Do brain cancer rates correlate with ambient exposure levels of criteria air pollutants or hazardous air pollutants (HAPs)?

Age-adjusted brain cancer rates vary more than fourfold among US counties, but risk factors underlying this variation have not been identified. The only known risk factor for brain cancer is ionizing radiation at therapeutic dose levels. Lifestyle factors are not known to affect brain cancer risk; hypothetical risks from environmental or occupational exposures to numerous chemicals have been suggested, but no consistent results have emerged in populations exposed to chemicals via oral intake or inhalation. We investigated whether county-by-county brain cancer incidence rates and mortality rates were correlated with patterns of local ambient air pollution. We utilized central-site ambient measurement data for criteria air pollutants, and inhalation exposure concentrations of hazardous air pollutants (HAPs) as predicted by the US EPA National Air Toxics Assessment. We selected 30 HAPs that had been variously hypothesized in the context of a link to brain cancer risk. We also included a regression on density of domestic cattle populations given that viruses have been implicated in brain cancer and cattle are a well-known reservoir for a variety of viruses. The analyses yielded correlations that were mostly null or negative, with similar results obtained for brain cancer incidence and mortality rates. The cattle density correlations were slightly positive and statistically significant. Given the limitations of this exploratory analysis, further research is needed to investigate the role of environmental chemical exposures in brain cancer risk. Our findings suggest that county-by-county differences in air pollution exposure concentrations are not likely to be explanatory factors for variations in brain cancer rates.