Ra. Morello-frosch et al., Air toxics and health risks in California: The public health implications of outdoor concentrations, RISK ANAL, 20(2), 2000, pp. 273-291
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.