Men living in areas with polluted air may be more likely to develop lung cancer, according to scientists.
Heavily polluted areas may carry a health risk
Norwegian experts who looked at more than 16,000 men over almost three decades found that those in more polluted areas were more at risk.
The stronger the concentration of nitrogen dioxide, the greater the chance of developing the disease.
However, smoking remains the biggest risk factor for developing lung cancer, say experts.
It has always been tough to pin down the health effects of air pollution, although evidence linking it to cancer has emerged before.
However, the large scale study has produced clear signs that pollution may influence lung cancer.
During the follow-up period of 27 years, more than 400 men from the group developed lung cancer.
The researchers altered their results to take account of smoking habits - but found that, for every rise in concentration of the pollutant nitrogen dioxide (NO2) around their home compared to their home at the start of the study, there was roughly an 8% increase in risk.
However, there was no increase in cancer risk linked to sulphur dioxide (SO2) levels.
This still means their overall risk of lung cancer is relatively small - but the researchers, from the Norwegian Institute of Public Health, believe that it is clear evidence.
In the journal Thorax, they wrote: "Urban air pollution may increase the risk of developing lung cancer in men.
"Compared with smoking the association between air pollution and lung cancer is weak but of expected size."
However, they said that further, similar research was needed to make sure no other factor was at work in these cases.
Professor Stephen Spiro, of the British Thoracic Society, said: "This is yet
another study which reinforces a small but significant link between urban air
pollution and the risk of developing lung cancer.
"Air pollution does not discriminate between young and old - it is therefore
essential that we all work together to minimise pollution levels.
"We must also conduct further in-depth studies investigating this link."