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.
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