People who have never smoked, but who live in areas with higher air pollution levels, are roughly 20 percent more likely to die from lung cancer than people who live with cleaner air, researchers conclude in a new study.
"It's another argument for why the regulatory levels (for air pollutants) be as low as possible," said Francine Laden, a professor at the Harvard School of Public Health, who was not involved in the research.
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.
The participants lived in all 50 states and in Puerto Rico, and based on their zip codes, the researchers estimated how much air pollution they were exposed to -- measured in units of micrograms of particles per cubic meter of air.
Pollution levels in different locations ranged from a low of about six units to a high of 38. The levels dropped over time, however, from an average of 21 units in 1979 - 1983, to 14 units in 1999 - 2000, producing an overall average pollution level of 17 units across the study period.
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 percent.
The increased risk for lung cancer associated with pollution is small in comparison to the 20-fold increased risk from smoking.
And the study team didn't prove that the pollution caused the cancer cases, but "there's lots of evidence that exposure to fine particles increases cardiopulmonary mortality," Turner told Reuters Health.
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.
Previous research has suggested similar conclusions. A study of people in China, for example, found an increased risk of lung cancer attributed to indoor air pollution from burning coal and wood to heat homes (see Reuters story of December 7, 2009). And several European studies have linked levels of soot and vehicle exhaust to lung cancer in non-smokers.
Laden noted that the pollution levels associated with the increased risk of cancer in the current study are not uncommon in the U.S.
"These levels are within the (regulatory) standards," Laden told Reuters Health. "We're not talking about people who live in a really polluted place with no pollution control.