INTRODUCTION: General. Bioaerosol monitoring is a rapidly emerging area of industrial hygiene. Bioaerosol monitoring includes the measurement of viable (culturable and nonculturable) and nonviable microorganisms in both indoor (e.g., industrial, office or residential) and outdoor (e.g., agricultural and general air quality) environments. In general, indoor bioaerosol sampling need not be performed if visible growth is observed. Monitoring for bioaerosols in the occupational environment is one of the many tools the industrial hygienist uses in the assessment of indoor environmental quality, infectious disease outbreaks, agricultural health, and clean rooms. Contamination (microbial growth on floors, walls, or ceilings, or in the HVAC system) should be remedied. If personnel remain symptomatic after remediation, air sampling may be appropriate, but the industrial hygienist should keep in mind that false negative results are quite possible and should be interpreted with caution. Other exceptions for which bioaerosol sampling may be appropriate include epidemiological investigations, research studies, or if situations indicated by an occupational physician and/or immunologist. Sampling for fungi and bacteria (including Actinomycetes) is included in this chapter. Less developed methods for bioaerosols such as viruses, protozoa, antigenic fragments, algae, arthropods, and mycoplasmas are not addressed at this time. Indoor and Outdoor Bioaerosols. In general, indoor microflora concentrations of a healthy work environment are lower than outdoor concentrations at the same location [ACGIH 1989, Step two; Macher et al. 1995]. If one or more genera are found indoors, in concentrations greater than outdoor concentrations, then the source of amplification must be found and remedied. Bioaerosol sampling is often performed out of doors for pollen and fungi to assist allergists in their treatment of patients by identifying taxa distribution and concentration in air over time. On occasion, outdoor bioaerosol sampling is conducted in an occupational environment (e.g., agricultural investigations and sewage treatment plants). Indoor bioaerosol sampling is often conducted in occupational (industrial and office environments) and nonoccupational (residential and educational buildings) settings. When sampling is indicated, it is advisable to sample before, during, and after the sampling area is occupied, including times when the heating, ventilating, and air conditioning system is activated and inactivated. Viable and Nonviable Bioaerosols. Viable microorganisms are metabolically active (living) organisms with the potential to reproduce. Viable microorganisms may be defined in two subgroups: culturable and nonculturable. Culturable organisms reproduce under controlled conditions. Information regarding environmental conditions and media to culture microorganisms is shown in Sections 3.a. and 3.c. Nonculturable organisms do not reproduce in the laboratory because of intracellular stress or because the conditions (e.g., culture medium or incubation temperature) are not conducive to growth. As the name implies, viable bioaerosol sampling involves collecting a bioaerosol and culturing the collected particulate. Only culturable microorganisms are enumerated and identified, thus leading to an underestimation of bioaerosol concentration. Nonviable microorganisms are not living organisms; as such, they are not capable of reproduction. The bioaerosol is collected on a "greased" surface or a membrane filter. The microorganisms are then enumerated and identified using microscopy, classical microbiology, molecular biological, or immunochemical techniques. When sampling for culturable bacteria and fungi, the bioaerosol is generally collected by impaction onto the surface of a broad spectrum solid medium (agar), filtration through a membrane filter, or impingement into an isotonic liquid medium (water-based). Organisms collected by impaction onto an agar surface may be incubated for a short time, replica-plated (transferred) onto selective or differential media, and incubated at different temperatures for identification and enumeration of microorganisms [Tortora et al. 1989]. Impingement collection fluids are plated directly on agar, serially diluted and plated, or the entire volume of fluid is filtered through a membrane filter. The membrane filter is then placed on an agar surface and all colonies may be replicaplated. Culturable microorganisms may be identified or classified by using microscopy, classical microbiology, or molecular biology techniques such as restriction fragment length polymorphic (RFLP) analysis. Classical microbiology techniques include observation of growth characteristics; cellular or spore morphology; simple and differential staining; and biochemical, physiological, and nutritional testing for culturable bacteria. Analytical techniques which may be applied to both nonviable and viable microorganisms, but not distinguish between them, include polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA). Such methods may be used to identify specific microorganisms and to locate areas of contamination. Though these latter methods are generally qualitative, current research efforts involve modifying the methods to obtain semiquantitative or quantitative results.