Diesel exhaust#Concerns regarding particulates

The primary products of petroleum fuel combustion in air are carbon dioxide, water, and nitrogen. The other components exist primarily from incomplete combustion and pyrosynthesis.{{cite journal |last1=Scheepers|first1=P. T.|last2=Bos|first2=R. P.|s2cid=4721619|date=1992-01-01|title=Combustion of diesel fuel from a toxicological perspective. I. Origin of incomplete combustion products|journal=International Archives of Occupational and Environmental Health|volume=64|issue=3|pages=149–161|issn=0340-0131|pmid=1383162|doi=10.1007/bf00380904|bibcode=1992IAOEH..64..149S }} While the distribution of the individual components of raw (untreated) diesel exhaust varies depending on factors like load, engine type, etc., the table below shows a typical composition.

The physical and chemical conditions that exist inside any such diesel engines under any conditions differ considerably from spark-ignition engines, because, by design, diesel engine power is directly controlled by the fuel supply, not by control of the air/fuel mixture, as in conventional gasoline engines.{{cite book |author=Song, Chunsham |date= 2000 |title=Chemistry of Diesel Fuels |place=Boca Raton, FL, US |publisher=CRC Press |page=4 |url=https://books.google.com/books?isbn=1560328452 |access-date=24 October 2015}} As a result of these differences, diesel engines generally produce a different array of pollutants than spark-driven engines, differences that are sometimes qualitative (what pollutants are there, and what are not), but more often quantitative (how much of particular pollutants or pollutant classes are present in each). For instance, diesel engines produce one-twenty-eighth the carbon monoxide that gasoline engines do, as they burn their fuel in excess air even at full load.{{cite journal |title=The Toxicity of Diesel Exhaust: Implications for Primary Care |volume=21 |issue=1 |journal= The Journal of the American Board of Family Medicine|date=January 2008 |author=Krivoshto, Irina N. |author2=Richards, John R. |author3= Albertson, Timothy E. |author4=Derlet, Robert W. |name-list-style=amp |pages=55–62 |doi=10.3122/jabfm.2008.01.070139 |pmid=18178703|doi-access=free }}{{cite book |url=https://books.google.com/books?id=z5VTXj_got4C&q=diesels%20burn%20their%20fuel%20in%20%22excess%20air%20even%20at%20full%20load%22&pg=PA230 |title=Alternative Transportation Fuels: Utilisation in Combustion Engines |publisher=CRC Press |work=Book |date=18 June 2013 |access-date=24 October 2015 |author1=Gajendra Babu, M.K. |author2=Subramanian, K.A. |pages=230 |isbn=9781439872819}}{{cite web |url=https://www.dieselnet.com/tech/emi_intro.php |title=What Are Diesel Emissions |date=2012 |publisher=Ecopoint Inc. |access-date=5 June 2015 |author=Majewski, W. Addy}}{{Third-party inline|date=October 2015}}

However, the lean-burning nature of diesel engines and the high temperatures and pressures of the combustion process result in significant production of NO_x (gaseous nitrogen oxides), an air pollutant that constitutes a unique challenge with regard to their reduction.{{not verified in body|date=October 2015}} While total nitrogen oxides from petrol cars have decreased by around 96% through the adoption of exhaust catalytic converters as of 2012, diesel cars still produce nitrogen oxides at a similar level to those bought 15 years earlier under real-world tests; hence, diesel cars emit around 20 times more nitrogen oxides than petrol cars.{{cite news|url=https://www.theguardian.com/environment/2012/jul/08/pollutionwatch-diesel-petrol-electric-cars|title=Diesel cars emit more nitrogen oxides than petrol cars|date=Jul 8, 2012|work=The Guardian|quote=New diesels produce similar nitrogen oxides to those bought 15 years ago. Typical modern diesel cars emit around 20 times more nitrogen oxides than petrol cars. |last1=Fuller |first1=Gary |access-date=5 June 2015}}{{cite news |url=https://www.telegraph.co.uk/motoring/green-motoring/10190942/Why-is-killer-diesel-still-poisoning-our-air.html |title=Why is killer diesel still poisoning our air? |date=Jul 19, 2013 |work=The Telegraph |quote=Much of the problem is down to EU emission standards, which have long allowed diesel engines to emit much more nitrogen dioxide than petrol ones. |last1=Lean |first1=Geoffrey |access-date=5 June 2015}}{{cite book |title=Trends in NOX and NO2 emissions and ambient measurements in the UK |author=Carslaw D., Beevers |author2=S., Westmoreland E. |author3=Williams, M.|author3-link=Martin Williams (environmental scientist) |author4=Tate, J. |author5=Murrells, T. |author6=Stedman, J. |author7=Li, Y. |author8=Grice, S. |author9= Kent A |author10=Tsagatakis, I. |name-list-style=amp |publisher=Department for Environment, Food and Rural Affairs |year=2011 |location=London |quote=However, vehicles registered from 2005–2010 emit similar or higher levels of NOx compared with vehicles before 1995. In this respect, NOx emissions from diesel cars have changed little over a period of about 20 years.}} Modern on-road diesel engines typically use selective catalytic reduction (SCR) systems to meet emissions laws, as other methods such as exhaust gas recirculation (EGR) cannot adequately reduce NO_x to meet the newer standards applicable in many jurisdictions.

