That’s according to a new study by the American Cancer Society and the National Cancer Institute, published Wednesday in the New England Journal of Medicine.

Historically, men have had a higher rate of lung cancer, due in large part to men being more likely to be cigarette smokers. That remained true through the ‘90s—between 1995 and 1999, the number of cases of lung cancer per 100,000 women per year was 26% lower than for men. The same age group, 44 to 49, saw a change starting in 2010: the incidence of lung cancer was suddenly 8% higher in women than men. This was particularly true for non-Hispanic white and Hispanic women.

Before you start to worry, though, the overall rate of lung cancer has actually decreased in both groups. The difference is that the declines have been more marked in men than women. But while women have begun to take up smoking at a higher rate than before, the researchers have not found that smoking patterns fully explain the change.

They do have at least one hypothesis. The risk of developing lung cancer drops after one quits smoking: by 25% in the first five years, 50% after ten, and as much as 90% after more than 20 years of not smoking. But the risk for one type of lung cancer, adenocarcinoma, drops more slowly than the others—and women are more likely to to develop adenocarcinoma than men.

Approximately 10% of men and 15% women who develop lung cancer have never smoked—meaning that it’s also possible that women could be more susceptible to non-smoking-related lung cancer, reports Buzzfeed.

The researchers examined nationwide data on the incidence of lung cancer according to sex, race or ethnic group, age, year of diagnosis, and year of birth. This included all cases of invasive lung cancer diagnosed in people ages 30 to 54 from 1995 to 2014 across 46 states and Washington, D.C.