What this research shows, for the first time, is that the closer children live to a main road, the more likely they are to develop asthma symptoms

Donna Covey, National Asthma Campaign

Donna Covey, chief executive of the National Asthma Campaign, told the BBC: "What this research shows, for the first time, is that the closer children live to a main road, the more likely they are to develop asthma symptoms."

She said there was no proof pollution caused asthma, but that it was known if a person suffered from the condition, pollution made it worse.

The NAC is calling for more research into the causes of asthma, and wants the government to make the condition a priority.

Ms Covey advised parents who lived near busy roads: "The most important thing to say is if you're worried about your children, go and see a doctor."

She added: "Exercise is very good for children, and it's very good for children with asthma, so keeping children with asthma inside isn't the answer either."

Increased incidence

Asthma is the most common chronic disease of childhood and it is on the increase in developed countries.

It is estimated that 5.1m people in the UK are being treated for asthma - one in eight children and one in 13 adults.

The number of asthma attacks seen by GPs has soared over recent years.

Incidence measured in this way is now three to four times higher in adults and six times higher in children than it was 25 years ago.

In 2000, GPs in the UK saw over 18,000 cases relating to new asthma attacks each week.

Carbon dioxide and ozone contained in pollution from cars have been linked to asthma.

The research was published in the American Journal of Respiratory and Critical Care Medicine.