Moreover, the fine particles (fine particulate matter) in diesel exhaust (e.g., soot, sometimes visible as opaque dark-colored smoke) has traditionally been of greater concern, as it presents different health concerns and is rarely produced in significant quantities by spark-ignition engines. These especially harmful particulate contaminants are at their peak when such engines are run without sufficient oxygen to fully combust the fuel; when a diesel engine runs at idle, enough oxygen is usually present to burn the fuel completely.{{cite journal |year=2015 |title=Recent Studies on Soot Modeling for Diesel Combustion |journal=Renewable and Sustainable Energy Reviews |volume=48 |pages=635–647 |doi=10.1016/j.rser.2015.04.019 |author1=Omidvarbornaa, Hamid |author2=Kumara, Ashok |author3=Kim, Dong-Shik|bibcode=2015RSERv..48..635O }} From the particle emission standpoint, exhaust from diesel vehicles has been reported to be significantly more harmful than those from petrol vehicles.

Diesel exhausts, long known for their characteristic smells, changed significantly with the reduction of the sulfur content of diesel fuel, and again when catalytic converters were introduced in exhaust systems.{{not verified in body|date=October 2015}} Even so, diesel exhausts continue to contain an array of inorganic and organic pollutants, in various classes, and in varying concentrations (see below), depending on fuel composition and engine running conditions.

The following are classes of chemical compounds that have been found in diesel exhaust.{{cite web |url=https://www.arb.ca.gov/toxics/dieseltac/de-fnds.htm |title=The Report on Diesel Exhaust |last=Board |first=California Air Resources |website=www.arb.ca.gov |access-date=2016-10-11 |quote=Diesel exhaust includes ... acetaldehyde; antimony compounds; arsenic; benzene; beryllium compounds; bis(2-ethylhexyl)phthalate; dioxins and dibenzofurans; formaldehyde; inorganic lead; mercury compounds; nickel; POM (including PAHs); and styrene.}}

The following are classes of specific chemicals that have been found in diesel exhaust.{{Verify source|date=October 2015}}{{update after|2015|10|22}}{{page needed|date=October 2015}}

^§ Includes all regioisomers of this aromatic compound. See ortho-, meta-, and para-isomer descriptions at each compound's article.

{{Expand section|with=a general introduction with citations that covers current international agreements and federal regulations in English-speaking countries|date=October 2015}}

{{Further|Emission standard|Non-road diesel engine#Emission standards}}

To rapidly reduce particulate matter from heavy-duty diesel engines in California, the California Air Resources Board created the Carl Moyer Memorial Air Quality Standards Attainment Program to provide funding for upgrading engines ahead of emissions regulations.{{cite web |publisher= Bay Area Air Quality Management District |url= http://www.baaqmd.gov/Divisions/Strategic-Incentives.aspx |title=Strategic Incentives Division}} In 2008, the California Air Resources Board also implemented the 2008 California Statewide Truck and Bus Rule which requires all heavy-duty diesel trucks and buses, with a few exceptions, that operate in California to either retrofit or replace engines in order to reduce diesel particulate matter.{{citation needed|date=October 2015}}

Unlike international shipping, which had a sulfur limit of 3.5% mass/mass outside ECA until 2020, when it reduced to 0.5% outside ECA, diesel for on road use and off-road (heavy equipment) has been limited throughout the EU since 2009.

Diesel and gasoline have been limited to 10 ppm sulfur since 2009 (for on-road vehicles) and 2011 (non-road vehicles). Mandatory specifications also apply to more than a dozen fuel parameters.{{Cite web|url=https://www.transportpolicy.net/standard/eu-fuels-diesel-and-gasoline/|title=EU: Fuels: Diesel and Gasoline {{!}} Transport Policy|language=en-US|access-date=2019-12-24}}

Emissions from diesel vehicles are more harmful than those from petrol vehicles.{{cite news|last1=Vidal|first1=John|title=Diesel fumes more damaging to health than petrol engines|url=https://www.theguardian.com/uk/2013/jan/27/diesel-engine-fumes-worse-petrol|access-date=5 June 2015|work=The Guardian|date=Jan 27, 2013}}{{Cite web |last=Dewan |first=Pandora |date=2024-02-23 |title=Breathing in diesel fumes is crippling our immune systems |url=https://www.newsweek.com/breathing-diesel-fumes-crippling-immune-system-1872785#:~:text=Previous%20research%20from%20the%20University,times%20more%20than%20electric%20cars. |access-date=2024-09-29 |website=Newsweek |language=en}}{{Cite web |title=The health costs of air pollution from cars and vans |url=https://www.globalactionplan.org.uk/news/the-health-costs-of-air-pollution-from-cars-and-vans |access-date=2024-09-29 |website=Global Action Plan |language=en |quote=the health damage associated with diesel vehicle emissions are …… at least 5 times greater than those associated with petrol vehicles.}} Diesel combustion exhaust is a source of atmospheric soot and fine particles, which is a component of the air pollution implicated in human cancer, heart and lung damage, and mental functioning. Moreover, diesel exhaust contains contaminants listed as carcinogenic for humans by the IARC (part of the World Health Organization of the United Nations), as present in their List of IARC Group 1 carcinogens.{{cite web |author=IARC |title=Diesel Engine Exhaust Carcinogenic |url=https://www.iarc.fr/en/media-centre/pr/2012/pdfs/pr213_E.pdf |access-date=June 12, 2012 |publisher=International Agency for Research on Cancer (IARC) |format=Press release |quote=After a week-long meeting of international experts, the International Agency for Research on Cancer (IARC), which is part of the World Health Organization (WHO), today classified diesel exhaust as probably carcinogenic to humans (Group 1), based on enough evidence that exposure is associated with an increased risk of lung cancer.}}

