Current World Environment

Introduction

In the last few decades, the problems of airborne bacteria and fungi in indoor environments have significantly increased worldwide¹. The quality of the living spaces and occupants' health and well-being can be influenced directly or indirectly by microbes in buildings^2-6. The problems of indoor microbes in the built environment have been taken seriously into account by biologists in collaboration with building designers^{7, 8}. Kembel et al. summarised the three essential concerns associated with indoor microbes. Firstly, indoor bacteria and fungi interact with one another and with the environment in the complex ecosystem of the built environment^10-12. Secondly, people in the built environment have significant potential contact with indoor bacteria and fungi^13,14. Thirdly, many cases show that indoor bacteria and fungi affect human health^15-17.

Numerous studies on airborne bacteria and fungi in the built environment conducted in different countries, including Korea, China, Ethiopia, Iran, Italy, Portugal, Jordan, France, Egypt, Malaysia, Pakistan, Greece and Finland with varying indoor locations^15,17-28. Table 1 provides the details (i.e. country, location, finding, and genera) of the studies on indoor airborne conducted worldwide. Micrococcus, Staphylococcus, Bacillus and Pseudomonas are the most common bacteria found indoors⁵.

Table 1: Details of Studies on Indoor Airborne Conducted Worldwide.

No.	Country		Location		Finding		Dominant Genera			References
1.	Finland		School buildings		Fungal was influenced by hidden excess moisture and mould growth of other fungi		Fungal: Trichoderma species (i.e. Mycoparasitic and mycotrophic T. atroviride)			28
2.	Greece		Primary school		Green Roof System tend to improve indoor air quality, hence enhanced the students' comfort and performance.		Fungal: Penicillium, Cladosporium, and Aspergillus			17
3.	Pakistan		Cafeterias of a university campus		The outdoor fungal and bacterial concentrations were higher than that in indoor		- Fungal: 46% of Cladosporium spp., 8.6% of Geotrichium spp., 8.5% of Ulocladium spp., 7.9% of Alterneria spp., 6.6% of Fusarium spp., 3% of Curvularia spp., 2.4% of Aspergillus spp. and 1.3% of Penicillium spp. - Bacterial: Staphylococcus spp., Kocuria spp., Bacillus spp. and Micrococcus spp.			27
4.	Malaysia		Mosques		The bacterial and fungal counts and the average of PM10 concentrations were higher in air conditioning mosques compared to non-air conditioning mosques		Bacterial: Staphylococcus spp., Bacillus spp. and Micrococci spp. Fungal: Aspergillus niger.			26
5.	Egypt		Public buildings: libraries, faculties, schools, child daycare centres and hospitals		- The microbes contaminate the hospitals' operation - Temperature has no significance in bacterial growth - Particulate matter and relative humidity influential in fungal growth		- Bacterial: Bacillus subtilis and Bacillus atrophaeus - Fungal: Aspergillus, Alternaria, Penicillium, and Cladosporium			25
6.	France		Hospitals		The bacterial growth has no relationship with the sampling sites.		- Fungal: Penicillium, Alternaria, Cladosporium - Bacterial: Staphylococcus Aureus, Escherichia coli			24
7.	Jordan		Dwellings and educational building		The PAHs metabolite, which is active and carcinogenic products could influence the human		- Bacterial: Gram-positive bacteria, Gram-negative bacteria, Fungal: Penicillium/Aspergillus spp.			23
8.		Portugal		Homes, child day-care centres, primary schools, and elderly care centres		- The cause of the highest indoor bacterial and fungal concentrations in the user activities and density and the lack of ventilation.		Fungal: Penicillium and Cladosporium	22
9.		Italy		Library		The white efflorescence appeared in the book caused by the lack of ventilation system, a dusting programme, air-conditioning, and the hygroscopic behaviour of materials used in the book.		Fungal: Eurotium halophilicum behaved as a pioneer species	21
10.		Iran		Hospital		The outdoor bioaerosol influenced indoor hospital air quality		- Bacterial: Bacillus spp., Micrococcus spp., Streptomyces spp., and Staphylococcus spp. were the dominant bacteria indoors and outdoors on regular and dust event days.	20
11.		Ethiopia		Library university		Most bacterial and fungal isolated could cause sick building syndromes with significant allergy, rhinitis, asthma, and conjunctivitis.		- Bacteria: Micrococcus sp., Staphylococcus aureus, Streptococcus pyogenes, Bacillus sp., and Neisseria sp. - Fungal: Cladosporium sp., Alternaria sp., Penicillium sp., and Aspergillus sp.	15
12.		Hong Kong, China		Restaurants located on the hillside at The Chinese University of Hong Kong		Researcher’s identified the bacteria species in the skin and respiratory tract of humans and from the soil.		Bacterial: Gram-positive bacteria in which Micrococcus and Bacillus species were the most abundant	19
13.		Korea		Factories		- The environmental factors have a significantly positive relationship with the vitality of indoor airborne microorganisms.		- Bacterial: Staphylococcus spp., Micrococcus spp., Corynebacterium spp. and Bacillus spp. - Fungal: Cladosporium spp., Penicillium spp. and Aspergillus spp.	18

