Viable pathogens are generally present at every stage of wastewater treatment. Thus a potential airborne infectious pathway due to microbial aerosols exists, particularly at activated sludge treatment plants. In this study the concentration of sewage-borne indicator bacteria and viruses in effluent aerosols was determined and the exposure to such aerosols by treatment plant workers was estimated. Initial aerosol collection with fog-gage screen collectors yielded erratic results due especially to lack of control over sampled air volume and slit openings too large for effective collection. Standard Andersen and glass impinger collectors were used throughout the field study. Areas downwind of aeration basins yielded as much as 1 657 CFU/m^3 total bacteria and 1.8 CFU/m^3 coliphage. These aerosols were mostly 2.1 to 3.3 μm droplet nuclei with a high potential for inhalation and bronchial retention. The inhalation of microbial aerosols by Wahiawa treatment plant workers was estimated to be between 48 to 94 total bacteria/person/day above background levels. The inhaled total coliform level may average 8 to 27 CFU/person/day. This usually short duration and sometimes intense exposure could not be implicated directly to cause adverse health effects. Correlations of statistical tabulations of sick-leave illness with degree of exposure were not significant at the 0.01 or the 0.05 level.