The risk of lung cancer in nonsmokers has doubled, according to new research, and doctors are at a loss to explain the increase.

The new British study found that the rate of the most common type of lung cancer doubled in nonsmokers over six years and that two-thirds of the cases occurred in women.

The study confirms similar findings in the U.S., where researchers also have found that the rate of lung cancer in nonsmokers is rising.

The UK researchers analyzed the records of 2,170 patients who underwent lung cancer surgery between 2008 and 2014 and found that the rate of the disease in nonsmokers increased steadily over the six years, with women accounting for 67 percent of the increase in cases.

The researchers speculated that mutations in genetics might have something to do with the increase. Second-hand smoke, radon, or air pollution might be contributors, they say.

Lung cancer is the leading cause of cancer death in both men and women.

Current and former smokers are typically screened for the disease, but the new findings show that nonsmokers should also be considered at risk, said researchers, who presented their findings at the World Conference on Lung Cancer.