FRIDAY, March 2 (HealthDay News) -- Smokers are at higher risk of developing the autoimmune skin condition psoriasis than nonsmokers, a new study finds, possibly because smoking pushes the body's immune system into overdrive, one expert suggests.

The research doesn't directly prove that smoking causes psoriasis, and the wide majority of smokers would avoid developing the condition even if they faced an increased risk.

Still, the findings provide yet another reason for smokers to drop the habit, said study co-author Dr. Abrar Qureshi, an assistant professor of dermatology at Harvard Medical School. "It behooves us even more to advise individuals who are smoking to quit," he said, especially because psoriasis itself is linked to higher risks of diabetes and heart attacks.

Psoriasis, which tends to occur in adulthood, causes itchy and painful patches of thick, scaly and reddened or whitened skin. The patches often appear on the knees and elbows.

The condition appears when the immune system mistakenly attacks skin cells. Treatments are available, but they may not keep psoriasis at bay forever.

Dermatologists have wondered whether smoking makes psoriasis worse, Qureshi said. In the new study, investigators examined research projects that followed more than 185,000 medical professionals in the United States for as long as two decades. They looked specifically at 2,410 people who'd been diagnosed with psoriasis.

After adjusting their statistics so they wouldn't be thrown off by high or low numbers of people who were overweight or had other risk factors for psoriasis, the researchers found that current smokers were about 1.9 times more likely to have psoriasis than nonsmokers. Past smokers were nearly 1.4 times more likely to have the condition.

The study found a "graded reduction of risk" as time passed after a participant had quit smoking.

The study didn't calculate the actual percentages of smokers, nonsmokers and past smokers who developed psoriasis. However, Qureshi said about 2 percent to 3 percent of the general population has the condition.

It's possible that something other than smoking boosts the risk of psoriasis in smokers, Qureshi said. It would be unethical to confirm that smoking is the cause because that would require researchers to randomly assign some people to smoke, he said.

Even if researchers could confirm that smoking causes or worsens the condition, the question would remain how it might do so. Qureshi said that while it's possible that simply being around smoke could hurt the skin externally, "there are a number of autoimmune conditions that are exacerbated and caused by inhaled smoke."

Dr. Joel Gelfand, medical director of the University of Pennsylvania's department of dermatology clinical studies unit, said that "since psoriasis is an inflammatory disease, it is plausible that smoking lights the fire that leads to chronic inflammation of psoriasis in people who are susceptible."

Gelfand, who's familiar with the study results, said the research confirms previous findings.

"Importantly, the investigators showed that the risk of psoriasis increased with the amount of smoking and a reduction in risk of psoriasis was observed with an increase in time from when people quit smoking," he said. "Smoking is common among patients with psoriasis and extremely common among patients with a variant of psoriasis called 'palmar-plantar pustular psoriasis,' which can be severely disabling."

Smoking has already been associated with aging and wrinkling of the skin, Gelfand said. Psoriasis, he added, "is another reason to quit."

The study appears in a recent issue of the American Journal of Epidemiology.

More information

For more about psoriasis, visit the U.S. National Library of Medicine.