Air toxics and health risks in California: The public health implications of outdoor concentrations

Of the 188 hazardous air pollutants (HAPs) listed in the Clean Air Act, onl y a handful have information on human health effects, derived primarily fro m animal and occupational studies. Lack of consistent monitoring data on am bient air toxics makes it difficult to assess the extent of low-level, chro nic, ambient exposures to HAPs that could affect human health, and limits a ttempts to prioritize and evaluate policy initiatives for emissions reducti on. Modeled outdoor HAP concentration estimates from the U.S. Environmental Protection Agency's Cumulative Exposure Project were used to characterize the extent of the air toxics problem in California for the base year of 199 0. These air toxics concentration estimates were used with chronic toxicity data to estimate cancer and noncancer hazards for individual HAPs and the risks posed by multiple pollutants. Although hazardous air pollutants are u biquitous in the environment, potential cancer and noncancer health hazards posed by ambient exposures are geographically concentrated in three urbani zed areas and in a few rural counties. This analysis estimated a median exc ess individual cancer risk of 2.7E(-4) for all air toxics concentrations an d 8600 excess lifetime cancer cases, 70% of which were attributable to four pollutants: polycyclic organic matter, 1,3 butadiene, formaldehyde, and be nzene. For noncancer effects, the analysis estimated a total hazard index r epresenting the combined effect of all HAPs considered. Each pollutant cont ributes to the index a ratio of estimated concentration to reference concen tration. The median value of the index across census tracts was 17, due pri marily to acrolein and chromium concentration estimates. On average, HAP co ncentrations and cancer and noncancer health risks originate mostly from ar ea and mobile source emissions, although there are several locations in the state where point sources account for a large portion of estimated concent rations and health risks. Risk estimates from this study can provide guidan ce for prioritizing research, monitoring, and regulatory intervention activ ities to reduce potential hazards to the general population. Improved ambie nt monitoring efforts can help clarify uncertainties inherent in this analy sis.