Previous research had shown that gulls living close to steel mills in the Great Lakes region had increased rates of DNA mutations. The exact role of air pollutants was unclear, however, because the birds may have also been exposed to toxins in their water supply. In an attempt to elucidate the effect of air pollution, Christopher M. Somers of McMaster University in Hamilton, Ontario, and his colleagues housed two groups of mice in separate locations for 10 weeks. The first group was situated one kilometer downwind from two steel mills, and the second was placed in a rural setting 30 kilometers away. After being returned to the laboratory, the mice and their offspring were tested for genetic mutations. The researchers found that the mice that had lived near the mills had smaller litters, on average, than did those that lived in the country. What is more, the so-called steel mice exhibited twice as many DNA mutations as the control animals did, with the majority originating from the fathers. The authors conclude that "this is the first demonstration of heritable mutation induction in any organism as a result of ambient air pollution exposure."