Microbiology of the Built Environment

“Every breath you take, every move you make, every bond you break, every step you take, microorganisms will be watching you.” As you already probably know, microorganisms are literally everywhere – in the air, water and soil, and also in our built environment, which comprises houses, offices and vehicles, among others. But what are microorganisms exactly? How do they end up in interior air? Which airborne microorganisms do different sources emit into our living and working spaces? Do they affect our health and the quality of air that we breathe? These are some of the important questions related to the microbiology of the built environment and indoor air quality that this post will try to answer.

According to the American National Cancer Institute, the term “microorganisms” describes “organisms that can be seen only through a microscope” and it includes bacteria, fungi, protozoa and algae (Fig. 1). Sometimes it also includes viruses, despite the fact that they are not considered living creatures. [1] Due to the fact that we spend up to 90% of our time in indoor spaces [2], it is of great importance to identify the sources of interior air microorganisms so that we can mitigate potential health risks emerging from them. The study conducted by researchers at Virginia State University detected eight major generators of airborne microbes in the built environment – humans, pets, plants, plumbing systems, HVAC systems, water-damaged materials, dust and outdoor environment (Fig. 2). [3] While for most of them there is no source strength data available, research done on emission rates of airborne bacteria and fungi in an occupied classroom shows that we humans might potentially be one of the most significant factors that influence bioaerosol numbers and their community structure in indoor spaces. [3,4] Through respiration and shedding of skin cells, one person emits approximately 3,7 x 107 bacterial and 7,3 x 106 fungal genome copies per hour, which corresponds to the mass emission rate of 30 mg per person per hour. [4] If we look at the results of a study on bacterial contamination in healthy office buildings, we notice that the bacteria that appear most frequently in the air are cocci that originate from human skin, such as Staphylococcus epidermidis, Micrococcus luteus and Staphylococcus hominis. [5] These bacteria are not harmful for people with an intact immune system, but can sometimes lead to diseases under certain circumstances, which are, however, very specific and not related to airborne transmission. [6,7] Unlike in the case of bacteria, humans have only a minor effect on airborne fungi in interior spaces. Despite the fact that humans emit non-insignificant amounts of fungal genome and that one study found traces of skin-associated yeasts in classroom dust [8], like for example Rhodotorula, Candida and Cryptococcus, fungal microorganism communities in indoor environments are dominated by fungi from the outdoor air [3]. Moreover, it was also proven that the concentrations of fungal microbiome in indoor air were affected neither by the number of occupants nor by their behavior. [9]

Fig. 1: Categories of microorganisms. Source: https://www.toppr.com/ask/question/what-is-the-study-of-microorganisms-called/
Fig. 2: Sources of airborne microorganisms in the built environment. Source: https://link.springer.com/article/10.1186/s40168-015-0144-z/tables/1

Besides us humans, another very important source of airborne microorganisms for inside spaces is the outdoor environment. According to the research conducted by W. Nazaroff, the penetration efficiency of bioaerosols in a building with natural ventilation reaches almost 100%, which means that airborne organisms can enter the interior space through leaks and openings without any problems. [10] As already mentioned above, the airborne fungal communities inside are strongly affected by fungi in the outdoor air. A UC Berkeley study on dispersal of microbes showed that the two most dominant fungal species in residential homes were Cryptococcus victoriae and Cladosporium spp. [11], with the latter being responsible for mold growth, allergic reactions or the development of asthma. [12] When it comes to airborne microorganisms in indoor spaces where mechanical ventilation is deployed, further research undertaken at the UC Berkeley demonstrates that not only fungal, but also bacterial communities are strongly affected by outdoor air. [13] The observations resulting from that research show an overlap in the microbial taxa of indoor and outdoor aerosol samples, whereby the most abundant group of bacteria is Burkholderiales spp., which can endanger health of cystic fibrosis patients. [3,13,14] Although humans and outdoor air are the two most significant generators of airborne microorganisms in the built environment [3], the remaining six sources should not be forgotten either, since their contribution to indoor microbe communities is definitely not negligible. Some newer studies proved namely that the presence of a pet, or more precisely a dog, in a household can enrich the indoor air microbiome and have beneficial effects on children’s health. [3] Plants on the other hand can cause elevated levels of potentially hazardous, already above mentioned, airborne Cladosporium spp. [15] If we have a look at the data from several studies on contribution of plumbing systems to indoor bioaerosols concentrations, we can conclude that toilets, sinks and showers generate quite a significant amount of harmful fungi and bacteria which can stay in the air for hours, such as Salmonella spp. and Legionella. [16-19] Speaking of Legionella –HVAC systems that are not regularly cleaned and maintained provide ideal conditions for its growth and spread it all over the room in which they are situated. [20] Last but not least, while water-damaged materials are sources of very toxigenic fungus called Stachybotrys atra, dust resuspension leads to elevated levels of pathogen bacterial genus Staphylococcus in indoor air. [3]

