Atmospheric pollution discriminates between the sexes, a study has found.

It examined the effects of smog on the respiratory systems of boys and girls, and found that while polluting gases affected boys most, particle pollution affected girls.

Scientists have no explanation as to why this might be.

The study was undertaken for the California Air Resources Board and its results were published in the Sacramento Bee.

The finding is the latest surprise to emerge from a comprehensive look at the health effects of air pollution on US children.

The $15m, 10-year investigation of 5,000 children in Southern California is scheduled to end in 2003.

Respiratory symptoms

Researchers at the University of Southern California carried out the study.

They tracked respiratory symptoms, breathing capacity and school absenteeism among children in the fourth, seventh and 10th grades - aged nine to 18 - in a dozen California cities.

Children are particularly vulnerable to the effect of pollution

Investigators reported distinct gender differences in the way that boys and girls react to high levels of nitrogen oxide, ozone and particle pollution.

Jerry Martin, a spokesman for the air board, said: "It was unexpected. To my knowledge it has never been observed before in any other study, but there has never been a study that focused on children and long-term exposure to pollution."

The report found that boys were more likely to be affected by high levels of ozone gas.

Girls were more affected by high levels of particle pollutants like dust and nitrogen oxide, a smog-forming contaminant that comes mainly from vehicle exhaust.

Mystery

The investigators could not explain the difference, and said more testing and analysis was needed.

Mr Martin said: "It could be due to the fact that boys when they are outside exercise harder, or are more active. We just don't know," Martin said. "But there clearly are some other things happening, and obviously the pollutant and the gender are playing some role."

The study also showed both sexes suffered from pollution-related ill health.

Children in the most polluted communities covered by the research had lung growth about 5% lower than normal.

Coughing and wheezing among asthmatics was found to be more severe in areas with higher levels of nitrogen oxide and particle pollution.

The USC study's preliminary findings will be published in March in the American Review of Respiratory and Critical Care Medicine.