Diesel exhaust is a Group 1 carcinogen, which causes lung cancer and has a positive association with bladder cancer.{{cite web |date=June 12, 2012 |title=IARC: DIESEL ENGINE EXHAUST CARCINOGENIC |url=https://www.iarc.fr/en/media-centre/pr/2012/pdfs/pr213_E.pdf |access-date=August 14, 2016 |publisher=International Agency for Research on Cancer (IARC) |format=Press release |quote=The scientific evidence was reviewed thoroughly by the Working Group and overall it was concluded that there was sufficient evidence in humans for the carcinogenicity of diesel exhaust. The Working Group found that diesel exhaust is a cause of lung cancer (sufficient evidence) and also noted a positive association (limited evidence) with an increased risk of bladder cancer}}{{cite web |date=October 2, 2014 |title=Report on Carcinogens: Diesel Exhaust Particulates |url=https://ntp.niehs.nih.gov/ntp/roc/content/profiles/dieselexhaustparticulates.pdf |publisher=National Toxicology Program, Department of Health and Human Services |quote=Exposure to diesel exhaust particulates is reasonably anticipated to be a human carcinogen, based on limited evidence of carcinogenicity from studies in humans and supporting evidence from studies in experimental animals and mechanistic studies.}}{{cite web |date=2003-02-28 |title=Diesel engine exhaust; CASRN N.A. |url=https://cfpub.epa.gov/ncea/iris/iris_documents/documents/subst/0642_summary.pdf#nameddest=woe |publisher=U.S. Environmental Protection Agency |quote=Using U.S. EPA's revised draft 1999 Guidelines for Carcinogen Risk Assessment (U.S. EPA, 1999), diesel exhaust (DE) is likely to be carcinogenic to humans by inhalation from environmental exposures.}}{{cite journal |last1=Silverman |first1=Debra T. |last2=Samanic |first2=Claudine M. |last3=Lubin |first3=Jay H. |last4=Blair |first4=Aaron E. |last5=Stewart |first5=Patricia A. |last6=Vermeulen |first6=Roel |last7=Coble |first7=Joseph B. |last8=Rothman |first8=Nathaniel |last9=Schleiff |first9=Patricia L. |date=2012-06-06 |title=The Diesel Exhaust in Miners study: a nested case-control study of lung cancer and diesel exhaust |journal=Journal of the National Cancer Institute |volume=104 |issue=11 |pages=855–868 |doi=10.1093/jnci/djs034 |issn=1460-2105 |pmc=3369553 |pmid=22393209}}{{cite journal |last1=Attfield |first1=Michael D. |last2=Schleiff |first2=Patricia L. |last3=Lubin |first3=Jay H. |last4=Blair |first4=Aaron |last5=Stewart |first5=Patricia A. |last6=Vermeulen |first6=Roel |last7=Coble |first7=Joseph B. |last8=Silverman |first8=Debra T. |date=2012-06-06 |title=The Diesel Exhaust in Miners study: a cohort mortality study with emphasis on lung cancer |journal=Journal of the National Cancer Institute |volume=104 |issue=11 |pages=869–883 |doi=10.1093/jnci/djs035 |issn=1460-2105 |pmc=3373218 |pmid=22393207}} It contains several substances that are also listed individually as human carcinogens by the IARC.

A March 2025 report from the Centre for Research on Energy and Clean Air (CREA) reveals that the impact of diesel trucks is more harmful to public health than previously believed.{{Cite web |date=2025-03-25 |title=Heavy-duty harm |url=https://energyandcleanair.org/publication/heavy-duty-harm/ |access-date=2025-04-04 |website=Centre for Research on Energy and Clean Air |language=en-US}} CREA assessed the impacts of nitrogen oxide (NOX) emissions from medium and heavy-duty trucks manufactured and sold by Daimler, Traton, Volvo, and Paccar between 2014–2023. The analysis predicts that, through 2040, there could be 307,000 premature deaths, 217,000 new childhood asthma cases, 120 million lost workdays, and $1.4 trillion in economic costs linked to diesel truck emissions.

File:Diesel particulate matter monitors.png

Exposure to diesel exhaust and diesel particulate matter (DPM) is an occupational hazard to truckers, railroad workers, occupants of residential homes in the vicinity of a rail yard, and miners using diesel-powered equipment in underground mines. Adverse health effects have also been observed in the general population at ambient atmospheric particle concentrations well below the concentrations in occupational settings.

