Smokers have a higher risk of developing psoriasis, a study suggests.
Cigarette smoke contains hundreds of toxins
US researchers found that heavier smokers have a greater risk of the skin condition and this only falls back to normal 20 years after quitting.
The study of 79,000 nurses published in the American Journal of Medicine also found that people with psoriasis who smoke had more severe disease.
It is thought that toxins in cigarette smoke may affect parts of the immune system associated with psoriasis.
Psoriasis, which occurs when the skin replaces itself too quickly, affects more than one million people in the UK. There are many different forms.
It usually appears as red, scaly patches that when scraped or scratched reveal fine silvery scales.
Previous research has reported links between smoking and psoriasis but was unable to look at whether smoking occurred before the onset of the condition.
The researchers reported 887 cases of psoriasis in nurses who took part in the 14-year study.
Compared with women who never smoked, the risk of psoriasis was 37% higher among past smokers and 78% higher among current smokers.
The more people had smoked over the years, the higher their risk and it took a couple of decades after stopping smoking before the risk fell back to that of non-smokers.
The team also found that exposure to passive smoke during pregnancy or childhood was associated with an increased risk of psoriasis.
Study leader, Dr Hyon Choi, researcher at Harvard Medical School in Boston and the University of British Columbia in Vancouver, Canada, said the findings provided a clear incentive for those with psoriasis to stop smoking as well as those at risk of the condition.
"Beyond the potential effect on psoriasis, smoking cessation would lead to a better overall clinical outcome in psoriasis patients, who often suffer co-morbidities related to smoking," he said.
Stopping smoking may decrease the level of smoke induced inflammation in the body by lowering levels of circulating immune cells, he added.
Gladys Edwards, chief executive of the UK's Psoriasis Association, said: "We have always recommended that people with psoriasis should aim to cease smoking for their general health and to help improve their psoriasis.
"This study suggests that there is a stronger link between smoking and the risk of developing psoriasis and this clearly merits further research.
"Psoriasis, however, is an immensely complex condition - there are people with psoriasis who do not and have never smoked."