Industrial compost made from source separated household waste was investigated. The objective was to estimate the microbial risk potential of bioaerosols over the progressing stages of outdoor pile composting. Bulk samples of 75 kg were collected from piles after 1, 5 and 9 weeks of composting, and bioaerosols were generated experimentally in a rotating drum tester. Samples of bioaerosols and composted materials were quantified for total counts of microbial cells by epi-fluorescence microscopy, viable counts of microorganisms and endotoxin. Five groups of microorganisms were cultured in parallel: mesophilic bacteria, actinomycetes and fungi (25°C), Aspergillus fumigatus and thermophilic actinomycetes (55°C). Furthermore, the bulk samples were analysed for nine other groups of viable microorganisms. In general, the concentrations were increasing (p<0.05) over time of pile composting. The microflora was dominated by bacteria and actinomycetes to a total of 10⁹-10¹¹ cells/g bioaerosol or composted materials. In bulk samples the maximum concentrations of viable bacteria reached 2.1x10⁹ cfu/g and the thermophilic actinomycetes 1.8x10⁷ cfu/g. The maximum viable concentrations in bioaerosols were 1.7x10⁷ cfu/g for bacteria and 1.5x10⁷ cfu/g for thermophilic actinomycetes. Maximum endotoxin levels were equivalent to 14 μg/g and 110 μg/g in bioaerosols and bulk samples, respectively. The composition of the microflora differed from most other compost studies by low fungal concentrations, and Aspergillus fumigatus was only found sporadically in levels close to the limit of detection. Thermophilic actinomycetes were the predominant source for airborne spores which should be emphasized in risk assessments for this particular type of compost. Personal sampling is recommended to clarify the occupational exposure.