Robert Baker, president of the non-profit US Indoor Air Quality Association, said American scientists have found the air inside cars to be more contaminated than the air outside, even in urban areas.
This was due to unfiltered air from exhaust fumes and chemical smells from car seats, audio equipment and air fresheners.
"The air in an indoor space does not clean itself, unlike the outdoors, where air travels," Baker told a news conference at the launch of a Singapore Web site on car cabin air quality.
The site, www.healthycarcabin.org.sg, says prolonged exposure to bad cabin air can cause cancer and respiratory diseases. Drivers often install air cleaning devices, such as airconditioner filters.
"We release gases, bacteria and fungi into the air. The more people there are in an enclosed area, the more harmful it is," he said.
One solution was to open the car's windows, though the Web site recommends doing so only along country roads.