Diesel Engines and Public Health

With mounting evidence that diesel exhaust poses major health hazards, reducing diesel pollution has become a public priority.

Published Jul 15, 2005 Updated Jan 8, 2008

Health impacts of diesel pollution

Diesel-powered vehicles and equipment account for nearly half of all nitrogen oxides (NOx) and more than two-thirds of all particulate matter (PM) emissions from US transportation sources.  

Particulate matter or soot is created during the incomplete combustion of diesel fuel. Its composition often includes hundreds of chemical elements, including sulfates, ammonium, nitrates, elemental carbon, condensed organic compounds, and even carcinogenic compounds and heavy metals such as arsenic, selenium, cadmium and zinc.¹ Though just a fraction of the width of a human hair, particulate matter varies in size from coarse particulates (less than 10 microns in diameter) to fine particulates (less than 2.5 microns) to ultrafine particulates (less than 0.1 microns). Ultrafine particulates, which are small enough to penetrate the cells of the lungs, make up 80-95% of diesel soot pollution.

Particulate matter irritates the eyes, nose, throat, and lungs, contributing to respiratory and cardiovascular illnesses and even premature death. Although everyone is susceptible to diesel soot pollution, children, the elderly, and individuals with preexisting respiratory conditions are the most vulnerable. Researchers estimate that, nationwide, tens of thousands of people die prematurely each year as a result of particulate pollution. Diesel engines contribute to the problem by releasing particulates directly into the air and by emitting nitrogen oxides and sulfur oxides, which transform into "secondary" particulates in the atmosphere.

Diesel emissions of nitrogen oxides contribute to the formation of ground level ozone, which irritates the respiratory system, causing coughing, choking, and reduced lung capacity. Ground level ozone pollution, formed when nitrogen oxides and hydrocarbon emissions combine in the presence of sunlight, presents a hazard for both healthy adults and individuals suffering from respiratory problems. Urban ozone pollution has been linked to increased hospital admissions for respiratory problems such as asthma, even at levels below the federal standards for ozone.

Diesel exhaust has been classified a potential human carcinogen by the U.S. Environmental Protection Agency (EPA) and the International Agency for Research on Cancer. Exposure to high levels of diesel exhaust has been shown to cause lung tumors in rats, and studies of humans routinely exposed to diesel fumes indicate a greater risk of lung cancer. For example, occupational health studies of railroad, dock, trucking, and bus garage workers exposed to high levels of diesel exhaust over many years consistently demonstrate a 20 to 50 percent increase in the risk of lung cancer or mortality.²

Diesel pollution and public health solutions

The public-health problems associated with diesel emissions have intensified efforts to develop viable solutions for reducing these emissions. Both federal and state governments have taken steps to reduce diesel emissions, but more work needs to be done.

Cleaner Fuels – The EPA has adopted more stringent fuel standards to reduce the amount of sulfur allowed in diesel fuel. These requirements went into effect in late 2006 for on-road diesel vehicles, while off-road diesel fuel used in construction equipment and trains will take effect over the next five years. Lower sulfur diesel fuel allows the use of advanced emission control technologies, which when combined, can reduce emissions more than 85 percent. The fuel used in ships visiting our port cities, however, is not subject to EPA's regulation and remains a significant source of diesel pollution.

New Engine Standards – New engine standards for diesel cars, trucks and heavy equipment have traditionally lagged far behind those for gasoline powered vehicles. For example, diesel construction equipment faced no emissions standards as late as 1996. With mounting pressure to clean-up diesel engines, the EPA has adopted standards for both heavy-duty trucks and off-road construction equipment and more recently for marine vessels and trains, which will phase in over the coming decade. Under current regulations, passenger cars and trucks are subject to the same emission standards regardless of the fuel they use.  

Retrofitting – New engine standards only apply to the equipment in the dealer showrooms, not to the diesel engines that are already in operation. The combination of lagging emission standards and durability of diesel engines means there are many high polluting diesel trucks, buses, and off-road equipment that will continue to operate well in to the future. Retrofitting these diesel vehicles and equipment with advanced emission control devices can effectively reduce harmful tailpipe emissions.

More to be done

Faced with more stringent federal and state regulatory measures, diesel technology has advanced rapidly in recent years. Some diesel passenger cars are now starting to meet California's strict tailpipe standards, with more expected in the future. As vehicles equipped with advanced diesel emissions controls enter the market place, it will be important to ensure that emission levels are maintained throughout the life of the vehicle through periodic testing.


  1. Particulate Matter (TSP and PM-10) in Minnesota. Minnesota Pollution Control Agency. December 1997.
  2. Health Assessment Document for Diesel Engine Exhaust. National Center for Environmental Assessment, Office of Research and Development, US EPA. Washington D.C. May 2002. page 9-11. EPA/600/8-90/057F

Related resources