In March 2012, U.S. government scientists showed that underground miners exposed to high levels of diesel fumes have a threefold increased risk for contracting lung cancer compared with those exposed to low levels. The $11.5 million Diesel Exhaust in Miners Study (DEMS) followed 12,315 miners, controlling for key carcinogens such as cigarette smoke, radon, and asbestos. This allowed scientists to isolate the effects of diesel fumes.{{cite journal |last=Attfield|first=M. D.|author2=Schleiff, P. L.|author3=Lubin, J. H.|author4=Blair, A.|author5=Stewart, P. A.|author6=Vermeulen, R.|author7=Coble, J. B.|author8=Silverman, D. T.|title=The Diesel Exhaust in Miners Study: A Cohort Mortality Study With Emphasis on Lung Cancer|journal=JNCI Journal of the National Cancer Institute|date=5 March 2012|doi=10.1093/jnci/djs035|volume=104|issue=11|pages=869–883|pmid=22393207|pmc=3373218}}{{cite journal |last=Silverman|first=D. T. |author2=Samanic, C. M. |author3=Lubin, J. H. |author4=Blair, A. E. |author5=Stewart, P. A. |author6=Vermeulen, R. |author7=Coble, J. B. |author8=Rothman, N. |author9=Schleiff, P. L. |author10=Travis, W. D. |author11=Ziegler, R. G. |author12=Wacholder, S. |author13=Attfield, M. D.|title=The Diesel Exhaust in Miners Study: A Nested Case-Control Study of Lung Cancer and Diesel Exhaust|journal=JNCI Journal of the National Cancer Institute|date=5 March 2012|doi=10.1093/jnci/djs034 |volume=104 |issue=11 |pages=855–868 |pmid=22393209 |pmc=3369553}}

For over 10 years, concerns have been raised in the US regarding children's exposure to DPM as they ride diesel-powered school buses to and from school.{{cite web |url=http://www.nrdc.org/air/transportation/schoolbus/sbusinx.asp |title=No Breathing in the Aisles. Diesel Exhaust Inside School Buses |last1=Solomon |first1=Gina |last2=Campbell |first2=Todd |date= January 2001 |website=NRDC.org |publisher=Natural Resources Defense Council |access-date=19 October 2013}} In 2013, the Environmental Protection Agency (EPA) established the Clean School Bus USA initiative in an effort to unite private and public organizations in curbing student exposures.{{cite web |url=http://www.epa.gov/cleanschoolbus/csb-overview.htm |title=Clean School Bus |website=EPA.gov |publisher=United States Government |access-date=19 October 2013}}

File:Diesel-smoke.jpg

Diesel particulate matter (DPM), sometimes also called diesel exhaust particles (DEP), is the particulate component of diesel exhaust, which includes diesel soot and aerosols such as ash particulates, metallic abrasion particles, sulfates, and silicates. When released into the atmosphere, DPM can take the form of individual particles or chain aggregates, with most in the invisible sub-micrometer range of 100 nanometers, also known as ultrafine particles (UFP) or PM0.1.

The main particulate fraction of diesel exhaust consists of fine particles. Because of their small size, inhaled particles may easily penetrate deep into the lungs. The polycyclic aromatic hydrocarbons (PAHs) in the exhaust stimulate nerves in the lungs, causing reflex coughing, wheezing and shortness of breath.{{Cite web|url=https://www.sciencedaily.com/releases/2017/05/170522080928.htm|title=How diesel fumes could cause 'flare up' of respiratory symptoms|website=ScienceDaily|accessdate=25 July 2023}} The rough surfaces of these particles makes it easy for them to bind with other toxins in the environment, thus increasing the hazards of particle inhalation.{{verify source|date=October 2015}}

A study of particulate matter (PM) emissions from transit buses running on ULSD and a mixture of biodiesel and conventional diesel (B20) was reported by Omidvarborna and coworkers, where they concluded that PM emissions appeared lower in cases of mixed diesel/biodiesel use, where they were dependent on the engine model, cold and hot idle modes, and fuel type, and that heavy metals in PM emitted during hot idling were greater than those from cold idling; reasons for PM reduction in biodiesel emissions were suggested to result from the oxygenated structure of biodiesel fuel, as well as arising from changes in technology (including the use of a catalytic converter in this test system).{{cite journal |author1=Omidvarbornaa, Hamid |author2=Kumara, Ashok |author3=Kim, Dong-Shik |year= 2014 |title=Characterization of Particulate Matter Emitted from Transit Buses Fueled with B20 in Idle Modes|journal=Journal of Environmental Chemical Engineering|volume=2|issue=4, December|pages=2335–2342|doi=10.1016/j.jece.2014.09.020}} Other studies concluded that while in certain specific cases (i.e. low loads, more saturated feedstocks, ...), NO_x emissions can be lower than with diesel fuel, in most cases NO_x emissions are higher, and the NO_x emissions even go up as more biofuel is mixed in. Pure biodiesel (B100) even ends up having 10-30% more NO_x emissions compared to regular diesel fuel.{{Cite web|url=https://dieselnet.com/tech/fuel_biodiesel_emissions.php|title=Effects of Biodiesel on Emissions|website=dieselnet.com|accessdate=25 July 2023}}

