Aerosol-based modelling of infiltration of ambient PM2.5 and evaluation against population-based measurements in homes in Helsinki, Finland

Ambient particles (PM2.5) are estimated to be the leading source of environmental burden of disease. Modern populations spend more than 80%, often over 90% of their time indoors, where the exposures to ambient particles are modified by the building and ventilation system. The health risks, however, are routinely evaluated and controlled using only outdoor concentration measurements. The objective of the current work is to integrate a number of previously developed methods for estimation of the particle size distribution changes caused by infiltration of particles from outdoor air. The model is evaluated against population-based measurements conducted in 45 homes in Helsinki, Finland, with 3–5 repeated daily measurements. In comparison with previous sulphur-based estimation of PM2.5 infiltration rate, the aerosol model using 12-channel particle number size distribution from an electrical aerosol spectrometer (EAS) as input, was capable of capturing 85% of variance of indoor PM2.5 originating from outdoors. The estimated residential PM2.5 infiltration factor in Helsinki was 0.55±0.13 (mean±sd), while the corresponding values from the previously used elemental marker method were 0.58±0.15. These results demonstrate the importance of the particle size dependent aerosol processes in forming actual human exposures and doses, especially in showing that the exposures and doses are not equally modified by infiltration.