Ambient Concentration Limits

Air pollution control strategies for toxic air pollutants are frequently based on ambient concentration limits (ACLs). ACLs are also referred to as acceptable ambient limits (AALs) and acceptable ambient concentrations (AACs). A regulatory agency sets an ACL as the maximum allowable ambient air concentration to which people can be exposed. ACLs generally are derived from criteria developed from human and animal studies and usually are presented as weight-based concentrations in air, possibly associated with an averaging time.

The EPA uses this approach for criteria air pollutants but not for toxic air pollutants. The CAA Amendments of 1970 require the EPA to regulate toxic air pollutants through the use of national emission standards. The 1990 amendments continue and strengthen this requirement. However, state and local agencies make extensive use of ACLs for regulatory purposes. This extensive use is because, for most air pollutants, ACLs can be derived easily and economically from readily available health effects information. Also, the maximum emission rate for a source that corresponds to the selected ACL can be determined easily through mathematical modeling. Thus, the regulator can determine compliance or noncompliance. Lastly, the use of ACLs relieves regulators from identifying and specifying acceptable process or control technologies.

ACLs are frequently derived from occupational health criteria. However, ACLs are susceptible to challenge because no technique is widely accepted for translating standards for healthy workers exposed for forty hours a week to apply to the general population exposed for twenty-four hours a day. Another disadvantage of ACLs is that both animal and occupational exposures, from which health criteria are developed, are typically at concentrations greater than normal community exposures. This difference requires extrapolation from higher to lower dosages and often from animals to humans.

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