Exposure assessment of air pollutants: a review on spatial heterogeneity and indoor/outdoor/personal exposure to suspended particulate matter, nitrogen dioxide and ozone

This review describes databases of small-scale spatial variations and indoo r, outdoor and personal measurements of air pollutants with the main focus on suspended particulate matter, and to a lesser extent, nitrogen dioxide a nd photochemical pollutants. The basic definitions and concepts of an expos ure measurement are introduced as well as some study design considerations and implications of imprecise exposure measurements. Suspended particulate matter is complex with respect to particle size distributions, the chemical composition and its sources. With respect to small-scale spatial variation s in urban areas, largest variations occur in the ultrafine (<0.1 <mu>m) an d the coarse mode (PM10-2.5, resuspended dust). Secondary aerosols which co ntribute to the accumulation mode (0.1-2 mum) show quite homogenous spatial distribution. In general, small-scale spatial variations of PM2.5 were des cribed to be smaller than the spatial variations of PM10. Recent studies in outdoor air show that ultrafine particle number counts have large spatial variations and that they are not well correlated to mass data. Sources of i ndoor particles are from outdoors and some specific indoor sources such as smoking and cooking for fine particles or moving of people (resuspension of dust) for coarse particles. The relationships between indoor, outdoor and personal levels are complex. The finer the particle size, the better become s the correlation between indoor, outdoor and personal levels. Furthermore, correlations between these parameters are better in longitudinal analyses than in cross-sectional analyses. For NO2 and O-3 the air chemistry is impo rtant. Both have considerable small-scale spatial variations within urban a reas. In the absence of indoor sources such as gas appliances, NO2 indoor/o utdoor relationships are strong. For ozone, indoor levels are quite small. The study hypothesis largely determines the choice of a specific concept in exposure assessment, i.e. whether personal sampling is needed or if ambien t monitoring is sufficient. Careful evaluation of the validity and improvem ents in precision of an exposure measure reduce error in the measurements a nd bias in the exposure-effect relationship. (C) 2000 Published by Elsevier Science Ltd.