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.
More recent fuel and emission control system changes include:
Limits on gasoline volatility
Volatility is a measure of how easily a liquid evaporates. As described
earlier, some toxics such as benzene are present in gasoline and get into
the air when gasoline evaporates. Limits on gasoline volatility have been
imposed over the last several years to control evaporative emissions of both
hydrocarbon and toxic compounds (most air toxics are hydrocarbons so programs
designed to reduce hydrocarbon emissions also reduce air toxics).
Reformulated gasoline
The 1990 Clean Air Act requires reformulated gasoline to be introduced in
the nation's most polluted cities beginning in 1995. From 1995-1999, these
gasolines must provide a minimum 15% reduction in air toxics emissions over
typical 1990 gasolines. This increases to a 20% minimum reduction beginning
in the year 2000. The air toxics reductions will be achieved mainly by reducing
gasoline volatility and by reducing the benzene content of the gasoline.
Limits on diesel sulfur
Regulations limiting the amount of sulfur in diesel fuel took effect in
1993. Today's lower-sulfur diesel fuels are important in reducing emissions
of particulate matter and other air toxics from diesel-fueled buses and trucks.
Source: EPA.
