Nationally, just 8 chemicals account for 99% of estimated cancer risks. One pollutant - diesel emissions - accounts for almost 80% of the estimated lifetime cancer risk associated with outdoor hazardous air pollutant exposures. More on the public health impact of diesel emissions.

The situation is similar with noncancer hazards: just three pollutants were the main contributors to noncancer hazards.

The following sections briefly profile each high risk HAP, identify any outstanding uncertainties affecting these compound's rankings, and provide links to more detailed information elsewhere on Scorecard. Please also review our Caveats.

TOP 8 CANCER HAPS

• DIESEL EMISSIONS
• BENZENE
• CARBON TETRACHLORIDE
• CHROMIUM
• POLYCYCLIC ORGANIC MATTER (POM)
• 1,3 BUTADIENE
• FORMALDEHYDE
• COKE OVEN EMISSIONS

TOP 3 NONCANCER HAPS

• ACROLEIN
• FORMALDEHYDE
• DIESEL EMISSIONS

Diesel exhaust's cancer risk assessment value is more uncertain than chemical-specific values because it is a complex mixture of constituents of varying toxicity. Over 50 hazardous air pollutants have been detected as constituents of diesel emissions. EPA and Scorecard focus on the particulate matter component of diesel exhaust, because more is known about this fraction, and most researchers believe that diesel exhaust particles contribute the majority of the risk from exposures to diesel emissions. Scorecard's risk assessment value for diesel emissions was developed by California's Air Resources Board; U.S. EPA has not adopted an official cancer risk assessment value for the mixture.

For an accessible summary of what is known about the sources and health risks of diesel emissions, see the California Air Resources Board's Toxic Air Contaminant Summary. Extensive information about this chemical is provided in Scorecard's chemical profile.

BENZENE
Nationally, mobile sources account for 57% of estimated benzene concentrations, followed by background (23%), area sources (6%), and point sources (2%). The predominant sources of benzene emissions are gasoline fugitive emissions and gasoline motor vehicle exhaust. Background concentrations are significant because regional airsheds are generally contaminated by transportation sources. Benzene emissions also occur from area sources like gasoline service stations, agricultural burning, forest management burning, and wildfires. The primary point sources emitting benzene include crude petroleum and natural gas mining, petroleum refining, and electric services.

For an accessible summary of what is known about the sources and health risks of benzene, see the California Air Resources Board's Toxic Air Contaminant Summary. Benzene is a special focus chemical in EPA's 1993 Motor Vehicle-Related Air Toxics Study. Extensive information about this chemical is provided in Scorecard's chemical profile.

CARBON TETRACHLORIDE
Carbon tetrachloride is a stable gaseous compound in the lower atmosphere with a relatively long atmospheric lifetime of 50 years. Due to the lack of rapid atmospheric removal mechanisms, carbon tetrachloride accumulates to contribute to a global background concentration, which is estimated to be between 0.69 and 0.94 µg/m3. In the U.S., EPA believes that 99% of the estimated concentration of carbon tetrachloride comes from background levels, estimated to be 0.88 µg/m3. In the past, carbon tetrachloride was used for dry cleaning and as a grain-fumigant. However, it is no longer allowed to be used for these purposes in the United States. The primary sources that have reported emissions of carbon tetrachloride are chemical and allied product manufacturers, and petroleum refineries.

For an accessible summary of what is known about the sources and health risks of benzene, see the California Air Resources Board's Toxic Air Contaminant Summary. Extensive information about this chemical is provided in Scorecard's chemical profile.

CHROMIUM
Chromium is an odorless, steel-gray, hard metal that exists as thousands of compounds, exhibiting a wide range of chemical properties and toxicities. Nationally, the major contributor to estimated chromium concentrations are area sources (51%), followed by point sources (43%) and mobile sources (5%). Major area sources responsible for these emissions include chrome-plating shops and building cooling towers. Chromium is also emitted from oil combustion, sewer sludge incineration, cement production, municipal waste incinerators, and refractories.

Because emissions and exposure data for chromium do not identify specific compounds or valence states, there is greater uncertainty associated with risk estimation for this class of pollutants. Chromium was assigned a cancer risk assessment value based on the average proportion of the carcinogenic element chromium VI in environmental mixtures.

For an accessible summary of what is known about the sources and health risks of benzene, see the California Air Resources Board's Toxic Air Contaminant Summary. Extensive information about this chemical is provided in Scorecard's chemical profile.