 

Certain bacteria and fungi species tend to dominate various indoor spaces in buildings. These include schools, cafeterias, mosques, faculties, child day-care centres, hospitals, dwellings, educational buildings, homes, elderly care centres, libraries, restaurants, and factories.  The bacteria species were Bacillus spp. (24.1 %), Staphylococcus spp. (20.7 %), and Micrococcus spp. (20.7 %), Gram-positive bacteria (6.9 %), Kocuria spp. (3.4 %), Aureus (3.4 %), Escherichia coli (3.4 %), Gram-negative bacteria (3.4 %), Streptomyces spp. (3.4 %), Escherichia coli (3.4 %), Gram-negative bacteria (3.4 %), Streptococcus pyogenes (3.4 %), Neisseria spp. (3.4 %), and Corynebacterium spp. (3.4 %).  Meanwhile, fungi species were Penicillium spp. (25 %), Cladosporium spp. (21.9 %), Aspergillus spp. (21.9 %), Alterneria spp. (12.5 %), Trichoderma spp. (3.1 %), Ulocladium spp. (3.1 %), Geotrichium spp. (3.1 %), Fusarium spp. (3.1 %), Curvularia spp. (3.1 %), and Eurotium halophilicum (3.1 %).

Factors of Indoor Biological Contaminants in Buildings

Dispersal of Airborne Bacteria and Fungi by Outdoor Air

Outdoor air has become one of the main factors contributing to the complex system of indoor biological contaminants within a building^{25, 29, 30}. Air exchange in the built environment is the replacement of fresh outdoor air with indoor air.  A high air exchange rate from the outdoor environment with comfortable climate conditions can improve indoor air quality by reducing indoor air contaminants³⁰. Outdoor air can be exchanged with indoor air through infiltration (i.e. particle penetration through the leakage paths of the wall’s building), natural ventilation (i.e. doors, windows and other openings) and mechanical ventilation (i.e. fans) ^{30, 31}. Figure 1 shows the building air exchange³¹. However, indoor airborne bacteria and fungi accumulated from the dispersal from the outdoors that grew and resuspended in the built environment²⁹ can potentially influence the microbial community structure⁹.  Table 2 lists the study cases on outdoor air with microbial problems.

Figure 1: Air Exchange in Buildings through Infiltration, Natural Ventilation, and Mechanical Ventilation Adapted from Chen and Zhao31. Click here to view Figure

Table 2: Studies on Outdoor Air Dispersed into Indoor Buildings with Indoor Microbial Problems.

No.	Dispersion of Outdoor Air with Indoor Microbial Problems in the Built Environment	Dominant Genera	References
1.	From the ecological approach, fungi could be dispersed from the outdoors into the built environment when the building type is held constant.	The 100% sequence similarity of fungi in indoor air and outdoor air shows that Epicoccum sp., Aureobasidum pullulans, Cladosporium sp., Cryptococcus carnescens and Epicoccum sp. are the dominant genera.	29
2.	Outdoor bioaerosol can contribute to the lower indoor air quality in hospitals.	The dominant bacteria indoors and outdoors were Bacillus spp., Micrococcus spp., Streptomyces spp. and Staphylococcus spp.	20
3.	The indoor/outdoor (I/O) ratios of airborne microorganisms at old and new book libraries are less than one, indicating no microbial sources from indoor.	The dominant indoor bacteria from outdoors are Bacillus pseudomycoides and B. subtilis, whereas the prevalent outdoor bacteria are B. subtilis and Staphylococcus aspergillus and Penicillium.	30

Dust Present in Indoor Air

The accumulated dust (i.e. particulate matter) is one factor supporting airborne bacterial and fungal culturability in an indoor environment ²⁵. Indoor airborne particles sizes in the aerodynamic diameter range of 0.1–10 µm (i.e. coarse particles ≥ 7 µm and fine particles < 2.5 µm)³⁰. Figure 2 shows the airborne particles that can penetrate the human respiratory system from stage 0 to stage 7 with their respective particle size distribution³³. Pyrri et al.17 found that Aspergillus concentration is positively correlated with the coarse fraction of PM10 concentration indoors.  Besides, the bacterial and fungal components have contributed to the adverse health effects (i.e. inflammatory lung diseases (neutrophilic inflammation), chronic lung diseases (asthma, chronic obstructive pulmonary disorder (COPD), and lung cancer)) and environmental determinants (i.e. settled dust, floor dust, mattress dust, and from hoover dust collection bag)^34,35. Table 3 lists the study cases on dust particles with microbial problems.