Exposures have been linked with acute short-term symptoms such as headache, dizziness, light-headedness, nausea, coughing, difficult or labored breathing, tightness of chest, and irritation of the eyes, nose, and throat.{{cite web|url=https://toxtown.nlm.nih.gov/text_version/chemicals.php?id=11|title=Tox Town - Diesel - Toxic chemicals and environmental health risks where you live and work - Text Version|website=toxtown.nlm.nih.gov|language=en|access-date=2017-02-04|archive-date=2017-02-04|archive-url=https://web.archive.org/web/20170204170613/https://toxtown.nlm.nih.gov/text_version/chemicals.php?id=11|url-status=dead}} Long-term exposures can lead to chronic, more serious health problems such as cardiovascular disease, cardiopulmonary disease, and lung cancer.{{cite web |url=https://www.bbc.co.uk/news/health-18415532 |title=Diesel exhausts do cause cancer, says WHO - BBC News |publisher=Bbc.co.uk |date=2012-06-12 |access-date=2015-10-22}}{{cite web |url=http://www.medpagetoday.com/HematologyOncology/OtherCancers/33226 |title=WHO: Diesel Exhaust Causes Lung Cancer |publisher=Medpage Today |date= 2012-06-12|access-date=2015-10-22}}{{cite journal|title=Air pollution and lung cancer incidence in 17 European cohorts: prospective analyses from the European Study of Cohorts for Air Pollution Effects (ESCAPE)|journal=The Lancet Oncology|date=July 10, 2013|doi=10.1016/S1470-2045(13)70279-1|url=http://www.thelancet.com/journals/lanonc/article/PIIS1470-2045%2813%2970279-1/abstract|access-date=July 10, 2013|author=Ole Raaschou-Nielsen|quote=Particulate matter air pollution contributes to lung cancer incidence in Europe.|pmid=23849838|volume=14|issue=9|pages=813–22|display-authors=etal|archive-date=July 15, 2013|archive-url=https://web.archive.org/web/20130715195326/http://www.thelancet.com/journals/lanonc/article/PIIS1470-2045(13)70279-1/abstract|url-status=dead}}

Elemental carbon attributable to traffic was significantly associated with wheezing at age 1 and persistent wheezing at age 3 in the Cincinnati Childhood Allergy and Air Pollution Study birth cohort study.{{cite journal |last1= Bernstein |first1= David I. |date= Jul 2012 |title= Diesel Exhaust Exposure, Wheezing and Sneezing |journal= Allergy Asthma Immunol Res |volume= 4 |issue= 4 |pages= 178–183 |doi= 10.4168/aair.2012.4.4.178 |pmc=3378923 |pmid=22754710}} Ambient traffic-related air pollution is associated with decreased cognitive function in older men.{{cite journal |last1=Power |last2=Weisskopf |last3=Alexeeff |last4=Coull |last5=Spiro |last6=Schwartz |url=http://ehp03.niehs.nih.gov/article/info:doi/10.1289/ehp.100276 |archive-url=https://archive.today/20141121192550/http://ehp03.niehs.nih.gov/article/info:doi/10.1289/ehp.100276 |url-status=dead |archive-date=2014-11-21 |pmid=21172758 |doi=10.1289/ehp.1002767 |pmc=3094421 |volume=119 |issue=5 |title=Traffic-related air pollution and cognitive function in a cohort of older men |date=May 2011 |pages=682–7 |journal=Environmental Health Perspectives|bibcode=2011EnvHP.119..682P }}

The study of nanoparticles and nanotoxicology is in its infancy, and health effects from nanoparticles produced by all types of diesel engines are still being uncovered. It is clear, that diesel health detriments of fine particle emissions are severe and pervasive. Although one study found no significant evidence that short-term exposure to diesel exhaust results in adverse extrapulmonary effects, effects that are correlated with an increase in cardiovascular disease,{{cite web |url=http://www.blackwellpublishing.com/isth2005/abstract.asp?id=46528 |title= Congress of the International Society on Thrombosis and Haemostasis|website=www.blackwellpublishing.com |archive-url=https://web.archive.org/web/20090130091058/http://www.blackwellpublishing.com/isth2005/abstract.asp?id=46528 |archive-date=January 30, 2009}} a 2011 study in The Lancet concluded that traffic exposure is the single most serious preventable trigger of heart attack in the general public, as the cause of 7.4% of all attacks.{{cite journal |year= 2011 |title= Public health importance of triggers of myocardial infarction: comparative risk assessment |url= http://www.thelancet.com/journals/lancet/article/PIIS0140-6736%2810%2962296-9/abstract |journal= The Lancet |volume= 377 |issue= 9767 |pages= 732–740 |doi= 10.1016/S0140-6736(10)62296-9 |pmid=21353301 |last1= Nawrot |first1= TS |last2= Perez |first2= L |last3= Künzli |first3= N |last4= Munters |first4= E |last5= Nemery |first5= B|s2cid= 20168936 |url-access= subscription }}: "Taking into account the OR and the prevalences of exposure, the highest PAF was estimated for traffic exposure (7.4%)... "

:"... [O]dds ratios and frequencies of each trigger were used to compute population-attributable fractions (PAFs), which estimate the proportion of cases that could be avoided if a risk factor were removed. PAFs depend not only on the risk factor strength at the individual level but also on its frequency in the community. ... [T]he exposure prevalence for triggers in the relevant control time window ranged from 0.04% for cocaine use to 100% for air pollution. ... Taking into account the OR and the prevalences of exposure, the highest PAF was estimated for traffic exposure (7.4%) ... It is not possible to tell how much of this effect is due to the stress of being in traffic and how much is due to exposure to exhaust.{{citation needed|date=March 2012}}

