Development of the science of aerobiology has furnished a tool for the investigation of potential sources of microbial aerosols. An investigation of aerosols emitted by trickling-filter sewage treatment plants revealed that coliforms were indeed emitted and have been sampled to a distance of 0.8 mile (1.2 kilometers) downwind. Factors affecting survival of Escherichia coli are presented.

The aerosol stability of two particle forms, infectious and potentially infectious, of reovirus were examined under static conditions for a range of relative humidities at 21 and 24°C. Virus aerosolization efficiency was determined for two methods of dissemination: Collison nebulizer and Chicago atomizer. Suspensions of Bacillus subtilis var. niger spores were added to reovirus preparations that included both particle forms and disseminated into a dynamic aerosol toroid to estimate the physical decay of the aerosols. At 90 to 100% relative humidity, both reovirus particle forms showed less than 10-fold loss of infectivity after 12 h of aging. At lower relative humidities the aerosol decay curve showed rapid initial decay followed by a markedly lower decay rate. Our findings reveal that reovirus particles are relatively stable in the airborne state.

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The occurrence of animal viruses in the aerosol emissions of wastewater treatment facilities was evaluated by direct assay and by the use of coliforms and coliphages as indicator organisms. Coliforms and coliphages were compared and evaluated with regard to their suitability as indicators of airborne animal viral contamination from wastewater treatment facilities. Two plants, one with treatment by activated sludge and the other by trickling filtration, were studied. Field air sampling procedures used large-volume air samplers, with recirculation devices, and Andersen samplers. Airborne viruses were enumerated by a most probable number (MPN) procedure. Partially treated liquid sewage contained about 1.0x 102 pfu 1-~ of animal viruses assayed on Buffalo Green IVlonkey (BGM) cells, 3.6 x 105 and 5.0 x 105 pfu It of coliphages, depending upon the E. coli host strain used for assay, and 2.0 x 10 9 colonies 1-~ of coliform bacteria. No airborne animal viruses were recovered, airborne coliphage levels averaged 2.3 x 10-t and 3.0 x 10-t MPNm-3, coliforms from aerosol emissions were 2.1 x 102 colonies m-3. Ratios of coliphages to animal viruses indicate that wastewater treatment plants may be continuous sources of low level concentrations of animal virus aerosols. Evidence shows coliforms to be much less stable than coliphages in the airborne state. Coliphages may be a more acceptable indicator of airborne animal viral contamination than coliforms.

The emission (from wastewater treatment plants) of airborne coliphages that form plaques on two strains ofEscherichia coli was investigated. Two activatedsludge and two trickling-filter plants were studied. Field sampling procedures used large-volume air samplers with recirculation devices. Coliphages were enumerated by a most-probable-number (MPN) procedure. Temperature, relative humidity, windspeed, and presence of sunlight were monitored. Concurrent samples of sewage were taken during each air-sampling run. Average coliphage levels in the airborne emissions of trickling-filter beds and activated-sludge units were 2.84 x 10-l and 3.02 x 10-' MPN/m3, respectively, for all positive observations, and sewage liquor concentrations from the sources were 4.48 x 105 and 2.94 x 106 plaque-forming units/liter, respectively, depending upon the E. coli host used for assay. This work establishes minimal airborne-coliphage concentrations from the plants studied. The procedures employed will be useful in evaluating the animal virus levels in these emissions.