Figure 2: Sizes of Airborne Particles that can Penetrate the Human Respiratory System Adapted from Andrade-Lima et al.33. Click here ot view Figure

Table 3: Studies on Dust Particles with Indoor Microbial Problems.

No.	Dust Particles Causing Indoor Microbial Problems in the Built Environment	Dominant Genera	References
1.	Concentrations of bacteria and fungi within hospital buildings are high on dusty days.	The dominant indoors and outdoors bacteria on regular and dusty days are Bacillus spp., Micrococcus spp., Streptomyces spp. and Staphylococcus spp gram-positive bacteria exhibiting higher concentrations than gram-negative bacteria.	20
2.	Polycyclic aromatic hydrocarbons (PAHs) are active and carcinogenic products that have positive correlations with floor dust which contains bacteria and fungi.	Phenanthrene in PAHs has a moderate positive correlation with gram-positive bacteria, whereas fluoranthene and pyrene have significant positive correlations with gram-negative bacteria.	23
3.	Crawling infants tend to be exposed to floor dust resuspended from carpeted flooring, which produces a concentrated and localised cloud of microbial content.	Carpet flooring has concentrations of Proteobacteria and lower concentrations of Firmicutes.	36

Ventilation Problem

Indoor microorganism is also associated with the lack of ventilation systems, especially in air-conditioned buildings. Improperly designed or operated air-conditioning system within a building can contribute to many microorganisms in the air, which can worsen the indoor environment and threaten occupants’ health^37,38.  Lee et al.39 stated that microorganisms could be transferred into the indoor environment due to the air-conditioning system's faulty operation during the maintenance period and the temporary malfunction of the air-conditioning system. Table 4 provides the study cases on the inefficient ventilation system with indoor microbial problems.

Table 4: Studies on Inefficient Ventilation Systems with Indoor Microbial Problems.

No.	Inefficient Ventilation Systems as the Cause of Indoor Microbial Problems in the Built Environment	Dominant Genera	References
1.	Concentrations of bacteria are usually associated with indoor CO2 concentrations, which might be due to human occupancy and lack of a ventilation system.	The dominant indoor fungi are Penicillium and Cladosporium.	22
2.	The growth of fungi on library materials indoors is due to improper dusting, defective air-conditioning system and lack of a ventilation system.	The dominant indoor fungi on the library materials are Eurotium halophilicum.	21
3.	Microbial communities found in the Heating, Ventilation and Air Conditioning (HVAC) filter dust samples are the same in residences’ surface dust.	The amount of Proteobacteria and Gram-positive bacteria present on the HVAC filter dust samples occupied residences, respectively.	40

Humidity/Moisture Surface Problems

Dampness or moisture problems are associated with the growth of indoor microorganisms (i.e. mould exposures) in buildings, with the estimates ranging from 18% to 50%, and these affect human health^30,41-45. The relative humidity is one of the environmental factors contributing to airborne microorganisms' vitality with higher bacteria and fungi¹⁸. However, humidity with a lower percentage is the limiting factor for indoor microorganisms^46,47. The low-moisture condition in the air affects the germination and growth conditions of some fungi rather than bacteria^47-49. Dannemiller et al.50 found that bacteria tend to grow at 100% of relative humidity. By contrast, fungi grow at ≥80% in home carpet dust, and this surrounding condition results in a significant increase in microbes’ concentration. Haines et al.51 also reported that fungi could grow at high relative humidity even at short periods. To prevent mould growth, the home relative humidity level must remain below 60%, and proper building maintenance is needed to save occupants’ health^51,52.
Besides, microbial growth due to indoor dampness is always associated with building materials. Building materials with high porosity and rough surface (i.e. plasterboard and mortar) and moisture in building materials are favourable to the proliferation and growth of indoor microorganisms. They may cause high exposure to spores, spore fragments, secondary metabolites and cellular microbe components^53,54. Generally, fungal growth always gives off an unpleasant odour indoors. Table 5 provides details on how humidity/moisture surface problems influence indoor airborne.

Table 5: Studies on Humidity/Moisture Surface with Indoor Microbial Problems.

