abstract: Manual measurements of PM2.5 concentration are performed at two air quality monitoring stations located in the Warsaw agglomeration. At the first station, named MzWarszNiepodlKom, which is managed by the Voivodship Inspectorate of Environmental Protection in Warsaw, continuous measurements of PM2.5 concentration are carried out, whereas at the second station, MzWarszSGGW, belonging to the Division of Meteorology and Climatology at Warsaw University of Life Sciences, seasonal PM2.5 measurements are available. In this paper, preliminary results of the research on PM2.5 concentration at the considered stations for years 2004-2007 are presented. Estimation of PM2.5 fraction share in PM10 was also included in the contents of this paper. Using the methodology for air quality index determination (AQI), proposed by US EPA, the variability of classes of inhabitants exposure to the analysed particulate matter fraction was subject to considerations as well.