As you probably noticed while reading this post, microbiology of the built environment is still not a fully explored discipline, especially when it comes to understanding of airborne microbiome that requires some further research on emission rates from different sources [3]. Moreover, there is also more potential for creating a link between building design and this discipline in order to improve indoor air quality and other characteristics of interior spaces. [2] Until that happens, we should all be aware of good and bad microorganisms roaming around our homes and keep in mind that with every step we take and move we make, they will be watching us.


[1] American National Cancer Institute (n.d.). NCI Dictionary of Cancer Terms. Retrieved April 26, 2020, from https://www.cancer.gov/publications/dictionaries/cancer-terms/def/microorganism

[2] Kembel, S. W. et al. (2012). Architectural design influences the diversity and structure of the built environment microbiome. The ISME Journal, 6 (2012), 1469–1479. doi:10.1038/ismej.2011.211.

[3] Prussin, A. J. & Marr, L. C. (2015). Sources of airborne microorganisms in the built environment. Microbiome 3, 78 (2015). doi:10.1186/s40168-015-0144-z.

[4] Qian, J. et al. (2012). Size‐resolved emission rates of airborne bacteria and fungi in an occupied classroom. Indoor Air, 22 (2012), 339-351.

[5] Bouillard, L. et al. (2005). Bacterial contamination of indoor air, surfaces, and settled dust, and related dust endotoxin concentrations in healthy office buildings. Ann Agr Environ Med, 12 (2005), 187-192.

[6] Otto, M. (2011). Staphylococcus epidermidis – wie ein Hautkeim zum Problem wird. BIOspektrum, 17(11), 739-741.

[7] Albertson, D. et al. (1978). Septic Shock with Micrococcus luteus. Arch Intern Med, 138(3), 487-488. doi:10.1001/archinte.1978.03630270093032.

[8] Yamamoto, N. et al. (2015). Indoor emissions as a primary source of airborne allergenic fungal particles in classrooms. Environ Sci Technol. 49 (2015), 5098-5106.

[9] Adams, R. I. et al. (2013). Dispersal in microbes: fungi in indoor air are dominated by outdoor air and show dispersal limitation at short distances. The ISME Journal, 7 (2013), 1262-1273.

[10] Nazaroff, W. W. (2014). Indoor bioaerosol dynamics. Indoor Air, 26(1), 61-78. doi:10.1111/ina.12174.

[11] Adams, R. I. et al. (2013). Dispersal in microbes: fungi in indoor air are dominated by outdoor air and show dispersal limitation at short distances. The ISME Journal, 7 (2013), 1262-1273.

[12] Ogorek, R., Lejman, A., Pusz, W. (2012). Characteristics and taxonomy of Cladosporium fungi. Mikologia Lekarska, 19(2), 80-85.  

[13] Adams, R. I. et al. (2015). Chamber bioaerosol study: outdoor air and human occupants as sources of indoor airborne microbes. PLoS One, 10(7). doi:10.1371/journal.pone.0133221.

[14] Voronina, O. et al. (2018). On Burkholderiales order microorganisms and cystic fibrosis in Russia. BMC Genomics, 19(3), 77-90. doi: 10.1186/s12864-018-4472-9.

[15] Burge, H. A., Solomon, W. R., Muilenberg, M. L. (1982). Evaluation of indoor plantings as allergen exposure sources. Allergy Clin Immun, 70, 101-108.

[16] Johnson, D. et al. (2013). Aerosol generation by modern flush toilets. Aerosol Sci Technol, 47, 1047-1057.

[17] Gerba, C. P., Wallis, C., Melnick, J.L. (1975). Microbiological hazards of household toilets: droplet production and the fate of residual organisms. Appl Microbiol, 30, 229-237.

[18] Chang, C-W., Hung, P-Y. (2012). Evaluation of sampling techniques for detection and quantification of airborne legionellae at biological aeration basins and shower rooms. Aerosol Sci, 48, 63-74.

[19] Kinney, K., Bae, S., King, M. (2011). Dynamic monitoring of the aerosolized microbiome in shower units. AAAR Abstracts, 2011, 804.

[20] Ager, B., Tickner, J. (1983). The control of microbiological hazards associated with air-conditioning and ventilation systems. Ann Occup Hyg, 27, 341-358.