Women who smoke have a threefold risk of developing skin cancer, researchers say.

Men who smoke, on the other hand, had only a modest risk of basal or squamous cell skin cancers – the two types of non-melanoma skin cancer.

We don’t know why there’s a gender difference, said study lead author Dana Rollison.

About 700 study participants – 383 with skin cancer and 315 without – were queried about their smoking habits. The longer people smoked, the more likely they were to have skin cancer, researchers found, and women smokers were more likely to have squamous cell skin cancers. The study was conducted in Tampa, Florida, and the findings were published in the journal Cancer Causes Control.

Despite the study’s outcome, men remain more likely to develop skin cancer, researchers noted. That could be due to skin sensitivity to sun exposure, or due to a behavioral difference such as men being less likely to use sunscreen than women.

An estimated 2 million cases of non-melanoma skin cancer are treated each year in the U.S. It is the most common type of type of cancer, according the National Institutes of Health.