In order to investigate a specific area of short-term, non-occupational, human exposure to fine particulate air pollution, measurements of personal exposure to PM2.5 in transport microenvironments were taken in two separate field studies in central London, UK. A high flow gravimetric personal sampling system was used; operating at 16 l min(-1); the sampler thus allowed for sufficient sample mass collection for accurate gravimetric analysis of short-term travel exposure levels over typical single commute times. In total, samples were taken on 465 journeys and 61 volunteers participated. In a multi-transport mode study, carried out over 3-week periods in the winter and in the summer, exposure levels were assessed along three fixed routes at peak and off-peak times of the day. Geometric means of personal exposure levels were 34.5 microg m(-3) (G.S.D.= 1.7, n(s) = 40), 39.0 microg m(-3) (G.S.D. = 1.8, n(s) = 36), 37.7 microg m(-3) (G.S.D. = 1.5, n(s) = 42), and 247.2 microg m(-3) (G.S.D. = 1.3, n(s) = 44) for bicycle, bus, car and Tube (underground rail system) modes, respectively, in the July 1999 (summer) measurement campaign. Corresponding levels in the February 2000 (winter) measurement campaign were 23.5 microg m(-3) (G.S.D. = 1.8, n(s) = 56), 38.9 microg m(-3) (G.S.D. = 2.1, n(s) = 32), 33.7 microg m(-3) (G.S.D. = 2.4, n(s) = 12), and 157.3 microg m(-3) (G.S.D. = 3.3, n(s) = 12), respectively. In a second study, exposure levels were measured for a group of 24 commuters travelling by bicycle, during August 1999, in order to assess how representative the fixed route studies were to a larger commuter population. The geometric mean exposure level was 34.2 microg m(-3) (G.S.D. = 1.9, n(s) = 105). In the fixed-route study, the cyclists had the lowest exposure levels, bus and car were slightly higher, while mean exposure levels on the London Underground rail system were 3-8 times higher than the surface transport modes. There was significant between-route variation, most notably between the central route and the other routes. The fixed-route study exposure was similar in level and in variability to the 'real' commuters study, suggesting that the routes chosen and the number of samples taken provided a reasonably good estimate of the personal exposure levels in the transport microenvironments of Central London. This first comprehensive PM2.5 multi-mode transport user exposure assessment study in the UK also showed that mean personal exposure levels in road transport modes were approximately double that of the PM2.5 concentration at an urban background fixed site monitor.