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Vehicle Emission Basics

November 22, 2013 - 2:07pm


Vehicle emissions are the gases emitted by the tailpipes of vehicles that use internal combustion engines.  These vehicles can run on gasoline, diesel, biofuels, natural gas, or propane. Vehicle emissions are composed of varying amounts of:

  • water vapor
  • carbon dioxide (CO2)
  • nitrogen
  • oxygen
  • pollutants such as:
    • carbon monoxide (CO)
    • nitrogen oxides (NOx)
    • unburned hydrocarbons (UHCs)
    • volatile organic compounds (VOCs)
    • particulate matter (PM).

A number of factors determine the composition of emissions, including the vehicle's fuel, the engine's technology, the vehicle's exhaust aftertreatment system, and how the vehicle operates.  Emissions are also produced by fuel evaporation during fueling or even when vehicles are parked.

Why Are Vehicle Emissions a Concern?

Certain vehicle emissions are pollutants that can lead to the formation of ground-level ozone, which reduces air quality. Ozone can cause respiratory problems and asthma in children, the elderly, or those who are working or exercising outdoors on a smoggy day. Long-term exposure to ozone may lead to premature aging of the lungs and chronic respiratory illness. In addition, significant chronic exposure to the organic compounds found in vehicle exhaust can potentially lead to cancer.

Carbon dioxide is another major component of vehicle emissions. Carbon dioxide is a major contributor to global climate change, with transportation contributing about a third of the United States' greenhouse gas emissions.

How Are We Reducing Vehicle Emissions?

The federal government enacted the Clean Air Act in 1971 in response to the worsening air pollution in much of the United States. The Act sought to control both vehicle emissions and those from stationary sources such as power plants. The Environmental Protection Agency (EPA), under the U.S. National Ambient Air Quality Standards, sets the limits of allowable ambient levels of PM, NOx, and ozone.  The Clean Air Act and subsequent amendments have led to significant changes in fuels and vehicles that have reduced the amount of vehicle emissions and improved air quality.

While light-duty fuel economy standards have been in place since 1975, the EPA's Office of Transportation and Air Quality and the U.S. Department of Transportation's National Highway Transportation Safety Administration set new fuel economy standards in 2011 for both light and heavy-duty vehicles.  These new standards will both reduce the amount of fuel these vehicles use and the greenhouse gas emissions they produce.

The EERE Vehicle Technologies Office supports combustion research to develop advanced combustion strategies that produce near-zero emissions inside the engine.  It also supports research and development on exhaust aftertreatment technologies to control emissions coming out of vehicle tailpipes.  Learn more about our advanced combustion engine research and development efforts focused on increasing internal combustion engines' energy efficiency with minimal emissions.  In addition, the Vehicle Technologies Office also supports research on plug-in electric vehicles, which do not produce tailpipe emissions at all when they are running on electricity.