Traffic pollution may boost the risk of children getting asthma - if they have genes which make them vulnerable, a study says.
There is dispute about the relation between pollution and asthma
The University of Southern California team studied the health records and genetic profiles of 3,000 children.
Those with a gene variation were slightly more at risk but if they lived near a main road, the risk rose more, the Thorax journal reported.
But UK asthma experts said the link remained unclear.
Scientists exploring how respiratory diseases, including asthma, develop have highlighted the importance of genes which control key body chemicals linked to "clean-up" functions in the body.
Enzymes called EPHX1 and a gene called GSTP1 appear to have some responsibility for getting rid of harmful chemicals which we breathe in.
The researchers found that those who had high levels of EPHX1 were 1.5 times as likely to have been diagnosed with asthma, while those who also had variations in GSTP1 as well were four times as likely to have asthma.
However, living close to a main road appeared to make this effect even greater.
Children with very active EPHX1 who lived within 75 metres of a road had a doubled risk of asthma compared with those who had low EPHX1 levels.
Having active EPHX1, variations in the gene, and a home near a road meant a risk nine times greater.
Their conclusion was that while children with the wrong genes and enzyme activity were more prone to having asthma, living near a road seemed to compound that risk.
There has been a long-running dispute about a link between asthma and exhaust fumes.
And Leanne Male, Asthma UK's assistant director of research, said more work was needed.
"This study is very promising as it is one of the first to look specifically at how genetic susceptibility to respiratory disease and environmental traffic fumes can cause childhood asthma.
"People with asthma tell us that traffic fumes make their asthma worse and although this research only looks at individuals with a certain genetic make-up, we await further robust research in this new and exciting area to help us find better ways to treat asthma."