HVAC systems typically provide a mixture of outdoor air and recirculated indoor air at supply vents, but the systems themselves can be a source of airborne microorganisms due to contamination. Bernstein et al. Showed that improperly maintained HVAC systems supported abundant growth of Penicillium spp. (as an example) and resulted in 50 to 80 times higher concentrations of airborne fungi in an affected office compared to an unaffected one. Exposure to bioaerosols, including fungal ones, has been linked to a range of detrimental health effects; for example, moulds are associated with the onset of asthma in both infants and adults.