To study the adverse health effects associated with ambient respirable particulates it is necessary to assess population exposure. However, measurements are usually carried out by fixed-site air monitoring units and do not take into consideration the range of urban microenvironments. In particular, vehicular emissions are a major source of particulates in cities and it is therefore necessary to consider traffic density when investigating the adverse health effects of air particulates. This study assessed both PM(5) and total particle number (TPN) in areas of high and low traffic flow within the city of Cardiff, UK. Daytime measurements were made outdoors throughout the period January 2004-June 2005. Simultaneous indoor measurements were made in 73 participating households in areas of high traffic flow (exposed) and 50 households in areas of low traffic flow (controls). The median outdoor PM(5) concentration in the exposed area (16.7 microg cm(-3)) was 45% significantly higher than that in the unexposed area (11.5 microg cm(-3)) and the median TPN in the exposed area (18,130 particles cm(-3)) was almost double that in the unexposed area (9951 particles cm(-3)). The median indoor concentration of PM(5) in the exposed area (13.2 microg cm(-3)) was also 40% significantly higher than that in the unexposed area (9.4 microg cm(-3)), but there was almost no difference in indoor median TPN between the two areas: 8999 particles cm(-3) and 8549 particles cm(-3), respectively. The observed differences in respirable particles between areas of high and low traffic flow underline the importance of obtaining exposure measurements in microenvironments.