Adding to the list of possible health consequences from smoking, a large study suggests that smokers have an increased risk of developing the chronic skin condition psoriasis.
People with psoriasis develop thick, red, scaly patches on the skin, which are often itchy or sore.
Experts believe the disease is caused by an abnormal immune system attack on the body's own cells. Some studies have suggested that smokers are more vulnerable to psoriasis, possibly because the habit can affect immune activity.
But most have studied people at only one time-point, which makes it hard to be sure the smoking came before the psoriasis.
So for the new study, researchers used data from three large, long-running studies of U.S. health professionals.
Of nearly 186,000 men and women followed for 12 to 20 years, 2,410 developed psoriasis during that time. And the risk was greater among both current smokers and former smokers.
People who were current smokers at the study's start were almost twice as likely as lifelong non-smokers to develop psoriasis. And past smokers had a 39 percent higher risk than non-smokers.
The findings, reported in the American Journal of Epidemiology, do not prove that smoking, itself, causes psoriasis in some people.
But it is clear that the smoking came before the psoriasis, said senior researcher Dr. Abrar A. Qureshi, of Harvard Medical School and Brigham and Women's Hospital in Boston.
Past studies have found links between psoriasis and both obesity and heavy drinking. But after accounting for those factors, the smoking-psoriasis link remained, Qureshi told Reuters Health.
"I think if there's one message, it's that for now, smoking seems to be a risk factor for new-onset psoriasis," Qureshi said.
Other studies have pointed to some reasons that smoking could contribute to psoriasis -- mainly its effects on immune system activity and inflammation. Smokers, for instance, tend to have higher levels of "autoantibodies" -- immune defenses that are mistakenly aimed at the body's own cells.
About 7.5 million Americans have psoriasis, according to the National Psoriasis Foundation.
Type 1 psoriasis arises in teenagers and young adults, and it's strongly related to family history of the disease. In contrast, type 2 psoriasis arises later in life, and it tends to be milder and less related to heredity.
The men and women in the current study were middle-aged to older. So it's likely they had type 2 psoriasis, Qureshi's team says.
As for people who already have psoriasis, the current findings don't speak directly to whether quitting will help them, according to Qureshi.
But there are already plenty of reasons for any smoker to quit, he said.
In particular, people with psoriasis have been shown to have an increased risk of heart disease. Since smoking is a major risk factor for heart disease, Qureshi said, quitting seems especially important for people with psoriasis.