observatory 2}}
• {{annotated link|Soil gas}}

{{div col end}}

{{reflist|group=note}}

{{notelist}}

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{{Commons category}}

{{Library resources box |lcheading=Carbon dioxide}}

• [https://earth.nullschool.net/#current/chem/surface/level/overlay=co2sc/winkel3 Current global map of carbon dioxide concentration]
• [https://www.cdc.gov/niosh/npg/npgd0103.html CDC – NIOSH Pocket Guide to Chemical Hazards – Carbon Dioxide]
• [https://gml.noaa.gov/ccgg/trends/ Trends in Atmospheric Carbon Dioxide] (NOAA)
• [https://web.archive.org/web/20071007040239/http://www.shecco.com/about/history.php The rediscovery of {{CO2}}: History, What is Shecco?] - as refrigerant

{{Oxides}}

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{{Inorganic compounds of carbon}}

{{Global Warming|state=collapsed}}

{{Molecules detected in outer space}}

{{Oxygen compounds}}

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