Air Quality and Health and Welfare

2.7 Toxic Materials

2.7.1 Introduction
In addition to contributing to the health and welfare problems associated with exceedances of the air quality standards for ozone and PM10, emissions from almost all sources also include a number of air pollutants that increase the risk of cancer or have other negative health effects. These air pollutants include benzene, formaldehyde, acetaldehyde, 1,3-butadiene, and diesel particulate matter. All of these compounds are products of combustion; benzene is also found in non-exhaust emissions from gasoline-fueled vehicles.

There are hundreds of different compounds and elements that are known to be emitted from passenger cars, on-highway trucks, and various nonroad equipment. The US EPA has recently proposed a methodolofy for identifying which of these compounds and elemenyts are toxic and has developed a preliminary mobile source air toxics (MSAT) list.

In identifying compounds for the list of MSAT, EPA first compiled all available recent studies which speciated emissions from motor vehicles and their fuels. To do this, EPA reviewed a number of databases that contain information on the various species of compounds emitted from motor vehicles and their fuels. It identified recent (less than ten years old) speciation profiles for emissions from light-duty gas vehicles (LDGV), heavy-duty diesel vehicles (HDDV), heavy-duty gasoline vehicles (HDGV), gasoline powered nonroad engines, and turbine engine aircraft. Forty-four speciation studies were found that met this age criteria. All of these speciation profiles attempt to accomplish more or less the same objective: separating and identifying the compounds that comprise the hydrocarbon portion and particulate phase of mobile source emissions.

Then the EPA looked to the Integrated Risk Information System (IRIS), which is an EPA database of scientific information that contains the Agency consensus scientific positions on potential adverse health effects that may result from lifetime (chronic) or short-term (acute) exposure to various substances found in the environment (EPA IRIS Database, IRIS currently provides health effects information on over 500 specific chemical compounds. The information contained in the IRIS database includes an EPA finding for each compound that: (1) there is a health hazard, either cancer or noncancer, associated with exposure to the compound, (2) the compound is noncarcinogenic based on current data, or (3) the data is insufficient to determine if the compound is a hazard.

IRIS contains chemical-specific summaries of qualitative and quantitative health information. IRIS information may include the reference dose (RfD) for noncancer health effects resulting from oral exposure, the reference concentration (RfC) for noncancer health effects resulting from inhalation exposure, and the carcinogen assessment for both oral and inhalation exposure. Combined with information on specific exposure situations, the summary health hazard information in IRIS may be used in evaluating potential public health risks from environmental contaminants.

Before a substance is listed on the IRIS database, it goes through a thorough scientific evaluation. This consensus and review process, managed by EPA’s Office of Research and Development (ORD), consists of (1) an annual Federal Register announcement of the IRIS agenda and a call for scientific information from the public on the selected chemical substances, (2) a search of the current literature, (3) development of health assessment and draft IRIS summaries, (4) internal EPA peer review, (5) external peer review, (6) Agency consensus review and management approval within EPA, (7) preparation of final IRIS summaries and supporting documents, and (8) entry of summaries and supporting documents into the IRIS database.

By comparing the list of compounds in IRIS to the motor vehicle emissions identified in the speciation studies, EPA identified 21 MSAT as listed in table 2.7.1-1 below. Each of these pollutants are known, probable, or possible human carcinogens (Group A, B or C) or were considered by the Agency to pose a risk of adverse noncancer health effects.
2.7.1-1 Proposed List of Mobile Source Air Toxics (MSATs)

Notes about table 2.7.1-1:

1. Although the different species of the same metal differ in their toxicity, the onroad mobile source inventory contains emissions estimates for total compounds of the metal identified in particulate speciation profiles (i.e., the sum of all forms).

2. This entry refers to two large groups of chlorinated compounds. In assessing their cancer risks, their quantitative potencies are usually derived from that of the most toxic, 2,3,7,8-tetrachlorodibenzodioxin.

3. MTBE is listed due to its potential inhalation air toxics effects and not due to ingestion exposure associated with drinking water contamination.

4. Polycyclic Organic Matter includes organic compounds with more than one benzene ring, and which have a boiling point greater than or equal to 100 degrees centigrade. A group of seven polynuclear aromatic hydrocarbons, which have been identified by EPA as probable human carcinogens (benz(a)anthracene, benzo(b)fluoranthene, benzo(k)fluoranthene, benzo(a)pyrene, chrysene, 7,12-dimethylbenz(a)anthracene, and indeno(1,2,3-cd)pyrene) are sometimes used as a surrogate for the larger group of POM compounds.