Since the study of the detrimental health effects of nanoparticles (nanotoxicology) is still in its infancy, and the nature and extent of negative health impacts from diesel exhaust continues to be discovered, it remains controversial whether the public health impact of diesels is higher than that of petrol-fueled vehicles.{{cite journal |last1= Int Panis |first1= L |title= Diesel or Petrol ? An environmental comparison hampered by uncertainty |journal= Mitteilungen Institut für Verbrennungskraftmaschinen und Thermodynamik, Publisher: Institut für Verbrennungskraftmaschinen und Thermodynamik |volume= 81 |issue= 1 |pages= 48–54 |year= 2002 |url= https://www.researchgate.net/publication/232070437 |last2= Rabl |last3= De Nocker |first3= L |last4= Torfs |first4= R}}

The types and quantities of nanoparticles can vary according to operating temperatures and pressures, the presence of an open flame, fundamental fuel type and fuel mixture, and even atmospheric mixtures. As such, the resulting types of nanoparticles from different engine technologies and even different fuels are not necessarily comparable. One study has shown that 95% of the volatile component of diesel nanoparticles is unburned lubricating oil.{{cite journal |title=On-line measurements of diesel nanoparticle composition and volatility |doi=10.1016/S1352-2310(02)01017-8 |volume=37 |issue=9–10 |journal=Atmospheric Environment |pages=1199–1210|year=2003 |last1=Sakurai |first1=Hiromu |last2=Tobias |first2=Herbert J. |last3=Park |first3=Kihong |last4=Zarling |first4=Darrick |last5=Docherty |first5=Kenneth S. |last6=Kittelson |first6=David B. |last7=McMurry |first7=Peter H. |last8=Ziemann |first8=Paul J.|bibcode=2003AtmEn..37.1199S }} Long-term effects still need to be further clarified, as well as the effects on susceptible groups of people with cardiopulmonary diseases.

Diesel engines can produce black soot (or more specifically diesel particulate matter) from their exhaust. The black smoke consists of carbon compounds that have not burned because of local low temperatures where the fuel is not fully atomized. These local low temperatures occur at the cylinder walls, and at the surface of large droplets of fuel. At these areas where it is relatively cold, the mixture is rich (contrary to the overall mixture which is lean). The rich mixture has less air to burn and some of the fuel turns into a carbon deposit. Modern car engines use a diesel particulate filter (DPF) to capture carbon particles and then intermittently burn them using extra fuel injected directly into the filter. This prevents carbon buildup at the expense of wasting a small quantity of fuel.{{Cn|date=September 2024}}

When starting from cold, the engine's combustion efficiency is reduced because the cold engine block draws heat out of the cylinder in the compression stroke.{{Cite web |date=2023-04-12 |title=Understanding Diesel Engine Cold Start: Causes, Effects, And Solutions {{!}} FuelFlowPro |url=https://fuelflowpro.com/diesel-engine-cold-start/ |access-date=2024-04-12 |website=fuelflowpro.com |language=en-US}} The result is that fuel is not burned fully, resulting in blue and white smoke and lower power outputs until the engine has warmed. This is especially the case with indirect injection engines, which are less thermally efficient. With electronic injection, the timing and length of the injection sequence can be altered to compensate for this. Older engines with mechanical injection can have mechanical and hydraulic governor control to alter the timing, and multi-phase electrically controlled glow plugs, that stay on for a period after start-up to ensure clean combustion; the plugs are automatically switched to a lower power to prevent their burning out.{{Cn|date=September 2024}}

Wärtsilä states that there are two ways of forming smoke, on large diesel engines, one being fuel hitting metal and not having time to burn off. The other being, when too much fuel is in the combustion chamber.

Wärtsilä have tested an engine and compared smoke-output, when using conventional fuel system and common rail fuel system, the result shows improvement on all operation conditions when using the common rail system.{{Cite book|title=Pounder's marine diesel engines and gas turbines|date=2009|publisher=Elsevier/Butterworth-Heinemann|others=Woodyard, D. F. (Douglas F.)|isbn=978-0-08-094361-9|edition=9th|location=Amsterdam|pages=84, 85|oclc=500844605}}

{{Missing information|section|NOx emissions. They affect smog, ozone, acid rain, and pulmonary problems|date=January 2017}}

Experiments in 2013 showed that diesel exhaust impaired bees' ability to detect the scent of oilseed rape flowers.{{cite journal |title=Diesel exhaust rapidly degrades floral odours used by honeybees : Scientific Reports |journal=Scientific Reports |volume=3 |pages=2779 |doi=10.1038/srep02779 |pmid=24091789 |date= 2013-10-03|last1=Poppy |first1=Guy M. |last2=Newman |first2=Tracey A. |last3=Farthing |first3=Emily |last4=Lusebrink |first4=Inka |last5=Girling |first5=Robbie D. |pmc=3789406}}

Emissions from diesel engines contribute to the production of ground-level ozone, which can damage crops, trees, and other vegetation. Diesel exhaust also contributes to the formation of acid rain, which affects soil, lakes, and streams, and can enter the human food chain via water, produce, meat, and fish.{{Cite journal |last1=Reşitoğlu |first1=İbrahim Aslan |last2=Altinişik |first2=Kemal |last3=Keskin |first3=Ali |date=2015-01-01 |title=The pollutant emissions from diesel-engine vehicles and exhaust aftertreatment systems |journal=Clean Technologies and Environmental Policy |language=en |volume=17 |issue=1 |pages=15–27 |doi=10.1007/s10098-014-0793-9 |issn=1618-9558|doi-access=free |bibcode=2015CTEP...17...15R }}{{Creative Commons text attribution notice|cc=by4|from this source=yes}}

Diesel exhaust plays a role in climate change. Reducing greenhouse gas (GHG) emissions from diesel engines through improved fuel economy or idle reduction strategies can help address climate change, improve energy security, and strengthen the economies of countries. While diesel fuel contains slightly more carbon (2.68 kg CO₂/litre) than petrol (2.31 kg CO₂/litre), overall, the CO₂ emissions of a diesel car tend to be lower due to higher efficiency. In use, on average, this equates to around 200 g CO₂/km for petrol and 120 g CO₂/km for diesel.

