Asthma risk 'greater for smokers'
People who smoke are 33% more likely to develop asthma than those who do not, researchers from the Finnish Institute of Occupational Health have said.
The team told the European Respiratory Journal the 30-month study of 1,444 people gave the strongest evidence yet that smoking caused asthma.
Smoking has long been thought to aggravate the condition but whether it caused it has been disputed.
Anti-smoking groups said the study gave more weight to calls for workplace bans.
Cigarette smoke contains over 4,000 substances - many of which irritate the airways.
One of the standing complaints about studies into the effect of smoking is that they usually only cover a fixed population group at certain moments in time.
This makes it possible to identify apparent links between smoking and disease - but not to prove that the habit is directly to blame.
The Finnish team tackled this problem by only including people in the study who initially showed no signs of asthma or lung disease, and only developed symptoms during the study.
The 521 participants who started to show signs of developing asthma were then monitored over two-and-a-half years, and compared to 923 people with no signs of the allergy.
The study found that current smokers were 33% more likely to develop asthma but that people who had given up smoking had an even greater risk - 49% higher than those who never smoked.
The risk of asthma was proportional to the number of cigarettes smoked each day - and the total number smoked during a lifetime so far.
However, the finding did not apply to heavy smokers, who smoked more than 15 cigarettes a day, or who had smoked a lot of cigarettes during their life.
The researchers believe this finding, which they found surprising, might be explained by heavy smokers giving up or cutting down as soon as they have any breathing problems, before they have had a chance to be diagnosed with asthma.
They also stress that individual susceptibility to tobacco damage varies hugely from one person to another. It may be that people who smoke heavily are more tolerant of the irritants in tobacco.
The study also found that female smokers seem to be at a greater risk of developing asthma than their male counterparts.
Female smokers and ex-smokers had an increased risk of developing asthma of between 138% and 143%.
Lead researcher Dr Ritva Piipari said: "The heavier toll that smoking takes on women could be the result of a greater vulnerability to the ill-effects of smoke.
"In fact, we have noted that even non-smoking women have an increased risk (57%) of developing a new asthma compared to their male counterparts."
Professor Martyn Partridge, chief medical adviser to Asthma UK, said it had previously been difficult to show that smoking causes asthma as it only needed a few patients with lung disease to be wrongly classified as having asthma to skew the results.
He said: "This new study has tried very hard to exclude the previous problems with misdiagnosis, and it does suggest a possibility that smoking may be amongst the numerous lifestyle changes that may have enhanced the risk of adults developing asthma."
Professor Andrew Peacock, of the British Thoracic Society, said: "The evidence of just how harmful cigarettes are continues to grow and grow."
Amanda Sandford, of the charity Action on Smoking and Health, said: "This is an important study showing conclusively that tobacco smoke can trigger asthma in adults whether or not they smoke.
"It provides yet more evidence of the need for legislation to make all workplaces and public places smoke-free."