ACLs are typically derived from health criteria for the substance in question. They are usually expressed as concentrations such as micrograms per cubic meter (^g/cum). Health criteria are generally expressed in terms of dose— the weight of the pollutant taken into the body divided by the weight of the body. To convert a dose into a concentration, assumptions must be made about average breathing rates, average consumption of food and water, and the amount of each that is available to the body (adsorption factors). The EPA has a generally accepted procedure for this process (U.S. EPA 1988, 1989).
Other methods of deriving ACLs are based upon an absolute threshold (CMA 1988). These methods set ACLs at some fraction of an observed threshold or established guideline. A margin of safety is generally added depending on the type and severity of the effect on the body, the quality of the data, and other factors. Still other methods depend upon extrapolation from higher limits established for other similar purposes.
The health criteria felt most appropriate for deriving ACLs is the risk reference dose (RfD) established by the EPA (Patrick 1994). The EPA has developed RfDs for both inhalation and ingestion pathways (U.S. EPA 1986). They require much effort to establish and are generally designed for long-term health effects.
RfDs are developed for ingestion and inhalation exposure routes. If a relevant inhalation RfD is available, regulatory agencies should use it as the basis for deriving an ACL for an air pollutant. The EPA is currently deriving reference values for inhalation health effects in terms of micrograms per cubic meter. These risk reference concentrations (RfCs) provide a direct link with ACLs. Without more specific information on inhalation rates for the target population, regulators frequently assume the volume of air breathed by an average member of a typical population to be 20 cubic meters per day, which is considered a conservative value.
When an inhalation RfD is not available, regulators must derive an ACL from another source. One approach is to use an ingestion RfD to estimate an RfC. However, this technique can be inaccurate because absorption through the digestive system is different from absorption through the respiratory system.
RfDs and RfCs are available through the EPA's Integrated Risk Information System (IRIS). Many state and local regulatory agencies use the EPA-derived RfDs and RfCs to establish ACLs. These reference values are available through the EPA's National Air Toxics Information Clearing House (NATICH). Because of the large number of state and local agencies, NATICH does not always have the latest information. Therefore, the practicing engineer should get the latest information directly from the local agency.
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