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This study aims to determine the microbiological quality of particulate matter (PM) in an urban area in Jeddah, Saudi Arabia, during December 2012 to April 2013. This was achieved by the determination of airborne bacteria, fungi, and actinobacteria associated PM10 and PM2.5, as well as their relationships with gaseous pollutants, O-3, SO2 and NO2, and meteorological factors (T degrees C, RH% and Ws). High volume samplers with PM10 and PM2.5 selective sizes, and glass fiber filters were used to collect PM10 and PM2.5, respectively. The filters were suspended in buffer phosphate and aliquots were spread plated onto the surfaces of trypticase soy agar, malt extract agar, and starch casein agar media for counting of bacteria, fungi and actinobacteria-associated PM, respectively. PM10 and PM2.5 concentrations averaged 159.9 mu g/m(3) and 60 mu g/m(3), respectively, with the ratio of PM2.5/PM10 averaged similar to 0.4. The concentrations of O-3, SO2 and NO2 averaged 35.73 mu g/m(3), 38.1 mu g/m(3) and 52.5 mu g/m(3), respectively. Fungi and actinobacteria associated PM were found in lower concentrations than bacteria. The sum of microbial loads was higher in PM10 than PM2.5, however a significant correlation (r = 0.57, P <= 0.05) was found between the sum of microbial loads associated PM10 and PM2.5. Aspergillus fumigatus and Aspergillus niger were the common fungal types associated PM. Temperature significantly correlated with both PM10 (r = 0.44), and PM2.5 (r = 0.5). Significant negative correlations were found between O-3 and PM2.5 (r = -0.47), and between SO2 with PM10 (r = -0.48). Wind speed positively correlated with airborne microorganisms associated PM. The regression model showed that the inverse PM2.5 concentration (1/PM2.5) was a significant determinant of fungal count associated PM. Chemical processes and environmental factors could affect properties of PM and in turn its biological quality. (C) 2014 Elsevier B.V. All rights reserved.