Children playing outdoor team sports in areas of high ozone concentration could be at increased risk of developing asthma.

Research suggests that they are three times more likely to develop the condition than children who do not take part in sporting activities.

Our study provides evidence that, contrary to conventional wisdom, ozone is involved in the development of new onset asthma Dr Rob McConnell

Asthma is the most common chronic disease of childhood and it is on the increase in developed countries. However, scientists do not know why.

The new study suggests that pollution, which has long been suspected as a culprit, does indeed play a role.

A team from the University of Southern California, Los Angeles, examined around 3,500 children from schools in 12 communities in southern California.

During the five years of the study 265 children were diagnosed for the first time with asthma.

Bigger risk

Children who played three or more outdoor sports in high-ozone environments were more than three times as likely to develop asthma compared with children who did not play any sports.

There was no increased risk where ozone concentrations were low.

Children who spent time outside in areas with high ozone concentrations were 1.4 times more likely to develop asthma than children in areas of low ozone concentration.

Other environmental pollutants such as nitrogen dioxide, particulate matter were not associated with an increased asthma risk.

Researcher Dr Rob McConnell said: "Our study provides evidence that, contrary to conventional wisdom, ozone is involved in the development of new onset asthma in children who exercise heavily (and thereby increase the amount of ozone which gets into the lungs).

"It is by no means conclusive proof that air pollution causes asthma, but it may be a piece of the complicated asthma puzzle."

Dr McConnell said his study should not discourage parents from encouraging their children to take part in sport.

He said: "The bottom line is this: exercise is really healthy for children, for many reasons, and children should be encouraged to play team sports.

"But, on days when air pollution levels are expected to be high, children should limit prolonged outdoor exertion."

Not a trigger

Pollution, if shown to be a factor, may be only one amongst many Professor Martyn Partridge

Professor Martyn Partridge, chief medical adviser to the UK National Asthma Campaign, said until now most of the good scientific evidence suggests that outdoor pollution can make existing asthma worse but not actually trigger it in the first place.

He told BBC News Online: "This latest study does suggest an association between outdoor pollution and the development of asthma.

"However, this does not necessarily mean that one causes the other.

"Pollution, if shown to be a factor, may be only one amongst many that lead to more people having this condition now than 20-30 years ago.

"Nevertheless, this is a very important study adding significantly to our knowledge in this area"

Ozone, an unstable molecule composed of three oxygen atoms, provides an essential shield against dangerous ultra violet radiation in the upper atmosphere.

But at ground level, where it is produced from traffic fumes and industrial emissions, it is a highly toxic pollutant and a major component of city smog.

The gas is corrosive to lung tissue and can damage delicate bronchial branches and air sacs.

A study by the Committee on the Medical Effects of Air Pollution, a Department of Health expert advisory group, found three years ago that more than 12,000 people in Britain were dying prematurely each year because of exposure to ozone.