Key Words

Indoor and outdoor characteristics of particulate matter (PM) were studied in a naturally ventilated flat. Twenty-four-hour concentrations of indoor PM1, PM2.5 and PM10 and outdoor PM2.5 were measured. In winter all indoor concentrations of PM fractions were highly correlated (r0.97 to 0.98). High correlation was also found between ambient PM10 and outdoor PM2.5 (r0.99). In summer, the correlations between all PM fractions were lower. Human activity had a stronger effect on indoor PM concentrations in winter than in summer but only when the outdoor levels were low. During the second winter campaign higher outdoor concentra-tions were able to mask indoor sources. The presence of people corresponded to elevated levels of coarse particle fractions, cooking was associated with higher levels of fine particles. In summer with windows open, indoor PM values were not associated with human activity. A significant negative association was regis-tered between wind speed and indoor PM