Though smoking is the number one cause of lung cancer, about one in 10 people who develop lung cancer have never smoked.
"Lung cancer in 'never smokers' is an important cancer. It's the sixth leading cause of cancer in United States," said Michelle Turner, the lead author of the study and a graduate student at the University of Ottawa.
Previous estimates of how many non-smokers get lung cancer range from 14 to 21 out of every 100,000 women and five to 14 out of every 100,000 men.
The fine particles in air pollution, which can irritate the lungs and cause inflammation, are thought to be a risk factor for lung cancer, but researchers had not clearly teased apart their impact from that of smoking.
In this study, Turner and her colleagues followed more than 180,000 non-smokers for 26 years. Throughout the study period, 1,100 people died from lung cancer.
After the team took into account other cancer risk factors, such as second-hand smoke and radon exposure, they found that for every 10 extra units of air pollution exposure, a person's risk of lung cancer rose by 15 to 27 per cent.
The increased risk for lung cancer associated with pollution is small in comparison to the 20-fold increased risk from smoking.
Fine particles in air pollution can injure the lungs through inflammation and damage to DNA, Turner's team writes in its report, published in the American Journal of Respiratory and Critical Care Medicine