Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Diesel 101

On this page:

Emissions from diesel engines are a primary source of air pollution in the northeastern United States. They pose a significant risk to public health, and impose a high cost on society. Twenty-five counties in Connecticut, New Jersey, and New York fail to meet the health-based air quality standard for fine particles, and other urban areas in the Northeast only narrowly meet the standard. The fine particles in diesel exhaust can aggravate asthma, cause lung damage, and even lead to premature death. The Northeast has some of the highest asthma rates in the nation, including a childhood asthma rate above 10 percent in all six New England states and rates near 15 percent in areas of New York City. Nationally, EPA has taken critical steps to ensure that the diesel engines manufactured in the future will be significantly cleaner than those operating today; however, diesel engines are very durable, and older models will continue to be used and could pose health and environmental risks for decades.

Diesel engines in trucks, buses, locomotives, marine vessels, and construction equipment emit particulate matter, nitrogen oxides, greenhouse gases, and air toxics. These emissions contribute to unhealthy levels of air pollution in the Northeast, where millions of residents are affected.

Pollutants of Concern

Nitrogen Oxides (NOx)

  • Nitrogen oxides form when fuel burns at high temperatures. They cause a variety of health and environmental problems in locations far from their emissions source.
  • Nitrogen oxides react with volatile organic compounds when sunlight is present to form ground-level ozone, the primary constituent of smog. Ozone and smog can harm human health (causing lung damage and a variety of respiratory problems), damage the environment, and cause poor visibility.
  • Nitrogen oxide emissions contribute to the formation of particulate matter through chemical reactions in the atmosphere.

Particulate Matter (PM)

  • Particulate matter (PM) is the term for solid or liquid particles found in the air. Some particles are large or dark enough to be seen as soot or smoke. Fine particulate matter is tiny and is generally not visible to the naked eye.
  • Fine and ultrafine PM are a health concern because they can reach the deepest regions of the lungs. Health effects can include aggravated asthma, difficult or painful breathing, chronic bronchitis, and premature death in people with cardiopulmonary disease. Children and the elderly are especially at risk. Fine particulate matter associated with diesel exhaust is also thought to cause lung cancer.
  • Fine particulate matter can travel long distances on air currents and is a major cause of haze, which reduces visibility in cities and scenic areas throughout the United States.
  • Black carbon, a major component of PM emitted from diesel engines, is a potent contributor to global climate change.

Greenhouse Gases

  • Gases that trap heat in the atmosphere are referred to as greenhouse gases. By far, carbon dioxide is the most important greenhouse gas with respect to diesel vehicles.
  • Vehicle emissions are a major source of carbon dioxide, accounting for approximately 30% of the nation's total annual emissions.
  • Diesel fuel consumption accounts for approximately 21% of vehicle emissions.

Air Toxics

  • Air toxics are those pollutants known or suspected to cause cancer or other serious health effects (such as reproductive problems or birth defects) or adverse environmental effects.
  • Most air toxics originate from human activities, including fuel combustion in diesel vehicles, while some air toxics are released from natural sources such as volcanic eruptions. Examples of toxic air pollutants include benzene and toluene.

Reducing Diesel Pollution

The good news is that recently established regulations and programs greatly reduce harmful emissions from diesel engines. EPA's Clean Diesel Trucks and Buses Rule and Nonroad Diesel Rule establish more stringent emissions standards for new on-road diesel vehicles and diesel construction vehicles and equipment. Furthermore, new rulemaking will reduce emissions from the diesel locomotive and marine engines of the future. When the latest rules requiring the use of advanced emission control technology and clean diesel fuel with a sulfur content capped at 15 parts per million take effect, exhaust emissions will decrease by more than 90 percent. When fully implemented, these rules will achieve more than $100 billion in health benefits. However, these engine emissions rules will apply only to newly manufactured engines. With diesel engines lasting up to 30 years and with approximately 11 million engines in use today, the full benefits of these rules will not be realized for decades.

In order to address the emissions from these legacy engines, several strategies are available, including vehicle replacement, repowering, upgrading, retrofitting, using alternative fuels or employing anti-idling technologies. In the case of larger vehicles, the engines can be rebuilt with new components to lower emissions. In some cases it may be most cost effective to replace the entire vehicle itself. Refueling options may be as simple as substituting conventional diesel with ultra-low sulfur diesel (where it is not already required) or biodiesel, or may include changing the engine design to incorporate cleaner burning fuel such as natural gas. Finally, pollutants may be removed from the exhaust through retrofitting the vehicle with control devices such as filters and catalysts. The Manufacturers of Emissions Controls provides details on these strategies on their website at

Complementing these clean diesel rules is a range of federal, regional, and local programs that address emissions from diesel vehicles and equipment in use today. Please visit the sector pages for more information on these programs.