No.	Humidity/Moisture Surface Problems causing Indoor Microbial Issues in the Built Environment	Dominant Genera	References
1.	The airflow with moisture intrusion in the wall cavities	The fungal spores are dominated by Aspergillus versicolor, Cladosporium cladosporioides and Penicillium melinii.	30
2.	The increase of respiratory symptoms, asthma and rhinitis and decrease of remission caused by the dampness and mould at home and workplace building	Not available	55

Human Occupancy and Activities

Human occupants are also a prominent source of indoor biological contaminants in the built environment^{14,22,30,56-59}.  Al-Hunaiti et al.23 stated that bacterial and fungal concentrations are lower in the dwelling with a fewer number of occupants and the most recently built.  Noris et al.40 reported a high proportion of gram-positive bacteria in a high group of occupants. Indoor biological contaminants can spread in the built environment through natural shedding, which includes particles directly coming from human bodies (i.e. skin, respiratory tract, hair and nostrils) and clothing^{2,19,58,60,61} and through resuspension of spores from the room surfaces, whereby the microbial materials that previously settled onto or colonised indoor materials are disturbed by occupants’ movements^58,62.  Meadow et al.14 also found that microbes' transmission to indoor surfaces possibly occurs through direct and indirect contact.  Cao et al.63 found that the high indoor biological contaminants in the high occupancy (30 %) compared to low occupancy (10 %) of the older building as shown in Figure 3. Table 6 describes how human occupancy and activities influence indoor microbial problems.

Figure 3: The Higher Indoor Biological Contaminants in the Higher Levels of Human Occupancy in the Older Building Adapted from Cao et al.63. Click here to view Figure

Table 6: Studies on Human Occupancy and Activities with Indoor Microbial Problems.

No.	Human Occupancy and Activities Causing Indoor Microbial Problems in the Built Environment	Dominant Genera	References
1.	High human occupancy in diverse space types (i.e. classroom) is associated with several bacterial taxa, the human microbiome. By contrast, low human occupancy in various space types influenced by outdoor environments.	Lactobacillus and Staphylococcus are the dominant indoor bacteria, whereas methylobacterium is the dominant outdoor bacterium.	9
2.	The potential interactions between high occupancy and occupant activity with microbial communities in a hospital environment.	Not available	59
3.	Occupancy increases the total aerosol mass and bacterial genome concentration in the particulate matter.	All bacteria in samples of indoor air (17%), floor dust (20%) and ventilation duct supply air (17.5%) are associated with human taxa, which Proprionibacterineae, Staphylococcus, Streptococcus, dominate Enterobacteriaceae and Corynebacterineae.	2

Conclusion

The potential factors that can contribute to the growth of bacteria and fungi (i.e. outdoor air, dust, ventilation problems, humidity/moisture surface problems and human occupancy) should be observed clearly in a building’s characteristics and environment.  The dominant bacteria species found worldwide in various indoor buildings (i.e. schools, cafeterias, mosques, faculties, child day-care centres, hospitals, dwellings, educational buildings, homes, elderly care centres, libraries, restaurants, and factories) was Bacillus spp. (24.1 %), Staphylococcus spp. (20.7 %), and Micrococcus spp. (20.7 %) whereas for fungi species were Penicillium spp. (25.0 %), Cladosporium spp. (21.9 %), and Aspergillus spp. (21.9 %).  Buildings free from bacteria and fungi issues can contribute to a safe environment and protect occupants from poor indoor air quality.

Acknowledgement

The personal EACAR has supported this research to carry out this work.

Funding Source

There is no funding or financial support for this research work.

Conflict of Interest

The authors agree that this research was conducted in the absence of any self-benefits, commercial or financial conflicts and declare absence of conflicting interests with the funders.

