Childhood cancer incidence rates and hazardous air pollutants in California: an exploratory analysis - Children's Health

Environmental Health Perspectives, April, 2003 by Peggy Reynolds, Julie Von Behren, Robert B. Gunier, Debbie E. Goldberg, Andrew Hertz, Daniel F. Smith

When we restricted the analysis to urban block groups, the resulting point estimates were similar (data not shown). The confidence intervals were wider, because of the smaller number of census tracts included in the analysis, but the findings remained consistent. Similarly, when we included a measure of socioeconomic status (quartile of census tract median family income) in the original models, we observed no substantial differences in point estimates (data not shown).

Discussion

For all sites combined, we found no significant childhood cancer excess within census tracts in the highest exposure category for all HAPs combined. However, RRs for the leukemias appeared to be elevated in these tracts. The combined HAP exposure score provides an overall ambient air quality estimate because it includes emissions from mobile, area, and point sources. When calculating the exposure scores separately by emission source, we observed the most dramatically elevated childhood cancer and leukemia incidence rates within census tracts in the highest exposure category for point sources. Census tracts in the highest exposure categories for area- and mobile-source HAPs also had slightly elevated leukemia rates. Although these elevated rates were not statistically significant, the point estimates were similar in magnitude to those we observed for the combined score and point sources.

Given the highly correlated nature of the exposure data, we cannot ascertain clearly whether any one source or any one chemical is driving the observed associations. However, it is interesting to note that we observed the highest RRs for leukemia in census tracts ranked highest in exposure to HAPs emitted from point sources for which the main compounds contributing to the exposure scores were benzene and perchloroethylene. This more striking increased risk association for leukemia with point-source emissions is consistent with the known benzene exposure risk for leukemia in adults (IARC 1982; Rinsky et al. 1987). However, mobile (benzene) and area (perchloroethylene) sources also emit these same compounds in large quantities. We noted some, but not extensive, overlap between tracts categorized in the highest exposure groups for mobile, area, and point sources. For example, 36% of the highest point-source census tracts were also classified in the highest exposure category for area sources. However, only 19% of the highest point-source tracts fell into the highest exposure category for mobile sources. The census tracts in the highest exposure category for point sources were concentrated in heavily industrialized, urban counties; 70% of the tracts were in Los Angeles County alone.

In this study, the magnitude of the theoretical cancer risk, as estimated by the exposure scores used in our analysis, did not predict the resulting RRs for childhood malignancies. On the basis of the exposure scores alone, we would have expected the highest RRs to be for mobile sources and all sources combined, rather than for point sources as we observed. It is important to keep in mind that these theoretical risks are predicated primarily on cancer potency values derived from animal data and limited human health studies of adult cancers, which include tumor types quite different from those most common in children. Because of this and because of the uncertainties involved in extrapolating from high exposure levels in animal and occupational studies to lower exposure levels in ambient air, we are uncertain to what degree these estimates might apply to childhood cancers.