Diesel engines provide consumers with fuel efficiency, reliability, and durability, but they also pollute the environment by emitting nitrogen oxides (NOx), volatile organic compounds (VOCs), particulate matter (PM), sulfur dioxide, ketones, sulfates, cyanides, ammonia and other toxic substances such as arsenic, benzene, nickel, and formaldehyde into the air. Diesel engines are different from other internal combustion engines in that they use high pressure instead of spark plugs to ignite the fuel. Diesel operations require high temperatures causing oxygen and nitrogen from the intake air to combine as NOx. Diesel engines are used in heavy-duty trucks (large pickups and tractor-trailers), buses, and cars. They can also be found in non-road mobile sources such as: tractors, graders, combines, cranes, bulldozers, forklifts, compressors, and other construction and industrial vehicles. Emissions from diesel engines have played a significant role in hindering the achievement of Clean Air Act health standards in many regions across the country.
When NOx and VOCs react with heat and sunlight, ground-level ozone (smog) is formed. Elevated levels of ozone are harmful to human health. According to government sources, nonroad diesel engines emit approximately ten percent, and heavy duty vehicles emit twelve percent of the nationwide emission of NOx. The detrimental health effects of ozone include: a decrease in lung function, an increase in airway inflammation and sensitivity to other irritants, an impairment of lung defenses, chest pain, coughing, and wheezing. High level exposure to NOx can lead to respiratory illness, cause bronchial tube constriction in lungs, and may increase bronchitis in children. NOx emissions also contribute to acid precipitation which damages ecosystems within lakes and streams, harms trees, and reduces agricultural crop yield. NOx is especially problematic because it is transported across large distances, meaning that high levels of NOx in one region may affect ozone and acid precipitation in nearby regions.
Particulate matter can be found in visible pollutants such as smoke and soot as well as invisible aerosols formed from gaseous pollutants. Combustion produces particulates including sulfates, organic carbon aerosols, ammonium, nitrates, and carbon soot which are very small (2.5-10 microns; 10 microns is equivalent to 1/2500 of an inch). Diesel particulates are generally smaller than 1 micron. Exposure to particulate matter may affect breathing and other functions of the respiratory system, aggravate existing respiratory and cardiovascular diseases, alter the body's defense mechanisms against foreign material, and cause direct and indirect damage to lung tissue. Since diesel particulates are smaller than average, they can penetrate and damage lung tissue easier. Studies have shown that diesel powered engines emit fifty to eighty times more particulate matter than a typical gasoline powered cars. Particulate matter is especially dangerous in cities because of the large volume of traffic. It has been estimated by the U.S. Environmental Protection Agency and The Harvard School of Public Health that between 50,000 and 60,000 people in the United States die prematurely each year as a result of particulate air pollution. In 1995 The Harvard School of Public Health also reported that residents living in the nation's most polluted cities suffer from a mortality rate that is more than 15% higher than residents in cleaner air cities.
Diesel exhaust also contains many carcinogens including arsenic, benzene, and nickel. Other air toxics found in the exhaust include; 1,3-butadiene, cadmium, dioxins, dibenzofurans, formaldehyde, acetone, aluminum, ammonia, barium, copper, silver, sulfuric acid, and zinc. Sulfur is also a component of diesel exhaust found as sulfur dioxide (SO2). Studies have indicated that inhalation of SO2 can cause bronchial tubes to constrict and can also worsen preexisting respiratory diseases. SO2 also contributes to acid precipitation.
Many people are affected by diesel air pollution. Those at greatest risk are children, the elderly, those who exercise outdoors, and people with respiratory or cardiovascular diseases. Children are at risk because they often spend a great deal of time outdoors, and since they are generally smaller than adults, they inhale more air per pound of body weight and therefore more pollution. The elderly are at risk because their immune systems are not strong and because a disproportionate number suffer from cardiovascular weaknesses. Those who live in and around a metropolitan area and who exercise outdoors are also at risk because exercising increases heart rate and breathing rate. Increased amounts of pollutants are inhaled often through the mouth, bypassing nasal filters that protect humans from destructive substances such as particulate matter and toxic aerosols. People who suffer from asthma are particularly affected by particulates because the small particles disrupt their already disabled breathing. The inhalation of diesel particulates can also cause cancer, pneumonia, bronchitis, pleurisy, and other respiratory ailments.
Currently, new engine standards and regulations at the national level are being proposed that would force manufacturers of diesel engines to develop technologies that would reduce the amount of NOx, particulate matter, VOCs, and other substances emitted. These proposed standards would take effect in the year 2004. Since technology for the proposed regulations has not yet been developed, a technological review will occur in 1999 to determine progress and possible changes in standards if necessary. Since many heavy duty trucks travel interstate and NOx transports across regions, proposed rule changes will be national.
There are many alternatives to diesel engines. Many diesel city buses are being replaced by either electric or natural gas buses. Hybrid buses that combine internal combustion fuels like gasoline, diesel or natural gas and stored electric power, are currently being developed. Electric and natural gas cars are now on the market and offer a very "clean" alternative to other sources of fuel, but they comprise only a small percentage of the automobile market. The development of new technology will play a large role in reducing diesel and other mobile source pollution.