{{More science citations needed|section|date=October 2015}}

{{Third-party|section|date=October 2015}}

With emission standards tightening, diesel engines are having to become more efficient and have fewer pollutants in their exhaust.{{citation needed|date=October 2015}} Moreover, in recent years the United States, Europe, and Japan have extended emissions control regulations from covering on-road vehicles to include farm vehicles and locomotives, marine vessels, and stationary generator applications.{{cite journal |author1=Guan, B|author2=Zhan, R|author3=Lin, H|author4=Huang, Z.|date=2014|title=Review of state of the art technologies of selective catalytic reduction of NOx from diesel engine exhaust|journal=Applied Thermal Engineering|volume=66|issue=1–2|pages=395–414|doi=10.1016/j.applthermaleng.2014.02.021}} {{subscription required|date=October 2015}} Changing to a different fuel (i.e. dimethyl ether, and other bioethers as diethyl ether){{Cite web|url=https://www.researchgate.net/publication/258176856|title=Simultaneous reduction of NOx and smoke from a direct-injection diesel engine with exhaust gas recirculation and diethyl ether | Request PDF|accessdate=25 July 2023}} tends to be a very effective means to reduce pollutants such as NO_x and CO. When running on dimethyl ether (DME) for instance, particulate matter emissions are near-nonexistent, and the use of diesel particulate filters could even be omitted.{{Cite web|url=https://afdc.energy.gov/fuels/emerging_dme.html|title=Alternative Fuels Data Center: Dimethyl Ether|website=afdc.energy.gov|accessdate=25 July 2023}} Also, given that DME can be made from animal, food, and agricultural waste, it can even be carbon-neutral (unlike regular diesel). Mixing in bio ether (or other fuels such as hydrogen){{Cite journal|title=Effect of hydrogen-diesel fuel co-combustion on exhaust emissions with verification using an in–cylinder gas sampling technique|first1=Midhat|last1=Talibi|first2=Paul|last2=Hellier|first3=Ramanarayanan|last3=Balachandran|first4=Nicos|last4=Ladommatos|date=12 September 2014|journal=International Journal of Hydrogen Energy|volume=39|issue=27|pages=15088–15102|doi=10.1016/j.ijhydene.2014.07.039|doi-access=free|bibcode=2014IJHE...3915088T }}{{Cite web|url=https://edeninnovations.com/innovations/|title=Innovations | Eden Innovations|date=28 June 2016|accessdate=25 July 2023}} into conventional diesel also tends to have a beneficial effect on the pollutants that are emitted. In addition to changing the fuel, US engineers have also come up with two other principles and distinct systems to all on-market products that meet the U.S. 2010 emissions criteria,{{citation needed|date=October 2015}}{{update after|2015|10|22}} selective non-catalytic reduction (SNCR), and exhaust gas recirculation (EGR). Both are in the exhaust system of diesel engines, and are further designed to promote efficiency.{{citation needed|date=October 2015}}

Selective catalytic reduction (SCR) injects a reductant such as ammonia or urea — the latter aqueous, where it is known as diesel exhaust fluid (DEF) — into the exhaust of a diesel engine to convert nitrogen oxides (NO_x) into gaseous nitrogen and water. SNCR systems have been prototyped that reduce 90% of the NO_x in the exhaust system, with commercialized systems being somewhat lower.{{citation needed|date=October 2015}} SCR systems do not necessarily need particulate matter (PM) filters; when SNCR and PM filters are combined, some engines have been shown to be 3-5% more fuel efficient.{{citation needed|date=October 2015}} A disadvantage of the SCR system, in addition to added upfront development cost (which can be offset by compliance and improved performance),{{citation needed|date=October 2015}} is the need to refill the reductant, the periodicity of which varies with the miles driven, load factors, and the hours used.{{cite web |url=http://www.dieselforum.org/about-clean-diesel/what-is-scr- |title=What is SCR? | Diesel Technology Forum |publisher=Dieselforum.org |date=2010-01-01 |access-date=2015-10-22 |archive-date=2015-10-08 |archive-url=https://web.archive.org/web/20151008073809/http://www.dieselforum.org/about-clean-diesel/what-is-scr- |url-status=dead }}{{full citation needed|date=October 2015}}{{better source needed|date=October 2015}}{{Third-party inline|date=October 2015}} The SNCR system is not as efficient at higher revolutions per minute (rpm).{{citation needed|date=October 2015}} SCR is being optimized to have higher efficiency with broader temperatures, to be more durable, and to meet other commercial needs.

