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Background Long-term surveillance of airborne bioaerosols in health care facilities is required to protect the health of patients and health care workers. Feasible methods to measure airborne bioaerosol concentrations and determine associated environmental factors may help to avoid nosocomial tuberculosis (TB). Objectives To describe the concentrations and size of airborne bioaerosols and to identify the potential contributors to indoor airborne bioaerosols in TB high-risk areas in health care facilities. Methods We conducted a cross-sectional study in 7 large health care facilities located in Bangkok and nearby in central Thailand using a 6-stage Andersen cascade impactor to collect viable airborne bioaerosols that were quantified using culture techniques. Environmental parameters were determined using a tracer gas technique with an indoor air quality meter. Other potential factors were assessed using a questionnaire. Results The mean indoor airborne bacterial and fungal concentrations were 596.1 and 521.2 colony-forming units (cfu)/m3, respectively, and the mean outdoor airborne bacterial and fungal concentrations were 496.5 and 650.1 cfu/m3, respectively. The majority of airborne bioaerosols were in respirable sizes. The indoor-to-outdoor ratios were 1.2 for bacteria and 0.8 for fungi. Air change rate was inversely correlated with indoor airborne bioaerosol concentrations, whereas emergency department central-type air conditioners and relative humidity were positively correlated with the indoor airborne bioaerosol concentrations (P < 0.05). Conclusions High indoor bioaerosol concentrations found in the health care facilities suggest that it is imperative to improve the indoor air quality. Improved air change rate and avoiding use of central-type air-conditioning systems may reduce bioaerosol concentrations.