Previous studies have provided evidence for an association between exposure to high levels of air pollution and increased DNA damage in human sperm. In these studies DNA damage was measured using the sperm chromatin structure assay (SCSA) wherein the percentage of sperm with abnormal chromatin/fragmented DNA is determined and expressed as % DNA fragmentation index (%DFI). Here we extend these observations to address the following hypothesis: men who are homozygous null for glutathione-S-transferase M1 (GSTM1-) are less able to detoxify reactive metabolites of carcinogenic polycyclic aromatic hydrocarbons (c-PAHs) found in air pollution. Consequently they are more susceptible to the effects of air pollution on sperm chromatin. Using a longitudinal study design in which men provided semen samples during periods of both low (baseline) and episodically high air pollution, this study revealed a statistically significant association between GSTM1 null genotype and increased SCSA-defined %DFI (beta=0.309; 95% CI: 0.129, 0.489). Furthermore, GSTM1 null men also showed higher %DFI in response to exposure to intermittent air pollution (beta=0.487; 95% CI: 0.243, 0.731). This study thus provides novel evidence for a gene-environment interaction between GSTM1 and air pollution (presumably c-PAHs). The significance of the findings in this study with respect to fertility status is unknown. However, it is biologically plausible that increases in %DFI induced by such exposures could impact the risk of male sub/infertility, especially in men who naturally exhibit high levels of %DFI.