Diesel exhaust is a mixture containing over 450 different components, including vapors and fine particles. Over 40 chemicals in diesel exhaust are considered toxic air contaminants by the State of California. According to the American Lung Association, diesel engines account for an estimated 26 percent of the total hazardous particulate pollution (PM10) from fuel combustion sources in our air, and 66 percent of the particulate pollution from on-road sources. Diesel engines also produce nearly 20 percent of the total nitrogen oxides (NOx) in outdoor air and 26 percent of the total NOx from on-road sources. Nitrogen oxides are a major contributor to ozone production and smog.

High emissions of toxic air pollutants and fine particles from diesel buses and trucks contaminate the air and endanger people's health. Exposure to diesel emissions may result in cancer, exacerbation of asthma, and other health problems. Pennsylvania has the seventh highest emissions of diesel soot nationwide. In the Philadelphia region, road congestion and major fleets of public transit buses, school buses, delivery trucks and long-haul trucks provide the recipe for a significant diesel emissions problem.

The following contaminants from diesel emissions greatly contribute to hazardous health impacts and environmental degredation:

Diesel Particulates

Particulate matter, or PM, is the term for particles found in the air, including dust, dirt, soot, smoke, and liquid droplets, which are suspended in the air for long periods of time. Some particles are large or dark enough to be seen, such as soot or smoke. Others are so small that individually they can only be detected with an electron microscope. Deisel particulates are small (less than 2.5 microns) and complex substances.

Particulates consist of an uncombusted carbon core, absorbed hydrocarbons from engine oil and fuel. They also contain absorbed sulfates, water, and inorganic materials, such as those produced by internal engine abrasion.

Due to their chemical composion and extremely small size, diesel particulates are dangerous to human health. They contribute and aggravate chronic lung diseases, such as asthma, bronchitis, and emphysema. There is also a potential cancer risk from exposure to diesel particulates, as they contain toxic hydrocarbons. In addition to health risks, diesel exhaust impairs visibility, soils buildings and contributes to their structural damage through corrosion.

In 1993, the U.S. Environmental Protection Agency released a report entitled Motor Vehicle-Related Air Toxic Study. The report listed diesel particulates as one of the most serious hazardous pollutants emitted from mobile sources. The International Agency For Research on Cancer also concluded that diesel particulate is carcinogenic to humans, meaning that it is capable of increasing the incidence of malignant tumors.

NOx Emissions

NOx emissions from diesel engines pose a number of health and environmental concerns. Once in the atmosphere, NOx reacts with volatile organic compounds (VOCs) in the presence of sunlight to form ozone. Ozone is corrosive and contributes to many pulmonary function problems. It is particulatly harmful to children and the elderly. The American Lung Association reported that thousands of people are annually hospitalized during high ozone season because of elevated ozone levels. NOx compounds themselves can damage respiratory systems and lower resistance to respiratory infection.

Carbon Monoxide

Carbon monoxide (CO) is a colorless, odorless and poisonous gas produced by the burning of fuels. It is a component of motor vehicle exhaust, which contributes about 56 percent of all CO emissions nationwide. Other non-road engines and vehicles (such as construction equipment and boats) contribute an additional 22 percent. Higher levels of CO generally occur in areas with heavy traffic congestion. In cities, 85 to 95 percent of all CO emissions may come from motor vehicle exhaust.

When CO enters the bloodstream, it reduces the delivery of oxygen to the body's organs and tissues. Health threats are most serious for those who suffer from cardiovascular disease, particularly those with angina or peripheral vascular disease. Exposure to elevated CO levels can cause impairment of visual perception, manual dexterity, learning ability and performance of complex tasks.

Volatile Organic Compounds

Volatile Organic Compounds (VOCs) are evaporative pollutants of hydrogen and carbon atoms resulting from unburned fuel. VOCs contribute to the formation of ozone which is responsible for the choking, coughing, and stinging eyes associated with smog. Ozone damages lung tissue, aggravates respiratory disease, and makes people more susceptible to respiratory infections. Children are especially vulnerable to ozone's harmful effects, as are adults with existing disease.