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.Douwes, J., et al., Bioaerosol health effects and exposure assessment: Progress and prospects. Annals of Occupational Hygiene, 2003. 47(3): p. 187-200. Size of bioaerosol particles varies from below 1 μm to 100 μm in aerodynamic diameter;Sanchez-Monedero, M.A., et al., Effect of the aeration system on the levels of airborne microorganisms generated at wastewater treatment plants. Water Research, 2008. 42(14): p. 3739-3744. viable bioaerosol particles can be suspended in air as single cells or aggregates of microorganism as small as 1–10 μm in size.Pastuszka, J.S., et al., Bacterial and fungal aerosol in indoor environment in Upper Silesia, Poland. Atmospheric Environment, 2000. 34(22): p. 3833-3842. Since bioaerosols are potentially related to various human health effectsChao, H.J., et al., Populations and determinants of airborne fungi in large office buildings. Environmental Health Perspectives, 2002. 110(8): p. 777-782.Peccia, J., et al., A role for environmental engineering and science in preventing bioaerosol-related disease. Environmental Science & Technology, 2008. 42(13): p. 4631-4637.Wouters, I.M., et al., Increased levels of markers of microbial exposure in homes with indoor storage of organic household waste. Applied and Environmental Microbiology, 2000. 66(2): p. 627-631.Burge, H., Bioaerosol - prevalence and health effects in the indoor environment. Journal of Allergy and Clinical Immunology, 1990. 86(5): p. 687-701. 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.Chen, Q. and L.M. Hildemann, The Effects of Human Activities on Exposure to Particulate Matter and Bioaerosols in Residential Homes. Environmental Science & Technology, 2009. 43(13): p. 4641-4646.Park, J.H., et al., Predictors of airborne endotoxin in the home. Environmental Health Perspectives, 2001. 109(8): p. 859-864. Several studies have identified human activities as an important source for indoor bioaerosols.Kalogerakis, N., et al., Indoor air quality - bioaerosol measurements in domestic and office premises. Journal of Aerosol Science, 2005. 36(5-6): p. 751-761.Li, C.S. and P.A. Hou, Bioaerosol characteristics in hospital clean rooms. Science of the Total Environment, 2003. 305(1-3): p. 169-176. 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.Sanchez-Monedero, M.A., E.I. Stentiford, and C. Mondini, Biofiltration at composting facilities: Effectiveness for bioaerosol control. Environmental Science & Technology, 2003. 37(18): p. 4299-4303.Bauer, H., et al., Bacteria and fungi in aerosols generated by two different types of wastewater treatment plants. Water Research, 2002. 36(16): p. 3965-3970.
Factors influencing indoor bioaerosol generation According to previous studies,Verreault, D., S. Moineau, and C. Duchaine, Methods for sampling of airborne viruses. Microbiology and Molecular Biology Reviews, 2008. 72(3): p. 413-444.Dekoster, J.A. and P.S. Thorne, Bioaerosol concentrations in noncomplaint, complaint, and intervention homes in the Midwest. American Industrial Hygiene Association Journal, 1995. 56(6): p. 573-580.Law, A.K.Y., C.K. Chau, and G.Y.S. Chan, Characteristics of bioaerosol profile in office buildings in Hong Kong. Building and Environment, 2001. 36(4): p. 527-541. 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.
