A new study links smoking with an increased risk of psoriasis. Heavy smokers, and those who smoke for many years, are at greatest risk.

Psoriasis, which causes thick, red scaly patches on the skin, is believed to be caused by an abnormal immune system attack on the body’s cells, according to Reuters. Some previous studies have suggested smoking may increase psoriasis risk because it affects the immune system. However, most of these studies have only looked at one point in time, making it difficult to pinpoint an association between smoking and the skin disease.

The new study included data from three long-running studies that included almost 186,000 adults who were followed from 12 to 20 years. The researchers found 2,410 people developed psoriasis over this period. Current and former smokers were at greatest risk.

People who smoked at the start of the study were almost twice as likely as people who had never smoked to develop psoriasis, and past smokers had a 39 percent higher risk compared with nonsmokers.

“I think if there’s one message, it’s that for now, smoking seems to be a risk factor for new-onset psoriasis,” said researcher Abrar A. Qureshi of Harvard Medical School and Brigham and Women’s Hospital in Boston.

The study appears in the American Journal of Epidemiology.