POLYCYCLIC ORGANIC MATTER (POM)
Polycyclic organic matter consists of over 100 compounds and can be divided into the subgroups of polycyclic aromatic hydrocarbons (PAHs) and PAH-derivatives. Nationally, 95% of estimated POM concentrations come from area sources, and 5% from point sources. POM is produced by the incomplete combustion of fossil fuels and vegetable matter. POM has been detected in motor vehicle exhaust, smoke from residential wood combustion, and fly ash from coal-fired electric generating plants. Stationary emission sources for POM include petroleum refineries, industrial machinery manufacturers, and the wholesale trade in petroleum and petroleum products.

POM's cancer risk assessment value is more uncertain than chemical-specific values because it is a complex mixture of constituents of varying toxicity. Scorecard's value was developed by US EPA by adapting three methodologies that combine toxicity values for benzo(a)pyrene and other polyaromatic hydrocarbon (PAH) constituents.

For an accessible summary of what is known about the sources and health risks of POM, see the California Air Resources Board's Toxic Air Contaminant Summary. Extensive information about this chemical is provided in Scorecard's chemical profile.

1,3 BUTADIENE
Nationally, 84% of estimated butadiene concentrations come from mobile sources, followed by area sources (13%) and point sources (3%). The majority of 1,3-butadiene emissions are from incomplete combustion of gasoline and diesel fuels. Vehicles that are not equipped with functioning catalytic converters emit greater amounts of 1,3-butadiene than vehicles with functioning pollution control devices. Other sources of 1,3-butadiene include vehicle tire wear, petroleum refining, styrene-butadiene copolymer production and biomass burning, including residential wood combustion, agricultural burning, and managed forest fires.

For an accessible summary of what is known about the sources and health risks of 1,3 butadiene, see the California Air Resources Board's Toxic Air Contaminant Summary. Butadiene is a special focus chemical in EPA's 1993 Motor Vehicle-Related Air Toxics Study. Extensive information about this chemical is provided in Scorecard's chemical profile.

FORMALDEHYDE
Nationally, the major sources responsible for formaldehyde concentrations are mobile sources (67%), background levels (19%), area sources (13%), and point sources (1%). Formaldehyde is both directly emitted into the atmosphere and produced through secondary formation as a result of photochemical oxidation of organic chemicals in polluted atmospheres containing ozone and nitrogen oxides. The primary source of formaldehyde is vehicular exhaust. Formaldehyde is also a product of incomplete combustion. Catalytic cracking, coking operations, and fuel combustion are major sources of formaldehyde from refineries. Stone, clay, and glass production use fuel combustion sources such as boilers, furnaces, and engines in manufacturing processes which also generate formaldehyde.

For an accessible summary of what is known about the sources and health risks of formaldehyde, see the California Air Resources Board's Toxic Air Contaminant Summary. Formaldehyde is a special focus chemical in EPA's 1993 Motor Vehicle-Related Air Toxics Study. Extensive information about this chemical is provided in Scorecard's chemical profile.

COKE OVEN EMISSIONS Nationally, 100% of coke oven emissions are released by point sources, usually manufacturing facilities which use coke to extract metal from ores like iron. Coke oven emissions are a benzene-soluble fraction of total particulate matter produced during destructive distillation of carbonization of coal to produce coke. It consists of coal and coke particles, vapors, and tars that contain polycyclic aromatic hydrocarbons (PAH), benzene, naphthylamine, cadmium, arsenic, beryllium, and chromium.

For an accessible summary of what is known about the health risks of coke oven emissions, see the California Air Resources Board's Toxic Air Contaminant Summary. Extensive information about this chemical is provided in Scorecard's chemical profile.

ACROLEIN
Nationally, the major sources responsible for acrolein concentrations are mobile sources (75%), area sources (22%), and point sources (3%). Combustion of fossil fuels, tobacco smoke, and pyrolyzed animal and vegetable fats contribute to the environmental prevalence of acrolein. Acrolein is also a secondary formation product derived from atmospheric reactions of 1,3-butadiene. Acrolein is also a byproduct of fires. At point sources, acrolein is principally used as a chemical intermediate in the production of acrylic acid and its esters. Acrolein is also used directly as an aquatic herbicide and algicide in irrigation canals, as a microbiocide in oil wells, liquid hydrocarbon fuels, cooling-water towers and water treatment ponds, and as a slimicide in the manufacture of paper.

For an accessible summary of what is known about the sources and health risks of formaldehyde, see the California Air Resources Board's Toxic Air Contaminant Summary. Extensive information about this chemical is provided in Scorecard's chemical profile.