Domestic sewage sludges that have been stabilized to reduce pathogen content are termed biosolids; their beneficial reuse for agricultural applications constitutes approximately 60% of the 7 million dry tons generated each year in the United States (6). Driven by health complaints from residents living near application sites, concern has been expressed that a hazardous exposure is created when biosolid aerosols are spread onto fields, incorporated into soils, and released during high-wind events (23,36). Research in the last 5 years has made significant progress in estimating the human exposure to total biosolid aerosols and biosolid indicator microorganisms. Advances include both the development and calibration of theoretical and empirically based microbial aerosol models to assess off-site exposure during land application operations (8,16,31,54) and the application of culture and DNA-based microbial-source tracking methods to these aerosols (5, 14). However, the paucity of information on the viral and bacterial pathogen content of biosolids has limited model use and effectiveness.The U.S. Environmental Protection Agency (EPA) classifies end-product biosolids as class A (pathogen free) or class B (contains pathogens) based on indicator content and/or technologies used for stabilization (48). It is therefore important to distinguish between regulatory classes and treatment classes when describing the pathogen load in biosolids. In large U.S. municipalities, biosolids are most commonly stabilized with mesophilic anaerobic digestion (MAD) to produce a class B product, but current trends show that U.S. utilities have or are considering options for upgrading stabilization technology to produce class A biosolids. This includes the conversion to temperature-phased anaerobic digestion (TPAD) operations and/or composting the biosolids after anaerobic digestion (COM) (20). Meeting class A status requires the monitoring of fecal coliforms for estimating pathogen content, while the monitoring of enteric viruses, helminth ova, and Salmonella spp. is not required as long as "time and temperature" requirements are met. The majority of U.S. class A operations choose to meet class A status with time and temperature treatmentbased alternatives. Based on these regulatory monitoring targets, most culture-or PCR-based biosolid studies have focused on these microorganisms and similar enteric pathogens (i.e., Listeria monocytogenes, enterovirus, and Clostridium perfringens) but have not diversified to other relevant airborne pathogens, like Legionella pneumophila, that may proliferate during stabilization where enteric pathogens cannot (1,9,22,28,34,40,41,53). Only recently, and in limited sample sizes, have researchers begun to directly compare the diverse sludge stabilization methods available, including MAD, liming, and composting, to understand how selected enteric pathogens and indicators are removed (18, 22, 32, 34).