Residential traffic exposure and childhood leukemia: a systematic review and meta-analysis

Abstract

Context: Exposure to elevated concentrations of traffic-related air pollutants in the near-road environment is associated with numerous adverse human health effects, including childhood cancer, which has been increasing since 1975. Results of individual epidemiologic studies have been inconsistent. Therefore, a meta-analysis was performed to examine the association between residential traffic exposure and childhood cancer.

Evidence acquisition: Studies published between January 1980 and July 2011 were retrieved from a systematic search of 18 bibliographic databases. Nine studies meeting the inclusion criteria were identified. Weighted summary ORs were calculated using a random effects model for outcomes with four or more studies. Subgroup and sensitivity analyses were performed.

Evidence synthesis: Childhood leukemia was positively associated (summary OR=1.53, 95% CI=1.12, 2.10) with residential traffic exposure among seven studies using a postnatal exposure window (e.g., childhood period or diagnosis address) and there was no association (summary OR=0.92, 95% CI=0.78, 1.09) among four studies using a prenatal exposure window (e.g., pregnancy period or birth address). There were too few studies to analyze other childhood cancer outcomes.

Conclusions: Current evidence suggests that childhood leukemia is associated with residential traffic exposure during the postnatal period, but not during the prenatal period. Additional well-designed epidemiologic studies that use complete residential history to estimate traffic exposure, examine leukemia subtypes, and control for potential confounding factors are needed to confirm these findings. As many people reside near busy roads, especially in urban areas, precautionary public health messages and interventions designed to reduce population exposure to traffic might be warranted.

Figure 1

Forest plot of case-control studies examining the association between residential traffic exposure assessed during the postnatal period^a and childhood leukemia, and the random effects weighted summary OR and 95% CI Note: The weighted summary OR is derived from the random effects model. For each study, the center of the box denotes the OR, the horizontal line denotes the 95% CI, and the size of the box is proportional to the study’s weight in the calculation of the overall effect. The weighted summary OR is denoted by the center of the diamond and the 95% CI is denoted by the points of the diamond. ^aIncludes studies that assessed residential traffic exposure throughout the childhood period, at the time of diagnosis, or at the address of longest duration between birth and diagnosis ^bBecause of asymmetric 95% CIs due to rounding, either the lower (Amigou and Langholtz) or upper (Raaschou-Nielsen and Steffen) confidence limit was selected to calculate the SE and the opposite confidence limit Thus, the 95% CI values shown for these four studies are not identical to those of the published results.