There are four major types of human pathogenic organisms found in biosolids: (1) bacteria, (2) viruses, (3) protozoa, and (4) helminths (parasitic worms). The concentration of organisms in biosolids depends on the type and concentration of pathogens present in the source sewage and the degree to which those organisms are removed or killed by the pre -treatment process. The hazard that is associated with Class B biosolids is a function of the number and type of pathogens in the treated sludge relative to the minimum infective dose and the exposure level. Under the EPA biosolids rule (40 CFR 503), Class B biosolids must contain less than two million colony forming units (CFU) of fecal coliform per gram of total solids (dry weight). To protect public health, the EPA rule prescribes a restricted period of up to one year to limit public access to lands where Class B biosolids have been applied. These EPA restrictions do not apply to occupational access. The risk of worker exposure to infectious agents is likely greatest during and immediately after land application of the biosolids. Because the concentration of pathogens decays through natural processes, the potential for pathogen exposure decreases over time. During investigations at a Class B biosolids land application and at a Class B biosolids storage site: NIOSH interviewed five employees at a Class B biosolids land application operation. All five employees reported at least one episode of gastrointestinal illness after working with the biosolids, either at the treatment plant or during land application. The environmental sampling results indicated that the gastrointestinal illnesses were possibly of occupational origin. NIOSH collected bulk samples from different locations within the biosolids storage site. The mean fecal coliform concentration of the bulk samples was 220,000 CFU per gram of sample (wet weight). Fecal coliforms are used as an indicator for the presence of other enteric microorganisms. There remains a significant exposure risk. This example illustrates the importance of minimizing exposure to Class B biosolids during the restricted period. NIOSH collected air samples at the Class B biosolids land application and storage site. Enteric bacteria were detected in the air. The presence of enteric bacteria in air samples is sufficient justification to implement engineering controls and work practices that minimize employee exposure to biosolid aerosols. The detection of enteric bacteria in a limited number of air and bulk samples confirms the potential for workers to be exposed to organisms which have been associated with gastrointestinal symptoms and illness. Operations where employees are potentially exposed to Class B biosolids include transport, loading, unloading, and application activities. Other potentially exposed workers include compost workers, surface miners working around reclamation sites, and farmers.