AbstractA significant fraction of daily personal exposure to air pollutants occurs during commuting in transport microenvironments (TMEs). We carried out systematic mobile monitoring on a pre-defined route to assess personal exposure levels of particulate matter (PM) in four TMEs (bus, car, cycle and walk). Measurements were made during morning peak (MP), afternoon off-peak (OP) and evening peak (EP) hours in a typical UK town, Guildford. The objectives were to quantify the real-time exposure to fine and coarse particles, identify the factors influencing their spatiotemporal variation and estimate the respiratory deposition doses (RDD). The mean PM₁₀ concentrations were 90 ± 63, 23 ± 9, 14 ± 17 and 63 ± 76 μg m⁻³ for bus, car, cycle and walk modes, respectively. The average ratios of PM_2.5/PM₁₀ were 0.32, 0.90, 0.67 and 0.36 for bus, car, cycle and car journeys, respectively. The mean concentrations of coarse particles (PM_2.5-10) followed the trend: bus > walk > cycle > car. In contrast, mean concentrations of submicron (PM₁) and fine particles (PM_2.5) were usually high in the car while lowest for cyclists. RDD depend on the physical activity, particle size distribution and thus deposited fraction are not always proportional to the ambient concentration. RDD for coarse particles were largest for the walk mode (56 ± 14 μg h⁻¹), followed by buses (31 ± 2 μg h⁻¹), cycle (12 ± 3 μg h⁻¹) and cars (1.2 ± 0.3 μg h⁻¹). The corresponding RDD of fine particles were comparable for both walk (5.5 ± 0.3 μg h⁻¹) and cycle (5.1 ± 1.2 μg h⁻¹), followed by bus (4.1 ± 0.7 μg h⁻¹) and car (2.0 ± 0.2 μg h⁻¹). Car mode experienced both the least concentrations and RDD for coarse particles. It also had the lowest RDD for fine particles despite high concentrations. Physical activity of car commuters is modest compared with walking and cycling, which makes the rank ordering of RDD different than those of exposure concentrations. Hence the management of commuting exposures should consider potential dose and not just exposure concentration for curtailing adverse health effects related to commuting. RDD for pedestrian and cycle modes were not the lowest among the measured modes but opportunities such as an increased distance between the heavily trafficked roadways and pedestrians/cyclists should be considered in urban planning to reduce potential doses.