In a study of 6,989 cancer cases among children in California between 1988 and 1994, researchers looked for a correlation with the level of exposure to 25 hazardous air pollutants. Using U.S. Environmental Protection Agency (EPA) data that take into account emissions of pollutants and weather patterns to estimate exposure in a given census tract, the researchers matched exposure levels with the specific home address of each cancer patient. The sources of pollution included mobile sources (e.g., motor vehicles, airplanes), area sources (e.g., dry cleaners, gas stations), and point sources (large industrial manufacturing facilities).

When all cancers were analyzed, rates of cancers occurring in census tracts with the highest pollution levels were a modest 6% higher than those in the lowest pollution levels. However, when childhood leukemias‹the most common cancer type in the study‹were analyzed separately, the incidence was 32% higher for those children exposed to more point source pollutants. The incidence of acute nonlymphocytic leukemia was 46% higher at the highest exposure level to all polluants. Overall risk was further elevated among children under age five. The greatest exposure to the air pollutants studied occurred in the most densely populated areas of the state, including Los Angeles and the San Francisco Bay area.

"Because childhood cancers are likely to have shorter latency periods than tumors found in adults, they lend themselves more easily to this type of epidemiologic analysis," the study authors wrote. "Our findings suggest an association between increased childhood leukemia rates and high hazardous air pollutants exposure, but studies involving more comprehensive exposure assessment and individual-level exposure data are important for elucidating this relationship."

Research to date has failed to firmly establish risk factors for childhood cancer, with a few exceptions. However, changes to the U.S. Clean Air Act in 1990 allowed the EPA to develop a database that combines emissions with meteorological data into a dispersion model that estimates the annual average concentrations of 189 separate hazardous air pollutants. The highest cancer incidence rates in this study correlated with the highest exposures from point sources.

Commenting on the study, Dr. Jim Burkhart, science editor for EHP, says, "As we look for ways to improve air quality around the country, this is a significant study. It suggests rather plainly that our daily exposure to pollutants, particularly in urban areas, may be having an impact on the health of our children and in the rates of some of the most serious childhood illnesses."

The study was conducted by Peggy Reynolds, Julie Von Behren, Robert B. Gunier, and Daniel F. Smith of the California Department of Health Services, along with Debbie E. Goldberg of the Public Health Institute and Andrew Hertz of Impact Assessment, Inc.

EHP is the journal of the National Institute of Environmental Health Sciences, part of the U.S. Department of Health and Human Services. More information is available online at http://www.ehponline.org/.