Background Human exposure in transport micro-environments (MEs) is of particular interest because many citizens and commuters spend a substantial component of their time in these spaces. Fixed monitoring stations measuring outdoor air quality are used to estimate exposure levels, however, this approach may significantly underestimate the exposure of particular population subgroups, pointing to the need for direct personal exposure measurements. Aims Ultrafine particles (UFPs), size-fractionated particulate matter (PM) and carbon monoxide (CO) were measured in the central area of Milan, in order to assess the influence of time and space on individual exposures. Methods Individual exposures to CO (passive analyser - electrochemical cell), UFPs and PM (Condensation and Optical Particle Counters) were continuously collected along an established urban pathway, moving afoot or by different private and public means of transport during three week-long periods (summer, autumn and winter) and through three sampling sessions per day. Results The analysis of the exposure measures showed statistically significant differences between different MEs and sampling periods. The highest mean concentrations were observed during time spent along busy streets (walking or moving by motorized vehicles), in metro trains and on working-day mornings; the lowest exposures were observed in green areas and in indoor environments. PM and CO showed different seasonal behaviours: the highest mean concentrations were observed in summer for CO and for the coarser fractions of PM, and in winter for the finer fractions of PM. Conclusions Appreciable differences among MEs and sampling periods were observed. Exposure patterns appeared to be mostly related to the proximity to traffic sources, to peculiarities of MEs, as well as to time periods during the day, days of the week, and seasons of the year.
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