The report, compiled from studies in Britain, Europe and the United States by Earth Resources Research, says that far from sitting in a protective cocoon, car users can be exposed to pollution levels that are up to 18 times higher than those experienced by cyclists and pedestrians.
Ventilation does not help, although conditions inside cars improve with air conditioning and deteriorate when fan heaters are switched on. Concentrations of pollutants can also be increased by congested traffic, lower speeds, older vehicles and faulty exhausts.
The levels of benzene, a carcinogen for which the World Health Organisation says there is no safe threshold, were found to be 2 to 18 times higher inside cars than outside. Levels of carbon monoxide were 2 to 14 times higher and those of nitrogen dioxide 1.3 to 2.5 times higher, both exceeding recommended safety limits.
Charlie Kronick, transport campaigner for Greenpeace, said that the Government refused to acknowledge the severity of the health threat posed by traffic pollutants and which would be exacerbated as traffic increased.
'There is no safe haven from traffic pollution,' he said. 'Winding up the window won't help; drivers cannot simply shut themselves away from the effects of the pollution they create. The only real solution is to cut the number of cars on our roads.'
Exposure to benzene and carbon monoxide is particularly critical on urban roads and in congested, slow-moving traffic. Concentrations of nitrogen dioxide appear to rise during long motorway journeys.
The report says that recent data from the United States, where the majority of cars are fitted with catalytic converters, shows similar levels of pollutants inside vehicles.
Gill Felton of the Cycle Campaign Network said: 'I am pleased that cyclists are breathing cleaner air than motorists, but we still need much stricter pollution controls on vehicles.'