NATURALLY OCCURING ASBESTOS IN CALIFORNIA AND MESOTHELIOMA: IS THERE A LINK?

Background: An association between mesothelioma and residential proximity to ultramafic rock, considered a proxy for natural occurring asbestos (NOA), was reported in a California study. Many factors can influence asbestos exposure and mesothelioma that should be considered in assessing health risks. If the reported association truly reflects a health risk, it could have broad public health and land use implications.

Methods: We examined various factors to assess how they might affect an association between ultramafic rock, residential location, and mesothelioma including: 1) analysis of population migration patterns; 2) evaluation of “usual job title” for characterizing historical occupational asbestos exposure; 3) assessment of wind monitoring data, meteorological conditions, and air dispersion patterns; and 4) analysis of simulated data to assess the effects of environmental exposure and occupational misclassification.

Results: Population migration patterns indicate that up to 72% of observed mesothelioma cases likely had relevant asbestos exposure at an address prior to their residence at diagnosis. Our simulation studies show that the ecologic type analysis used in the California study can yield a significant association between mesothelioma and proximity to ultramafic rock as an artifact of occupational exposure misclassification combined with differential migration patterns of mesothelioma cases and controls. Wind monitoring data revealed patterns with dominant directionality that should be accounted for in exposure assessment.

Discussion: Severe exposure misclassification is likely in studies of mesothelioma that use current address as a proxy for asbestos exposure. Population migration, prior occupational history, and weather/physical factors evaluated here as well as simulation study results suggest that the reported association between residential proximity to ultramafic rock and mesothelioma, is not indicative of causal association or a health effect attributable to NOA. Future research must characterize human activities, wind and weather patterns, and indoor and outdoor airborne asbestos concentrations to assess potential health risks of NOA.