Childhood leukemia: bad air linked to increased risk. (Science Selections).

Researchers in the Environmental Health Investigation Branch of the California Department of Health Services have discovered a possible association between exposure to hazardous air pollutants (HAPs) and the incidence of childhood leukemia [EHP 111:663-668]. Their epidemiologic evaluation suggests that children living in areas of high ambient air pollution are at increased risk of developing leukemia.

Peggy Reynolds and her coauthors set out to evaluate whether childhood cancer rates were elevated in areas estimated to have high exposure to potentially carcinogenic HAPs. They used the population-based California Cancer Registry to gather information on all cancer cases diagnosed in children under age 15 from 1988 to 1994. They used a geographic information system to map nearly 7,000 childhood cases within individual California census tracts. Their analysis also examined the incidence of the most common childhood cancers--acute lymphocytic leukemia, acute nonlymphocytic leukemia, and gliomas (brain tumors).

On the pollution side of the equation, the investigators focused on 25 of the 189 HAPs identified in 1990 as potential human carcinogens by the U.S. Environmental Protection Agency (EPA). These 25 compounds--which included benzene, dioxins, lindane, and vinyl chloride--were selected because they had the best information on their potential to cause cancer via inhalation. The investigators also utilized an EPA dispersion model that combined 1990 emissions inventories with meteorologic data to estimate the annual HAP concentration for each census tract in the country.

Following the EPA model, they estimated which California census tracts had the greatest HAP exposures. They calculated census tract emission scores for all sources combined, as well as for three distinct source categories: mobile sources (such as motor vehicles, planes, trains, and ships), area sources (such as dry cleaners, gas stations, residences, farm pesticide use, and forest fires), and point sources (large industrial manufacturing facilities). For each of these emission source groups, they further calculated exposure scores for each census tract by multiplying the modeled air concentration by the corresponding inhalation unit risk factor for each HAP. The inhalation unit risk factor combines the cancer potency for each compound with standard assumptions for body weight and breathing rate.

When they ran the exposure score data and the cancer case incidence data through statistical analysis, they found little evidence of an increased risk of gliomas. However, they did find the risk of both types of leukemia to be elevated by 21% in census tracts with the greatest overall HAP exposure. More disturbingly, they found the most dramatically elevated childhood leukemia incidence rates--a 32% increase--within census tracts with the highest HAP exposure from industrial facilities. The association was even greater in children aged 0-4 years, which the investigators speculate may be due to the fact that younger children tend to spend more time at home than older ones.

Of course, many other factors could contribute to the development of cancer in children, including individual susceptibility and exposure to indoor pollutants such as environmental tobacco smoke. The authors acknowledge the inherent limitations of their investigation, but conclude that it "suggests that background air quality, as estimated by HAPs, may be associated with incidence of childhood leukemia." They have begun a follow-up study focusing more closely on the relationship between cumulative exposure to HAPs and childhood leukemia, which will include questionnaire information on personal activity patterns and indoor pollution sources.