The comet assay as a tool in human biomonitoring exposure to combustion-derived air pollution - a systematic review and meta-analysis.

Humans are exposed to environmental or occupational air pollution from combustion emissions in outdoor and indoor environments. Irrespective of the sources, combustion emissions are characterized by being a complex mixture of particles, volatile compounds and gases. The present systematic review summarizes results on DNA strand breaks measured by the comet assay in leukocytes, from studies on human exposure to traffic-related vehicle exhaust, biomass combustion and coke oven work environments. These exposures have in common the combustion of fuel, which generates particles and polycyclic aromatic hydrocarbons. Standardized mean differences (SMDs) have been calculated by random effects models. Meta-analyses show increased levels of DNA strand breaks in studies on traffic-related exhausts (SMD = 0.62, 95% CI: 0.36, 0.89, n = 21), biomass combustion (1.73, 95% CI: 0.72, 2.74, n = 10) and coke oven emission (0.84, 95% CI: 0.30, 1.37, n = 10). Studies from high-income countries have reported much smaller differences in DNA strand break levels than have studies from middle-income countries. These differences may be attributed to higher exposures related to less strict emission control, and more susceptible populations in middle-income populations; unrecognized confounding despite efforts to match subjects on traditional confounders; or higher risk of comet assay measurement bias and exposure misclassification. In conclusion, this systematic review and meta-analysis show that exposure to combustion-derived air pollution, with clear exposure gradients in terms of particulate matter or polycyclic aromatic hydrocarbons, is associated with increased levels of DNA strand breaks in human leukocytes.