A new study suggests secondhand smoke may be to blame for a high percentage of women who develop lung cancer.

A survey of more than a million people found up to 20 percent of women who develop lung cancer have never smoked. The same was true for just 8 percent of men.

One of the study’s leaders said because more men smoke than women, women may be more likely to be exposed to secondhand smoke even when they are classified as having never smoked.