Diesel exhaust is a complex mixture of gases and fine particles. The primary pollutants emitted from diesel engines include:

• Particulate matter (PM)
• Carbon monoxide (CO)
• Nitrogen oxides (NOx)
• Hydrocarbons (HC)
• Volatile organic compounds (VOCs)
• Other chemicals that are classified as “hazardous air pollutants” under The Clean Air Act

Health studies show that exposure to diesel exhaust primarily affects the respiratory system and worsens asthma, allergies, bronchitis, and lung function. There is some evidence that diesel exhaust exposure can increase the risk of heart problems, premature death, and lung cancer.

Particulate Matter (PM)
Particulate matter is the term for solid or liquid particles.  Some particles are large or dark enough to be seen as soot or smoke, but most are fine particulate matter. Fine particulate matter is composed of very small objects found in the air, including dust, dirt, soot, smoke, and liquid droplets. Ninety percent of diesel particulate matter is fine - more commonly referred to as PM2.5 (less than 2.5 microns in diameter).

Particulate matter can travel deep into the lungs where it can aggravate asthma, chronic bronchitis, emphysema, and other lung conditions. Our respiratory system filters out larger particles, but smaller particles get trapped in the lungs, while the smallest are so tiny they pass through the lungs into the blood stream. Particles may trigger or cause significant health problems, such as:

• Coughing and difficult or painful breathing
• Aggravated asthma, bronchitis, emphysema
• Decreased lung function
• Weakening of the heart, heart attacks
• Premature death

Carbon Monoxide (CO)
Carbon monoxide is a colorless, odorless, poisonous gas produced by the incomplete burning of solid, liquid, and gaseous fuels. The main source of carbon monoxide in our air is vehicle emissions.

Nitrogen Oxides (NOx)
Oxides of nitrogen, or NOx, is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. Many of the nitrogen oxides are colorless and odorless. However, one common pollutant, nitrogen dioxide (NO2) along with particles in the air can often be seen as a reddish-brown layer over many urban areas. Nitrogen oxides form when fuel is burned at high temperatures, as in a combustion process. The primary manmade sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels. NOx can also be formed naturally.

Hydrocarbons (HC)
Hydrocarbons are chemical compounds that contain hydrogen and carbon. Most motor vehicles and engines are powered by hydrocarbon-based fuels such as gasoline and diesel. Hydrocarbon pollution results when unburned or partially burned fuel is emitted from the engine as exhaust, and also when fuel evaporates directly into the atmosphere. Hydrocarbons include many toxic compounds that cause cancer and other adverse health effects. Hydrocarbons also react with nitrogen oxides in the presence of sunlight to form ozone. Hydrocarbons, which may take the form of gases, tiny particles, or droplets, come from a great variety of industrial and natural processes. In typical urban areas, a very significant fraction comes from cars, buses, trucks, and nonroad mobile sources such as construction vehicles and boats.

Volatile Organic Compounds (VOCs)
Volatile organic compounds are emitted from a variety of sources, including motor vehicles, chemical plants, refineries, factories, consumer and commercial products, and other industrial sources. Volatile organic compounds also are emitted by natural sources such as vegetation. Hydrocarbons (HC) are a large subset of VOC, and to reduce mobile source VOC levels there are maximum emissions limits for hydrocarbon as well as particulate matter.

Greenhouse Gases
Some greenhouse gases occur naturally in the atmosphere, while others result from human activities. Naturally occurring greenhouse gases include carbon dioxide, methane, nitrous oxide, ozone, and water vapor. Certain human activities add to the levels of most of these naturally occurring gases:

• Carbon dioxide (CO₂)
  Carbon Dioxide is released to the atmosphere when solid waste, fossil fuels (oil, natural gas, and coal), and wood and wood products are burned.
  
  
• Methane (CH₄)
  Methane is emitted during the production and transport of coal, natural gas, and oil
  
  
• Nitrous oxide (N₂O)
  Nitrous Oxide is produced during agricultural and industrial activities, as well as during combustion of solid waste and fossil fuels.

Carbon Dioxide (CO₂)
Carbon dioxide enters the atmosphere by burning fossil fuels (oil, natural gas, and coal), solid waste, trees and wood products, and also as a result of manufacturing processes. CO2 from fossil fuel combustion is responsible for almost all greenhouse gas emissions from mobile sources, which include both onroad sources and nonroad equipment such as agricultural and construction vehicles. Carbon dioxide is removed from the atmosphere when absorbed by plants as part of the biological carbon cycle.

Ozone
Ground-level ozone, the main ingredient in smog, is formed by complex chemical reactions of volatile organic compounds (VOC) and NOx in the presence of heat and sunlight. Ozone forms readily in the lower atmosphere, usually during hot summer weather.

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