London Underground passengers are regularly exposed to high levels of airborne particulate pollutants.
The first comparative study of personal exposure to PM2.5 - fine particles of soot and other substances of about 2.5 microns in diameter - has found that commuters travelling in cars, buses or on bikes or the tube are encountering more pollution than previously detected by ambient monitoring stations.
PM2.5 enters the body via the lungs and, through an unknown mechanism, is strongly linked to respiratory and cardiovascular diseases.
Mark Nieuwenhuijsen, Roy Colvile and colleagues at Imperial College, London, looked at personal exposure levels on three separate routes through central London. They investigated different modes of transport at different times of day throughout the year 1999-2000.
There was more pollution in summer than in winter.
The level of PM2.5 exposure on the underground was five to ten times higher than above ground. The exact make-up of the particles is not known, though they probably contain a high proportion of iron generated by the trains.
Wind speed strongly influenced exposure levels both above and below ground, with high winds cutting pollution significantly.
The amount of traffic and time of day seemed to have little effect as the PM2.5 persisted for many hours around busy roads.
However, the route taken by the commuter can have a major impact, with higher levels in the city centre.
Dr Nieuwenhuijsen said the findings, which are published in the journal Atmospheric Environment , should be of concern to commuters, though the health hazard that the exposure constitutes has yet to be ascertained.
A parallel study into the biological impact of PM2.5 has indicated that lung-lining fluid is the body's principal line of defence, agglomerating the tiny particles into larger ones.
This research suggests that those individuals with impaired lung-linings, perhaps through conditions such as asthma, are particularly at risk.