Toxic air contaminants in urban atmospheres: Experience in California

In addition to the criteria gaseous and particulate air pollutants which have been the subject of intensive regulation for many years in the U.S., there exists in the atmosphere of cities and surrounding areas a number of trace toxic contaminants which are of increasing public health and regulatory concern. In California, these Toxic Air Contaminants (TACs) are assessed and regulated by a multi-step process required by legislation. Risk assessment for chemicals which are considered potential TACs involves the gathering and analysis of information on emissions, exposures, toxicology, and epidemiology by two California Agencies, the Air Resources Board and Department of Health Services (now linked by the California Environmental Protection Agency) and an independent Scientific Review Panel. Eighteen chemicals have been designated as TACs since the process started in 1982, including perchloroethylene, formaldehyde, vinyl chloride, and 1,3-butadiene which are mentioned in some detail in this review. Future challenges for risk assessment and management are posed by such issues as gross mixtures, for example, from products of incomplete combustion; transport and deposition out of the originating air basin; contributions of natural sources to ambient levels; and the impact of the list of 189 hazardous air pollutants in the 1990 U.S. Clean Air Act Amendments on California's TAC identification-regulation process. The issues involved in a vigorous pursuit of risk reduction from TACs are discussed based upon experiences in California.