Personal monitoring of particulate matter (PM) exposure in infants is difficult. Indirect, microenvironment modelling methods are more practical. Infants spend most of their time indoors at home and the aim of this study was to investigate spatial variations in PM concentrations within homes. Three size fractions of PM - particles with an aerodynamic diameter of less than 10 mu m (PM10), less than 2.5 mu M (PM2.5) and total suspended particulates (TSP) - were monitored in the homes of 77 infants (0-2 years) using a multi-stage virtual impactor. In all homes PM was monitored simultaneously in the main living room at heights of 1.4 in and 0.2 m from the floor. In 26 of these homes monitoring was also conducted simultaneously in the infant's bedroom. Further, PM10 was measured simultaneously in the living room, bedroom and child's cot in 14 homes using a real-time photometer. All homes in the study were non-smoking households. On average, there were no significant differences between concentrations of any of the different PM size fractions measured at the two heights (living room) and between living room and bedroom concentrations. However, there were only moderate correlations in concentrations between the different microenvironments and in some homes there was considerable variation between sampling sites. From the real-time measurements there seemed to be good agreement between concentrations measured in different rooms and in the cot and short-term peak concentrations at one sampling site were often mirrored at other sites. These results suggest that, although large variations in PM concentrations between rooms within homes can occur, a single monitoring station can provide a reasonable estimate of indoor concentrations. (c) 2007 Elsevier B.V. All rights reserved.