{{Main|Exhaust gas recirculation#In diesel engines}}

{{See also|Water injection (engine)}}

Exhaust gas recirculation (EGR), on diesel engines, can be used to achieve a richer fuel to air mixture and a lower peak combustion temperature. Both effects reduce NO_x emissions, but can negatively impact efficiency and the production of soot particles. The richer mix is achieved by displacing some of the intake air, but is still lean compared to petrol engines, which approach the stoichiometric ideal. The lower peak temperature is achieved by a heat exchanger that removes heat before re-entering the engine, and works due to the exhaust gases' higher specific heat capacity than air. With the greater soot production, EGR is often combined with a particulate matter (PM) filter in the exhaust.Bennett, Sean (2004). Medium/Heavy Duty Truck Engines, Fuel & Computerized Management Systems 2nd Edition, {{ISBN|1401814999}}.{{full citation needed|date=October 2015}}{{page needed|date=October 2015}}{{full citation needed|date=October 2015}} In turbocharged engines, EGR needs a controlled pressure differential across the exhaust manifold and intake manifold, which can be met by such engineering as use of a variable geometry turbocharger,{{citation needed|date=October 2015}} which has inlet guide vanes on the turbine to build exhaust backpressure in the exhaust manifold directing exhaust gas to the intake manifold. It also requires additional external piping and valving, and so requires additional maintenance.{{citation needed|date=October 2015}}{{cite book |chapter-url=http://papers.sae.org/2013-01-2741/|access-date=2016-06-17|doi=10.4271/2013-01-2741|title=SAE Technical Paper Series|volume=1|year=2013|last1=Goswami|first1=Angshuman|last2=Barman|first2=Jyotirmoy|last3=Rajput|first3=Karan|last4=Lakhlani|first4=Hardik N.|chapter=Behaviour Study of Particulate Matter and Chemical Composition with Different Combustion Strategies}}

John Deere, the farm equipment manufacturer, is implementing a combined SCR-EGR design, in a 9-liter "inline 6" diesel engine that involves both system types, a PM filter and additional oxidation catalyst technologies.{{cite web |url=http://www.deere.com/en_US/docs/pdfs/emissions/large_engine_technology_final.pdf |title=Technology to Reduce Emissions in Large Engines |publisher=Deere.com |access-date=2015-10-22}}{{better source needed|date=October 2015}}{{Third-party inline|date=October 2015}} The combined system incorporates two turbochargers, the first on the exhaust manifold, with variable geometry and containing the EGR system; and a second a fixed geometry turbocharger. Recirculated exhaust gas and the compressed air from the turbochargers have separate coolers, and air merges before entering the intake manifold, and all subsystems are controlled by a central engine control unit that optimizes minimization of pollutants released in the exhaust gas.

{{Expand section|with=the further sourced content on such things as diesel particulate filters and use of alternate fuels, placed here with citations;: significant sentences only with citations, no more top-of-the-head content dumping|small=no|date=October 2015}}

A new technology being tested in 2016 has been created by Air Ink which collects carbon particles using a "Kaalink" cylindrical device that is retrofitted into a vehicle's exhaust system, after processing to remove heavy metals and carcinogens, the company plans to use the carbon to make ink.{{cite web |url=http://www.iflscience.com/environment/these-pens-use-ink-made-out-of-recycled-air-pollution/ |title=These Pens Use Ink Made Out Of Recycled Air Pollution |publisher=IFL Science |date=17 August 2016}}

In India, the Chakr Dual Fuel Kit retrofits a diesel generator set to operate on a mixture of both gas and diesel, with 70% natural gas and 30% fossil fuel.{{Cite web |title=Chakr Innovation launches Dual Fuel Kit to provide first Turnkey Solution to DG set ban in Delhi-NCR |url=https://news.webindia123.com/news/articles/Business/20221012/3992365.html |access-date=2023-05-30 |website=news.webindia123.com}}

There has been research into ways that troops in deserts can recover drinkable water from their vehicles' exhaust gases.{{Cite web|url=https://apps.dtic.mil/dtic/tr/fulltext/u2/a576788.pdf|title=Article title|accessdate=25 July 2023}}{{Cite web|url=https://patents.google.com/patent/WO2002059043A2/en|title=Recovery and purification of water from the exhaust gases of int ernal combustion engines|accessdate=25 July 2023}}{{cite book |chapter-url=https://www.sae.org/publications/technical-papers/content/2015-01-2806/ |chapter = Extraction of Liquid Water from the Exhaust of a Diesel Engine|doi = 10.4271/2015-01-2806|title = SAE Technical Paper Series|year = 2015|last1 = Barros|first1 = Sam|last2 = Atkinson|first2 = William|last3 = Piduru|first3 = Naag|volume = 1}}{{Cite web|url=https://patents.google.com/patent/US4656831A/en|title = Apparatus and method of recovering water from engine exhaust gases}}{{cite web |url=https://science.energy.gov/news/featured-articles/2011/06-23-11/ |title = Newsroom {{!}} Department of Energy}}

• Carl Moyer Memorial Air Quality Standards Attainment Program
• List of IARC Group 1 carcinogens
• List of IARC Group 2A carcinogens
• List of IARC Group 2B carcinogens
• List of IARC Group 3 possible carcinogens
• National Emissions Standards for Hazardous Air Pollutants
• Rolling coal - intentional creation of conspicuous excessive diesel exhaust
• Vehicle emissions control
• Volkswagen emissions scandal

{{Reflist|30em}}

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