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

Table 4 lists the distribution of exposure scores by census tract for each emission source group and the associated RRs obtained from Poisson regression. For each of the four source groups, we found the exposure scores for census tracts at the highest level ([greater than or equal to] 90th percentile) to be at least three times greater than the exposure scores for tracts at the lowest or reference level (< 25th percentile). Figure 2 shows the spatial distribution of the combined source exposure score by census tract in California. As expected, the most densely populated areas of the state have the highest combined exposure scores including Los Angeles and the San Francisco Bay area. Figure 3 is an enlarged map of the combined exposure score by census tract in Los Angeles and Orange Counties; local variations in exposure scores between census tracts are more apparent at this scale.

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For the combined source group, the cancer incidence rates in the highest HAP exposure census tracts were a modest 6% higher than those in the lowest HAP exposure areas [RR = 1.06; 95% confidence interval (CI), 0.97-1.16]. For the leukemias, the RR for the highest combined source group exposure category was 1.21 (95% CI, 1.03-1.42). The trend from the lowest to highest exposure levels for the combined source group and leukemias was statistically significant (p < 0.05). We also ran initial models for each of the emission source groups, adjusting only for age, race/ethnicity, and sex. For the point-source group, the RR for leukemia at the highest exposure level ([greater than or equal to] 90th percentile) was 1.32 (95% CI, 1.11-1.57). Again, the trends from the lowest to highest exposure levels were statistically significant (p < 0.05). We saw some suggestion of a stronger effect in younger children. Among children diagnosed with leukemia younger than 5 years, the RR for the highest point-source exposure category was 1.45 (95% CI, 1.17, 1.79), somewhat higher than the RR for children 5-14 years old, for whom the RR was 1.18 (95% CI, 0.88-1.58; data not shown).

Poisson regression using a spline (with six join points) for point-source exposure score conveys the same impression of an increasing leukemia rate with increasing score (Figure 4), although with a leveling off above a score of 100, which includes only seven census tracts (0.1%). A likelihood-ratio test for the spline terms (adjusting for age, race/ethnicity, and sex) has a p-value of 0.02. In contrast, the spline regression for point-source exposure and glioma shows no distinct trend (Figure 5), consistent with the glioma RRs for exposure categories in Table 4.

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Because childhood leukemias were the most common cancer type, we further examined these malignancies by specific subtypes. Among the leukemias, 1,938 were ALL and 368 were ANLL. The RR for ALL was 1.19 (95% CI, 1.00-1.43) for the highest exposure level of the combined source group. For ANLL, the RR was 1.46 (95% CI, 0.97-2.19) at the highest exposure level of the combined source group (data not shown).