Air pollution could increase the risk of infection by damaging the immune system, a study says.
Scientists have demonstrated for the first time that nano-sized particles in traffic fumes reduce the body's ability to kill viruses and bacteria.
While the potential link between car-choked streets and illness has been the subject of much debate, the work at Edinburgh Napier University is the first to show this effect and presents significant human health implications.
Dr Peter Barlow, who led the research, said: "This is an area of research that is very poorly understood.
"We were extremely concerned when we found that air pollution particles could inhibit the activity of these molecules, which are absolutely essential in the fight against infection.
"In light of these findings, we urge that strong action plans are put in place to rapidly reduce particulate air pollution in our towns and cities."