BOSTON, Oct. 30 (UPI) -- U.S. researchers have added psoriasis to the list of smoking-related diseases.

The study, published in The American Journal of Medicine, found the risk of developing psoriasis is more than two times higher for heavy smokers. Also, exposure to passive smoke during pregnancy or childhood was also associated with an increased risk of psoriasis.

The study included 14 years of follow-up on data from the Nurses Health Study II, a study of 116,608 female registered nurses from 15 states between the ages of 25 and 42.

The researchers at Massachusetts General Hospital, Brigham and Women's Hospital and the Harvard School of Public Health, all in Boston, documented 887 cases of psoriasis and measured smoking exposure in pack-years -- equal to smoking 20 cigarettes per day for one year -- and found as pack-years increased so did the risk of psoriasis.

Compared with women who never smoked, the risk of psoriasis was 78 percent higher among current smokers and didn't decreases to near that of never smokers until 20 years after cessation.