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Mobile Source Air Toxics: An Avoidable Threat

Mobile Source Air Toxics: An Avoidable Threat

What Are Mobile Source Air Toxics?

Mobile source air toxics are the most hazardous pollutants emitted by mobile sources such as cars, trucks, buses, boats, snowmobiles, construction equipment, and lawnmowers. EPA has identified 21 mobile source air toxics, including benzene, formaldehyde, 1,3-butadiene, acetaldehyde, and diesel particulate matter, all of which can cause cancer or other serious health problems.

Impacts on Public Health

Mobile source air toxics are pollutants designated (or listed) as "hazardous" by the Environmental Protection Agency under the Clean Air Act and emitted by mobile sources such as cars, trucks, buses, boats, snowmobiles, construction equipment, and lawnmowers. EPA has identified 21 mobile source air toxics, including benzene, formaldehyde, 1,3-butadiene, acetaldehyde, and diesel particulate matter, all of which can cause cancer or other serious health problems.

Each year, mobile sources emit more than 700,000 tons of benzene, formaldehyde, 1,3-butadiene, acetaldehyde, and diesel particulates. They also emit more than one million tons of other hazardous air pollutants (66 Fed. Reg. at 17238-17239). By volume, mobile sources account for about 40 percent of all emissions of hazardous air pollutants.

The detrimental health effects of this group of pollutants are severe and subject Americans to unacceptable levels of risk for cancer, neurological and reproductive disorders, blood disease, birth defects, developmental damage, kidney and liver damage, and respiratory disease.

The Law

As Congress recognized in 1990, toxic emissions from mobile sources can be substantially reduced. The Clean Air Act amendments of 1990 required EPA to complete by 1992 a study of mobile source air toxics and, in particular, of the health risks they pose (42 U.S.C. § 7521(1)(1)). Following the study, EPA was required to promulgate by 1995 regulations "based on the study...to control hazardous air pollutants from motor vehicles and motor vehicle fuels" (42 U.S.C. § 7521(1)(2)). EPA was required to include standards for fuels, vehicles, or both that "reflect the greatest degree of emission reduction achievable through the application of technology which will be available, taking into consideration [EPA's other mobile source regulations], the availability and cost of the technology, and noise, energy, and safety factors and lead time" (Id). EPA also was required to establish regulations that "at a minimum, apply to emissions of benzene and formaldehyde."

EPA's Failure to Implement the Law

Although EPA has had more than ten years to study the problem of mobile source air toxics, the agency has never completed a thorough analysis. Notably absent from EPA's study is any evaluation of the cumulative risk presented by mobile source air toxics; instead, the study discusses the risks posed by only one pollutant at a time. The study also omits any discussion of health risks other than cancer from mobile source air toxics. The study does not address risks resulting from exposure to these toxics through contaminated food and water. Finally, the study ignores the heightened risks to highly exposed and highly sensitive individuals.

Released in March 2001 -- six years late -- EPA's mobile source air toxics regulations are an even more serious breach of the Clean Air Act than EPA's study because they do not reflect the greatest degree of emissions reduction achievable. Recognizing that its regulations are far less than what the law requires, EPA has committed to promulgate another mobile source air toxics rule in 2004.  In 2004, Earthjustice filed litigation that challenged EPA’s failure to issue regulations limiting these emissions. Our suit led to a court order that requires EPA to set standards for tailpipe emissions by February 2007. In March 2006, EPA proposed a rule that requires major benzene reductions in gasoline and gasoline containers, a move that will clean up the air for millions of Americans and greatly reduce dangerous health threats. EPA estimates that the cost for this reduction will be less than one cent per gallon, and will result in a 45 percent benzene reduction overall.

Public Health Threats



  • All 21 mobile source air toxics can cause cancer or other serious health problems.

  • Benzene is a known human carcinogen. Formaldehyde, 1,3-butadiene, acetaldehyde, and diesel particulate are all probable human carcinogens.

  • EPA has indicated that, as a result of exposure to mobile source air toxics, about half of all Americans are exposed to significantly more than EPA's acceptable cancer risk of 1 in one million. Even worse, the 130 million people living in urban areas are exposed to a risk of 1 in 100,000.

  • About 800,000 Americans face a cancer risk of 1 in 10,000 due to exposure to mobile source air toxics.

  • EPA has stated that mobile source air toxics likely account for about half of all cancer cases caused by outdoor air toxics.

  • Other than cancer, the health effects of mobile source air toxics include serious and permanent damage to the nervous and reproductive systems, leukemia and other blood diseases, birth defects and damage to the development of children and unborn children, damage to the kidney and liver, and asthma and other respiratory disease.


Control Options for Mobile Source Air Toxics



  • Introduce an effective nationwide benzene cap. According to EPA, the cost of a benzene content standard for gasoline would be less than one tenth of one cent per gallon. Such a reduction could reduce benzene levels in some areas of the country by as much as 90 percent.

  • Use currently available emissions control technologies for what EPA classifies as "nonroad" gasoline engines, such as those on boats, personal watercraft, and large industrial gas engines. Such technologies are currently required by the California Air Resources Board. These types of gas engines emit approximately 35 percent of all benzene and 1,3 butadiene pollution in the United States. Catalysts for some categories of these gas engines are available at a cost of $5 per engine.

  • Require improved fuel efficiency. The less fuel used, the less pollution emitted.

  • Increase the effectiveness of existing inspection and maintenance programs and expand them as appropriate in new areas. By ensuring compliance with existing standards, large-scale reduction of mobile source air toxics can be achieved.

  • Promote advanced technology vehicles that do not rely solely on the combustion of fossil fuels.

  • Require on-board diagnostics for all heavy-duty engines, ensuring that vehicle emissions do not increase due to poor maintenance and tampering.

Program Area:  Healthy Communities
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