Traffic fumes are not the reason why people's asthma gets worse in the winter, say US scientists.
Pollution posed no real asthma threat, researchers said
Instead coughs and colds are to blame, according to their study of schoolchildren in Denver.
Respiratory infections doubled the chance of asthma worsening, the National Jewish Medical and Research Center team found.
But air pollution may still trigger summer asthma, they told the Journal of Allergy and Clinical Immunology.
Study author Dr Erwin Gelfand said: "It is well known that upper respiratory infections can cause problems for people with asthma, but the air pollution results were a surprise.
"We believe that careful monitoring of the children allowed us to filter out confounding factors that would have mistakenly suggested a significant impact of air pollution."
His team monitored 147 children aged six to 12 during three successive winters in Denver, Colorado, when air pollution was at its worst.
They recorded the number of times the children had asthma symptoms and were hospitalised for their asthma, as well as measuring their lung function, medication use and number of upper respiratory tract infections.
When they looked at the levels of six air pollutants, including particulate pollution, ozone and carbon monoxide, they found no obvious relation to asthma severity.
Higher carbon monoxide levels were marginally associated with increased use of asthma medication and daily asthma symptoms were marginally associated with ozone levels, but this was not significant.
Yet upper respiratory infections doubled the chances a child would suffer an asthma exacerbation and more than quadrupled the odds a child would suffer asthma symptoms.
Coughs and sneezes
Co-author Dr Nathan Rabinovitch said: "This is good news for parents. Instead of worrying about air pollution they can focus their efforts on preventing and treating the real wintertime threat to their children's health - colds and other respiratory infections."
But the researchers said air pollution might still be important in summertime asthma.
Children tend to spend more time outside in the summer and therefore may be exposed to higher levels of pollution.
Also, ozone rises to much higher levels during the summer.
The researchers said they would look at the effects in children with asthma next summer.
Katie Shepherd of Asthma UK said: "The findings of this research appear to confirm what people with asthma tell us: 90% say that colds and flu are triggers for their asthma symptoms.
"At the same time, 66% of people with asthma tell us that traffic fumes are a trigger and this does seem to be worse during the summer months when ground level ozone increases.
"If colds and flu make your asthma worse, prevention is the key to keeping your symptoms under control."