The mature male mice, genetically identical, were kept in a modified shed 2 km from two integrated steel mills and 1 km from a major highway in Ontario, Canada.

According to the researchers, the mutation induction from breathing particular air pollution was similar to that seen in mice after acute exposure to 0.5–1 Gy of X-rays.

The implications of the findings for human health are unclear, say the researchers. "Heritable mutation, germ-line DNA damage and epigenetic modifications have the potential to affect disease incidence in the descendents of exposed individuals," write the scientists in a paper in PNAS.

"Further work is needed before the potential repercussions to human health can be evaluated," added a spokesman for Health Canada.

The mechanism causing the mutation in mice is also uncertain. "It is unknown whether chemical agents in air pollution directly reach the gonads and cause DNA damage and mutation, or whether there is an indirect event elsewhere that results in physiological changes that destabilize the germ line," wrote the scientists.

The three most abundant metals in the air were iron, copper and manganese, while the three most plentiful polycyclic aromatic hydrocarbons (PAHs) were benzo(b)-fluoranthene, benzo(k)fluoranthene and indeno(123-cd)pyrene. The highest PAH concentration during the study was 8.3 ng/m3, the highest metal level was 3.6 µg/ m3, and the highest total suspended particles reading was 93.8 µg/ m3.

"Previous work has focused almost exclusively on the mutagenic hazard of PAHs in this environment," write the researchers. "Our findings suggest that other chemicals (eg metals), and particles in general, may play a more relevant role in germ-line mutagenesis than previously anticipated."

The researchers were from Health Canada, McMaster University, University of Lethbridge, all in Canada, Maastricht University in the Netherlands, and the US National Center for Toxicological Research.