During the composting process, meso- and thermophilic microorganisms are released into the ambient air. It is known that human health can be endangered by massive concentrations of bacteria and moulds in the aerosol of compost. The focus of this investigation was on the main pathogens of extrinsic allergic alveolitis (EAA), Saccharopolyspora sp., Saccharomonospora sp., Streptomyces sp. and Thermoacti- nomyces sp. (family Bacillaceae). From March 1996 until April 1997, airborne microorganisms were sampled in and around two different composting facilities (open and closed system) using Andersen 6-stage viable cascade impactors. Additional control sampling was conducted at various sampling locations in the urban and rural districts of Graz. There were no or only very low counts for thermophilic bacteria in the urban and rural areas of Graz (C). The median counts for actinomycetes were 31 CFU/m 3 air and for Thermoactinomyces sp. 13 CFU/m 3 air. In the open system (A), total actinomycete counts in the immediate vicinity of the facility at distance 3 m and 10 m were 54 CFU/m 3 air and 97 CFU/m 3 air (median values), respectively. From a distance of 10 m, Thermoactinomyces species occurred infrequently only. Sampling in the closed facility (B) showed the highest load of total actinomycetes during the postrot phase at 549 CFU/m 3 air (median) and 27 CFU/m 3 air for Thermoactinomyces sp. The maximum value was 4.356 CFU/m 3 air for thermophilic actinomycetes. Pathogenic species (EAA) were detected at all sampling locations in the area of the composting facilities. However, all of these counts are below the "threshold limits" discussed in previous studies. For an assessment of emissions, further studies are required, especially measurements covering a complete rotting process. In order to reduce the health risk to employees of a composting plant, measures relating to occupational medicine such as mouth and nose protection, air exhaust and cleaning of exhaust air, automated processing of compost etc. should definitely be taken (18). Furthermore, the main pathogens of EAA should be included in the establishment of standard values and limits.

        Key words: bioaerosols, airborne microorganisms, composting facilities, thermophilic actinomycetes