Human health effects Adverse health effects/diseases related to indoor bioaerosol exposure can be divided into two categories: those confirmed to be associated with bioaerosol and those suspected but not confirmed to be associated with bioaerosol. Bioaerosols have been revealed to cause certain human diseases, such as tuberculosis, Legionnaires' disease and different forms of bacterial pneumonia, coccidioidomycosis, influenza, measles, and gastrointestinal illness.Peccia, J. and M. Hernandez, Incorporating polymerase chain reaction-based identification, population characterization, and quantification of microorganisms into aerosol science: A review. Atmospheric Environment, 2006. 40(21): p. 3941-3961. Bioaerosols are also associated with some noninfectious airway diseases, such as allergies and asthma. As a known component of indoor bioaerosol, β(1→3)-glucan (cell wall components of most fungi) is proposed to be the causative agent of mold-induced nonallergic inflammatory reactions. It is reported that 25%-30% of allergenic asthma cases in industrialised countries are induced by fungi, which has been the focus of concerns about human exposure to airborne microorganisms in recent years.Vanhee, L.M.E., H.J. Nelis, and T. Coenye, Rapid Detection and Quantification of Aspergillus fumigatus in Environmental Air Samples Using Solid-Phase Cytometry. Environmental Science & Technology, 2009. 43(9): p. 3233-3239.
Some other human diseases and symptoms have been proposed to be associated with indoor bioaerosol, but no deterministic conclusions could be drawn due to the insufficiency of evidence. One example is the well-known sick building syndrome (SBS). SBS refers to non-specific complaints, such as upper-respiratory irritative symptoms, headaches, fatigue, and rash, which cannot be related to an identifiable cause but are building related.Redlich, C.A., J. Sparer, and M.R. Cullen, Sick-building syndrome. Lancet, 1997. 349(9057): p. 1013-1016. Over the last two decades, there have been many studies indicating association of indoor bioaerosol with sick building syndrome.Cooley, J.D., et al., Correlation between the prevalence of certain fungi and sick building syndrome. Occupational and Environmental Medicine, 1998. 55(9): p. 579-584.Gyntelberg, F., et al., Dust and the sick building syndrome. Indoor Air-International Journal of Indoor Air Quality and Climate, 1994. 4(4): p. 223-238.Teeuw, K.B., C. Vandenbrouckegrauls, and J. Verhoef, Airborne gram-negative bacteria and endotoxin in sick building syndrome - a study in Dutch governmental office buildings. Archives of Internal Medicine, 1994. 154(20): p. 2339-2345.Li, C.S., C.W. Hsu, and M.L. Tai, Indoor pollution and sick building syndrome symptoms among workers in day-care centers. Archives of Environmental Health, 1997. 52(3): p. 200-207. However, most of the related studies based their conclusions on statistical correlation between concentrations of certain types of bioaerosols and incidence of complaints, which has various drawbacks methodologically. For example, some studies have a small sample size, which critically undermines the validity of speculations based on the statistical results. Also, many studies were not able to exclude the influences of other factors beside bioaerosol in their analysis, which makes the statistical correlation theoretically inappropriate to support association of SBS with bioaerosols. Additional studies revealed that bioaerosol is unlikely to be the cause of SBS.Burge, P.S., Sick building syndrome. Occupational and Environmental Medicine, 2004. 61(2): p. 185-190.Harrison, J., et al., An investigation of the relationship between microbial and particulate indoor air pollution and the sick building syndrome. Respiratory Medicine, 1992. 86(3): p. 225-235. Recent epidemiological and toxicological studies continued to suggest a possible link between bioaerosol exposure and sick building syndrome, but methodological limitations remained in these studies.Laumbach, R.J. and H.M. Kipen, Bioaerosols and sick building syndrome: particles, inflammation, and allergy. Current Opinion in Allergy and Clinical Immunology, 2005. 5(2): p. 135-139.
The ability of bioaerosols to cause human disease depend not only on their chemical composition and biological characteristics, but also on the quantity of bioaerosol inhaled and their size distribution, which determines the site of bioaerosol deposition to human respiratory systems. Bioaerosols larger than 10 μm in aerodynamic diameter are generally blocked by the nasal region of the respiratory tract, those between 5-10 μm mainly deposit in the upper respiratory system and usually induce symptoms like allergic rhinitis, and particles with aerodynamic diameter less than 5 μm can reach the alveoli and hence lead to serious illnesses such as allergic alveolitis.