Ambient outdoor concentrations and size distributions of airborne microbial particles were measured approximately weekly for 2 years in a Washington, D.C., suburban area. The study objective was to characterize microbial air quality in the vicinity of a proposed sewage sludge composting facility. During the study, 379 samples were taken at 17 stations, using Andersen microbial samplers. Concentration ranges (in viable particles per cubic meter) were as follows: airborne mesophilic fungi, 0 to 7,220 with a geometric mean of 273; thermophilic fungi, 0 to 193 with a median of 2.1; Aspergillus fumigatus, 0 to 71 with a median of 1.0; aerobic bacteria, 4.2 to 1,640 with a geometric mean of 79; and fecal streptococci, 0 to 5.7 with a median of 0. No fecal coliforms were recovered. The potentially respirable fraction (less than 8 microns) averaged 34% for total bacteria, 56% for mesophilic fungi, 91% for thermophilic fungi, and 95% for A. fumigatus. The specific sampling location was not a major factor affecting microbial particle concentrations or size distributions. Conversely, the time of year was an important determinant of viable particle concentrations for all groups of microorganisms studied. The highest concentrations were observed in summer and fall, with significantly lower levels detected in winter. In general, the microbial data did not correlate with other variables, including weather conditions, measured in this study
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