Emissions Factsheet

Just the Basics

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What Are Exhaust Emissions?

In most heavily settled areas of the U.S., the personal automobile is the single greatest producer of harmful vehicle exhaust emissions. Exhaust emissions are generated by the fuel-air mixture burning in internal combustion engines, both gasoline-powered and diesel-powered. Emissions are also produced by fuel evaporation within the vehicle when it is stopped, and again during fueling.

The constituents of car (gasoline and diesel) and truck (diesel) emissions vary depending on fuel type and individual vehicle operating characteristics. The bulk of vehicular emissions are composed of water vapor, carbon dioxide, nitrogen, and oxygen (in unconsumed air). There are other pollutants, such as carbon monoxide, nitrogen oxides, unburned fuel, and particulate matter, in smaller quantities as well. Some of these substances play a considerable role in vehiclerelated air pollution including some that are "greenhouse gases," believed by many scientists to contribute to climate change. The table at right identifies the important gases and solids found in exhaust emissions that are cause for concern.

Why Are Exhaust Emissions a Problem?

Exhaust, evaporative, and refueling emissions are major contributors to air pollution and the health problems it causes. In addition, many of the compounds found in vehicle exhaust are known to be carcinogenic (cancer-causing) in significant, chronic exposures.

Other gases found in emissions— carbon monoxide and sulfur dioxide— contribute to the formation of ground-level, or tropospheric, ozone. Tropospheric ozone develops from the interaction between two or more precursor pollutants such as volatile organic compounds (VOCs) and nitrogen oxide (NOx) in the presence of ultraviolet light (sunlight). This type of ozone is responsible for much of the poor air quality that develops throughout the U.S., especially during the summer months.

What Is Being Done About Exhaust Emissions?

When it first became apparent during the 1960s that exhaust emissions were contributing to worsening air pollution problems in much of the U.S., the federal government enacted the Clean Air Act. Several revisions later, the Clean Air Act and ensuing legislation have led to significant modifications in fuels and vehicles that have produced impressive improvements in air quality, such as:

Major changes in U.S. fuel formulations, including removal of lead, addition of oxygenates, and reduction in sulfur content
Placement of catalytic converters on all U.S. passenger cars and light trucks, minivans, and SUVs manufactured after 1974, to reduce CO, NOx, HC, and VOC emissions
Mandatory vehicle emissions testing in more than half of U.S. states, with requirements for vehicle repair or removal from use
Installation of vapor recovery systems on gasoline fueling pumps in major metropolitan areas, to capture VOC emissions

While much has been achieved, more remains to be done. In the years since the Clean Air Act was first passed, the average number of vehicles per household has increased, many more vehicles use U.S. roads, vehicle miles per gallon of fuel are decreasing due to the popularity of sport utility vehicles (SUVs) and light trucks, and many drivers commute longer distances to and from work and school. Consequently, air quality is still unhealthy in much of the U.S., and the incidence of cardiopulmonary conditions such as heart disease, emphysema, and asthma is on the rise.

To help address these problems, the U.S. Department of Energy is funding research and development efforts that can improve fuel economy and engine efficiency, broaden the diversity of available vehicle fuels, and diminish the U.S.'s dependence on foreign petroleum while helping to reduce the amount of harmful vehicle emissions released into the air. These include:

Oxygenating fuel additives and devices
Alternative, non-petroleum-based fuels
Improved combustion processes
Technologies to replace internal combustion for transportation
Diesel fuel with reduced sulfur levels
Diesel oxidation catalysts
Hydrocarbon traps
Particulate traps

Where Can I Find More Information?

U.S. Department of Energy, Energy Efficiency and Renewable Energy— Air Emissions and Standards

U.S. Department of Energy— FreedomCAR and Vehicle Technologies Program

U.S. Environmental Protection Agency: Toxic Air Pollutants

U.S. Environmental Protection Agency: Mobile Source Air Toxics

Automobile Emissions: An Overview

U.S. Department of Transportation, Federal Highway Administration – Transportation Air Quality, Selected Facts and Figures

A Strong Energy Portfolio for a Strong America

Energy efficiency and clean, renewable energy will mean a stronger economy, a cleaner environment, and greater energy independence for America. Working with a wide array of state, community, industry, and university partners, the U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy invests in a diverse portfolio of energy technologies.

For more information, visit www.eere.energy.gov/vehiclesandfuels or call 1-800-DOE-3732.

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