Indoor bioaerosol is bioaerosol in an indoor environment. Bioaerosols are natural or artificial particles of biological (microbial, plant, or animal) origin suspended in the air. These particles are also referred to as organic dust. Bioaerosols may consist of bacteria, fungi (and spores and cell fragments of fungi), viruses, microbial toxins, pollen, plant fibers, etc. Size of bioaerosol particles varies from below 1 μm to 100 μm in aerodynamic diameter; viable bioaerosol particles can be suspended in air as single cells or aggregates of microorganism as small as 1–10 μm in size. Since bioaerosols are potentially related to various human health effects and the indoor environment provides a unique exposure situation, concerns about indoor bioaerosols have increased over the last decade.
Sources and influencing factors
Sources for indoor environments Indoor bioaerosols may originate from outdoor air and indoor reservoirs. Although outdoor bioaerosols cannot easily migrate into large buildings with complex ventilation systems, certain categories of outdoor bioaerosols (i.e., fungal spores) do serve as major sources for indoor bioaerosols in naturally ventilated buildings at specific periods of time (i.e., growing seasons for fungi). Major indoor sources for bioaerosols at residential homes include human occupants, pets, house dust, organic waste, as well as the heating, ventilation and air-conditioning (HVAC) system. Several studies have identified human activities as an important source for indoor bioaerosols. Human bodies can generate bioaerosols directly through activities like talking, sneezing, and coughing, while other residential activities (i.e., washing, flushing toilet, sweeping floor) can generate bioaerosols indirectly. Since microorganisms can accumulate and grow on dust particles, house dust is a potential source of bioaerosols. In a study by Wouters et al., they investigated the effects of indoor storage of organic household waste on microbial contamination among 99 households in the Netherlands in the summer of 1997, and indicated that "increased microbial contaminant levels in homes are associated with indoor storage of separated organic waste", which might elevate "the risk of bioaerosol-related respiratory symptoms in susceptible people". However, the analysis by Wouters et al. was based on the collected samples of settled house dust, which might not serve as a strong indicator for bioaerosols suspended in the air. Other materials in residential buildings, such as food stuffs, house plants, textiles, wood material and furniture stuffing can also become bioaerosol sources when water content is appropriate for microorganisms to grow. For non-residential buildings, some specific indoor environments, such as hospitals, wastewater treatment plants, composting facilities, certain biotechnical laboratories, have been revealed to have bioaerosol sources related to their particular environmental characteristics.
Factors influencing indoor bioaerosol generation According to previous studies, major indoor environmental factors influencing bioaerosol concentration include relative humidity, characteristics of air ventilation systems, seasonal variation, temperature, and chemical composition of the air. Other factors, such as the type of home, building material, geographical factors, do not seem to have significant impacts on respirable fungi and bacteria (important constituents of bioaerosols). Relative humidity is one of the most widely studied influencing factors for indoor bioaerosols. Concentrations of two categories of bioaerosols, endotoxin and airborne fungi, are both positively related to indoor relative humidity (higher concentration associated with higher relative humidity). Relative humidity also affects the infectivity of airborne viruses. Regarding the characterisation of air ventilation system, increased use of central air conditioning is found to be associated with lower fungal bioaerosol concentration.
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