Diesel exhaust contains a number of pollutants that can be harmful to public health and the environment, either alone or in combination with other substances. These include fine particulate matter, hydrocarbons, nitrogen oxides, carbon monoxide and toxic air contaminants known as hazardous air pollutants.

  • Fine Particles (PM2.5) are a mixture of solid particles and liquid droplets in the air. More than 90 percent of the particulates found in diesel exhaust are fine particles. Nitrogen oxides and sulfur dioxides are major sources of fine particles. Because of their small size, fine particles can be deposited deep in the lungs, where they can cause respiratory and cardiovascular problems, particularly among children and the elderly.  Particles are also a major cause of poor visibility and haze in many parts of the United States.  Learn more about particle pollution .
  • Hydrocarbons (HC) are formed by incomplete fuel combustion. When combined with NOx in the presence of sunlight, HC's produce ground-level ozone or "smog," which can irritate eyes, damage lungs and aggravate respiratory problems. Symptoms include coughing, shortness of breath and decreased lung function. Many hydrocarbons are also considered hazardous air pollutants.   
  • Nitrogen Dioxide and Other Oxides of Nitrogen (NO2 and NOx) are by-products of fuel combustion and contribute to the formation of ground-level ozone or "smog." Health effects include coughing, shortness of breath and decreased lung function.  Nitrogen oxides contribute to acid rain and the acidification of lakes and soils. They can also affect aquatic ecosystems by providing too many nutrients to aquatic plant life, which reduces dissolved oxygen levels and can ultimately harm the ecosystem. In addition, nitrogen oxides and hydrocarbons contribute to the formation of ground-level ozone or "smog," which is highly corrosive and damages crops and forests. Learn more nitrogen dioxide.
  • Carbon Monoxide (CO) is formed by incomplete fuel combustion. Carbon monoxide reduces the flow of oxygen in the bloodstream and is of particular concern to people with heart disease.  Learn more about carbon monoxide .
  • Hazardous Air Pollutants (HAPs) are substances listed by the U.S. Environmental Protection Agency (EPA) as air toxics. Diesel exhaust contains 40 of these substances, including 15 known or probable human carcinogens.  Of greatest concern are acetaldehyde, benzene, 1,3-butadiene, formaldehyde, and polycyclic aromatic hydrocarbons (PAHs). Learn more about air toxics .