Seasonal patterns of outdoor PM infiltration into indoor environments: review and meta-analysis of available studies from different climatological zones in Europe

Epidemiologists have observed higher risks for exposure to ambient particulate matter (PM) in the summer than in other seasons. This increased risk may be partly due to seasonal behaviour and higher exposures to indoor PM in the summer in relation to outdoor pollutant levels during winter when windows are kept closed and less time is spent outdoors. In this report, we analyse data from six European studies, based on three different methods of estimating outdoor to indoor infiltration factors, with the aim of characterizing the geographical and seasonal patterns of PM infiltration. The highest infiltration levels were observed for the summer in both a European combined dataset consisting of 382 observations of the average PM_2.5 infiltration factor for 1 day to 2weeks in regional data sets for Northern, Central and Southern Europe as well as for all ten cities individually. Th lowest values were observed for the winter, with spring and autumn displaying intermediate values. In all datasets and cities, the variability between residences and days within each season was much higher than the seasonal trend. PM₁₀ data were available from two studies, revealing that the PM₁₀ infiltration factors ranged from 70 to 92% of the corresponding PM_2.5 values. Some differences between the studies may be associated with the study designs and applied methods of determining the infiltration factor. The ratio of summer to winter PM_2.5 infiltration ranged from 1.3 in Rome to 2.3 in Helsinki, and the corresponding regional ratio ranged from 1.5 in Central Europe to 1.8 in Northern and Southern Europe. It is suggested that similar differences can be expected in epidemiological concentration–response relationships due to the modification in seasonal exposure associated with buildings and time spent indoors.