Smoking may be the number one cause of lung cancer, but one in ten people with the disease are non-smokers. A new study sheds light on what could be contributing to cases of lung cancer in people who have never smoked. The study found non-smokers who live in areas with high pollution levels are 20 percent more likely to die from lung cancer than people who live with cleaner air.

Researchers followed more than 180,000 non-smokers for 26 years. During that time, 1,100 people followed in the study died of lung cancer. Researchers then estimated how much air pollution they were exposed to. After taking into account second-hand smoke and radon exposure, the study found that for every 10 extra units of air pollution exposure, the risk of lung cancer increased by 15 to 27 percent.

The study in the American Journal of Respiratory and Critical Care Medicine did not prove that pollution causes cancer but researchers say there is evidence that exposure to pollution increases the risk of heart and lung problems. Authors of the latest study say the pollution levels connected with higher cases of lung cancer are not uncommon and are within regulatory standards.