What are Air Toxics?

Air toxics are air pollutants that cause adverse health effects. The U.S. Environmental Protection Agency (EPA) has focused most of its air toxics efforts to date on carcinogens, which are compounds that cause cancer. Non-cancer health effects such as reproductive and neurological problems are also of concern to EPA.

How dangerous are air toxics? It’s hard to say. Some air toxics have been proven to cause cancer in humans. However, most air toxics are identified through laboratory experiments in which animals receive
very high doses of the compound being studied. People almost never breathe such high doses. But lower exposures may still pose risks. One fact is clear, vehicles are such an integral part of our society that virtually everyone is exposed to their emissions.

Air Toxics from Vehicles and Their Fuels

Motor vehicles emit several pollutants that EPA classifies as known or probable human carcinogens. Benzene, for instance, is a known human carcinogen, while formaldehyde, acetaldehyde, 1,3-butadiene and diesel particulate matter are probable human carcinogens. Studies are underway to determine whether other toxic substances are present in mobile source emissions. For example, EPA and industry are investigating whether oxygen-containing fuel additives such as methyl tertiary butyl ether (MTBE) cause any adverse health effects. EPA is also working with the vehicle and fuel industries to test motor vehicle emissions for the presence of dioxin.

EPA estimates that mobile (car, truck, and bus) sources of air toxics account for as much as half of all cancers attributed to outdoor sources of air toxics. This estimate is not based on actual cancer cases, but on models that predict the maximum number of cancers that could be expected from current levels of exposure to mobile source emissions. The models consider available health studies, air quality data, and other information about the types of vehicles and fuels currently in use. Nonroad mobile sources (such as tractors and snowmobiles) emit air toxics as well.

How are Air Toxics from Motor Vehicles Formed?

Some toxic compounds are present in gasoline and are emitted to. The air. when gasoline evaporates or passes through the engine as unburned fuel. Benzene, for example, is a component of gasoline. Cars emit small quantities of benzene in unburned fuel, or as vapor when gasoline evaporates.

A significant amount of automotive benzene comes from the incomplete combustion of compounds in gasoline such as toluene and xylene that are chemically very similar to benzene. Like benzene itself, these compounds occur naturally in petroleum and become more concentrated when petroleum is refined to produce high octane gasoline.

Formaldehyde, acetaldehyde, diesel particulate matter, and 1,3-butadiene are not present in fuel but are by-products of incomplete combustion. Formaldehyde and acetaldehyde are also formed through a secondary process when other mobile source pollutants undergo chemical reactions in the atmosphere.

Reducing Air Toxics from Motor Vehicles

The emissions that come out of a vehicle depend greatly on the fuel that goes into it. Consequently, programs to control air toxics pollution have centered around changing fuel composition as well as around improving vehicle technology or performance. One of the first, and most successful programs has been the removal of lead from gasoline. The lead phaseout began in the mid-1970s. It will be complete January 1, 1996 when lead is banned from gasoline. The removal of lead from gasoline has essentially eliminated mobile source emissions of this highly toxic substance.

More recent fuel and emission control system changes include:

In summary, the many vehicle and fuel changes in the last 25 years have greatly reduced air toxics emissions from highway vehicles. New cars today are capable of emitting 90% less air toxics on a per-mile basis than the uncontrolled models of 1970; new trucks and buses are designed to emit less than half the air toxics of their 1970 counterparts. Overall air toxics emissions will continue to decrease through the 1990s as older vehicles leave the fleet and as new regulatory programs take effect. However, the number of vehicles on the road and the number of miles they travel is continuing to grow. Without additional controls, growth in vehicle travel will offset progress in reducing air toxics by early in the next century.

What More Can Be Done?

The 1990 Clean Air Act requires EPA to specifically regulate air toxics from motor vehicles in the form of standards for fuels, or vehicles, or both.

Additional hydrocarbon and particulate matter controls such as tightening tailpipe standards even further will reduce air toxics emissions somewhat. California for example, sets tighter hydrocarbon limits for vehicles sold in that state than the federal requirements for vehicles sold elsewhere. Other states can choose to adopt the California standards, which provide some air toxics benefits. Expansion of existing regulatory programs (such as more widespread use of reformulated gasoline or wider requirements for emission inspections) could also help reduce air toxics. Specific vehicle emission standards for one or more toxic compounds are also an option.

Changes in gasoline and diesel fuel composition (such as reducing sulfur, benzene or other aromatic chemical compounds) can also reduce air toxics emissions.

A switch to alternative, non-petroleum fuels that are cleaner than today’s gasoline and diesel fuels offers another strategy for reducing air toxics. Choices include alcohols, natural gas, propane, and electricity. These fuels are inherently cleaner than conventional gasoline and diesel because they do not contain toxics such as benzene. In addition, they are made of simpler chemical compounds which yield lower levels of complex combustion by-products such as 1,3-butadiene.

For Further Information:

The EPA National Vehicle and Fuel Emissions Laboratory is the national center for research and policy related to mobile source pollution. For additional information, contact the lab at 2565 Plymouth Road, Ann Arbor, MI 48105, or call (313) 668-4333.

Source: US EPA Office of Transportation and Air Quality

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Last Update – 20-Jul-98