Pollutants and Health

Pollutants emitted from burning conventional and alternative fuels fall into two categories: criteria and non-criteria pollutants.

The Clean Air Act (CAA) requires the U.S. Environmental Protection Agency (EPA) to set standards for six common pollutants. These standards are based on certain health and environmental "criteria"—hence the name. The six criteria pollutants are:

Although they are harmful to health and air quality, non-criteria pollutants are not regulated by CAA or EPA. They include:

For more information on pollutants, read the following detailed descriptions.

Criteria Pollutants

Carbon Monoxide (CO)

At low levels, CO can exacerbate cardiovascular disease. At high levels, it can damage the central nervous system. At extremely high levels, CO is poisonous and can cause death. In the United States, 56% of CO (up to 95% in cities) is emitted by on-road vehicles.

Oxides of Nitrogen (NOx)

NOx can cause damage to respiratory airways. The high diversity, mobility, and reactivity of NOx enable this pollutant to contribute to numerous environmental problems such as acid rain, climate change, deteriorated water quality, ground-level ozone, air toxics, and particulate matter. Approximately 55% of human-made NOx emissions come from motor vehicles.

Particulate Matter (PM)

PM can aggravate asthma, emphysema, bronchitis, heart disease, and lung disease. It is a carrier of many toxic compounds, contributes to haze, pollutes fresh and coastal waters, and contaminates farmland and natural ecosystems. PM is emitted directly from vehicles (especially diesel) and is formed through the atmospheric reactions of NOx and oxides of sulfur (SOx).

Ozone (O3)

Commonly referred to as smog, ozone is a powerful oxidant that can reduce lung function, aggravate asthma, increase chances for respiratory illness, and lead to permanent lung damage. It can also damage plant tissue, kill plants, and reduce farm yields.

Ozone is not a direct vehicle emission; it is formed in the air through reactions of NOx, volatile organic compounds (VOCs), and atmospheric air in the presence of sunlight. Generally, O3 formation in urban areas is more VOC-sensitive, while it is more NOx-sensitive in rural areas. Not all volatile organic compounds are equal when it comes to forming ozone. The following two metrics compare the potential between different VOCs.

  • Ozone Forming Potential: This metric, expressed in milligrams of ozone per mile driven, effectively weights the amount of VOCs that are emitted according to their propensity to form O3. For fuel-comparison purposes, this is the most important metric.

  • Specific Reactivity: This metric is expressed in terms of milligrams of ozone per milligram of nonmethane organic gases (a subset of volatile organic compounds). Therefore, it is a measure of the potency of the VOCs and does not take into consideration the quantity of VOCs emitted per mile of driving.

Oxides of Sulfur (SOx)

SOx can aggravate respiratory illness and heart and lung disease. It forms PM and is a primary cause of acid rain. Most vehicle emissions studies do not consider SOx because vehicles contribute such a small portion of the total amount emitted by human activity. In 2006, ultra-low sulfur diesel regulations reduced the contribution of SOx even further. Although SOx is not a major concern for conventional and alternative fuel vehicles, it is a concern for electric vehicles since electricity generation is the largest source of SOx.


Lead, which causes brain and nervous system damage in children, has been successfully removed from U.S. gasoline. Since no U.S. fuels contain lead, it is not considered in our emissions comparisons for alternative fuels.

Non-Criteria Pollutants

Some of the most harmful pollutants are not specifically regulated in the Clean Air Act. Scientists have documented negative effects of these emissions pollutants and grouped them into multiple overlapping categories. The following categories relevant to this web section are highlighted in the following illustration and descriptions.

Graphic showing five overlapping circles. The circles correspond to types of non-criteria pollutants: organic gases, air toxics, total hydrocarbons (THC), methane, and photoreactive volatile organic compounds (VOCs). The organic gases and THC circles overlap with each other and all other circles. Each of the other three circles (air toxics, methane, and VOCs) overlaps only with the organic gases and THC circles. The area where organic gases and THC overlap is labeled gaseous hydrocarbons (HC); the THC area outside of the gaseous HC area is labeled liquids and solids. The area where organic gases and air toxics overlap (and do not overlap with other circles) is labeled aldehydes.

Total Organic Gases (TOG)

Most of the non-criteria pollutants are TOGs. This broad category includes gases at atmospheric pressure and ambient temperatures. Various subsets of TOG cause headaches, dizziness, upper respiratory tract irritation, nausea, and cancer.

Volatile Organic Compounds

VOCs are defined in several ways. In the broadest sense, they are organic gases. EPA defines them as such and adds that they are photoreactive (meaning that they react to produce ozone when in the sunlight).

Total Hydrocarbons (THCs)

Hydrocarbons are a group of compounds comprised of hydrogen and carbon. Unlike toxic organic compounds, they do not have to be gaseous at room temperature. Most TOGs in vehicle emissions are hydrocarbons except for those containing nonhydrogen or carbon atoms as a fundamental part of their structure (not just a contaminant). Oxygenated hydrocarbons, such as alcohols and aldehydes, are not considered THCs. Therefore, the metric "hydrocarbon equivalent" was devised to include these compounds.


Methane is an organic gas and a hydrocarbon (it's also a VOC when used in a broader definition). It is neither photoreactive nor toxic, so it is often omitted from these three categories. This results in the "non-methane" (NM) counterparts (known as NMOG, NMHC, NMHCE, and NMVOC). Methane is also a greenhouse gas (GHG) that is 23 times more potent in warming the atmosphere than carbon dioxide.

Air Toxics

Air toxics are air pollutants that cause adverse health effects. EPA's Mobile Source Air Toxics webpage is a good source of information on these compounds. Of the 1,162 known toxic compounds emitted from vehicles, the following four compounds are the focus of most emissions studies.

  • 1,3-Butadiene: This compound is classified as a probable human carcinogen, but EPA is re-evaluating it based on studies associating it with leukemia. It is correlated to cardiovascular disease.

  • Formaldehyde: This compound is a probable human carcinogen with studies associating it to lung and airway cancer.

  • Benzene: This compound is a known human carcinogen that increases the incidence of leukemia. It also causes blood disorders in humans and is linked to impaired fertility in women. Benzene also has adverse effects on the development of animal fetuses. Both motor vehicle exhaust and evaporative gasoline emissions are major sources of benzene.

  • Acetaldehyde. This compound is a probable human carcinogen. Long-term exposure produces symptoms similar to those of alcoholism. Acetaldehyde has many sources, including wood burning, which is the biggest contributor.

Primary Toxics and the Risk Factors
Compound Normalized 1999 EPA Inhalation Risk Factor*
1,3-Butadiene 100.0
Formaldehyde 4.6
Benzene 3.0
Acetaldehyde 0.8

*Risk ratings are based on the concentration needed to cause adverse health effects.

Greenhouse Gases

Greenhouse gases are a global pollutant, so this web section assesses them on a life cycle basis. Carbon dioxide (CO2) is by far the most abundant GHG in the transportation sector. However, fluorinated refrigerants used in mobile air conditioners are also major contributors due to their high global warming potential (GWP). The breakdown of greenhouse gas emissions from the U.S. transportation sector, weighted by GWP, is shown in table below.

GWP-Weighted GHGs Emitted by the U.S. Transportation Sector
Greenhouse Gas CO2 Fluorinated Refrigerants* N2O CH4 Other
100-Year GWP 1 8100, 1300 296 23
% of Total 88.40% 8.90% 2.00% 0.20% 0.50%

*The fluorinated refrigerants used in U.S. mobile air conditioning is CFC-12 (GWP 8100) for model years 1991 and earlier, HFC-134a (GWP 1300) for model years 1995 and later, and a mix from 1992-1994 model year vehicles.
Source: IPCC Assessment Report, Chapter 5 - Transport and its Infrastructure