References

1. GÏŒrny R. L., Dutkiewicz J. Bacterial and Fungal Aerosols in Indoor Environment in Central and Eastern European Countries. Annals of Agricultural and Environmental Medicine. 2002; 9:17-23.
2. Hospodsky D., Qian J., Nazaroff W. W., Yamamoto N., Bibby K., Rismani-Yazdi H., Peccia J. Human Occupancy a Source of Indoor Airborne Bacteria. PloS One. 2012; 7(4):1-10.
   CrossRef
3. Lax S., Smith D. P., Hampton-Marcell J., Owens S. M., Handley K. M., Scott N. M., Gibbons S. M., Larsen P., Shogan B. D., Weiss S., Metcalf J. L. Longitudinal Analysis of Microbial Interaction Between Humans and the Indoor Environment. Science. 2014; 345(6200):1048-1052.
   CrossRef
4. Adams R. I., Bhangar S., Dannemiller K. C., Eisen J. A., Fierer N., Gilbert J. A., Green J. L., Marr L. C., Miller S. L., Siegel J. A., Stephens B., Waring M. S., Bibby K. Ten Questions Concerning the Microbiomes of Buildings. Building and Environment.2016; 109:224-234.
   CrossRef
5. Piecková E. Indoor Microbial Aerosol and its Health Effects: Microbial Exposure in Public Buildings–Viruses, Bacteria, And Fungi, in Viegas C., Viegas S., Gomes A., Täubel M., Sabino R. (Eds.), Exposure to Microbiological Agents in Indoor and Occupational Environments. Springer, Cham. 2017; 237-252.
   CrossRef
6. Al-abdalall A. H., Al-dakheel S. A., Al-Abkari, H. A. Impact of Air-Conditioning Filters' on Microbial Growth Indoor Air Pollution [online]. IntechOpen. https://www.intechopen.com/books/low-temperature-technologies/impact-of-air-conditioning-filters-on-microbial-growth-and-indoor-air-pollution. 2019: (Accessed 1 September 2020).
7. Corsi R. L., Kinney K. A., Levin H. Microbiomes of Built Environments: 2011 Symposium Highlights and Workgroup Recommendations. Indoor Air. 2012; 22(3): 171-172.
   CrossRef
8. Kelley S. T., Gilbert J. A. Studying the Microbiology of the Indoor Environment. Genome Biology. 2013; 14(2): 1-9.
   CrossRef
9. Kembel S. W., Meadow J. F., O’Connor T. K., Mhuireach G., Northcutt D., Kline J., Moriyama M., Brown G. Z., Bohannan B. J. M., Green J. L. Architectural Design Drives the Biogeography of Indoor Bacterial Communities. PloS One. 2014; 9(1): 1-10.
   CrossRef
10. Tang J. W. The Effect of Environmental Parameters on the Survival of Airborne Infectious Agents. Journal of the Royal Society Interface. 2009; 6(6): 737-746.
    CrossRef
11. Frankel M., Bekö G., Timm M., Gustavsen S., Hansen E. W., Madsen A. M. Seasonal Variations of Indoor Microbial Exposures and Their Relation to Temperature, Relative Humidity, and Air Exchange Rate. Applied and Environmental Microbiology. 2012; 78(23): 8289-8297.
    CrossRef
12. Kembel S. W., Jones E., Kline J., Northcutt D., Stenson J., Womack A. M., Bohannan B. J. M., Brown G. Z., Green J. L. Architectural Design Influences the Diversity and Structure of the Built Environment Microbiome. The ISME Journal. 2012; 6(8): 1469-1479.
    CrossRef
13. Klepeis N. E., Nelson W. C., Ott W. R., Robinson J. P., Tsang A. M., Switzer P., Behar J. V., Hern S. C., Engelmann W. H. The National Human Activity Pattern Survey (NHAPS): A Resource for Assessing Exposure to Environmental Pollutants. Journal of Exposure Science and Environmental Epidemiology. 2001; 11(3): 231-252.
    CrossRef
14. Meadow J. F., Altrichter A. E., Kembel S. W., Moriyama M., O’Connor T. K., Womack A. M., Brown G. Z., Green J. L., Bohannan B. J. Bacterial Communities on Classroom Surfaces Vary with Human Contact. Microbiome. 2014; 2(1): 1-7.
    CrossRef
15. Hayleeyesus S. F., Manaye A. M. Microbiological Quality of Indoor Air in University Libraries. Asian Pacific Journal of Tropical Biomedicine. 2014; 4: 312-317.
    CrossRef
16. Salin J. T., Salkinoja-Salonen M., Salin P. J., Nelo K., Holma T., Ohtonen P., Syrjälä H. Building-related Symptoms are Linked to the in Vitro Toxicity of Indoor Dust and Airborne Microbial Propagules in Schools: A Cross-Sectional Study. Environmental Research. 2017; 154: 234-239.
    CrossRef
17. Pyrri I., Zoma A., Barmparesos N., Assimakopoulos M. N., Assimakopoulos V. D., Kapsanaki-Gotsi E. Impact of a Green Roof System on Indoor Fungal Aerosol in a Primary School in Greece. Science of the Total Environment. 2020; 719: 1-25.
    CrossRef
18. Kim K. Y., Kim H. T., Kim D., Nakajima J., Higuchi T. Distribution Characteristics of Airborne Bacteria and Fungi in the Feedstuff-Manufacturing Factories. Journal of Hazardous Materials. 2009; 169(1-3): 1054-1060.
    CrossRef
19. Chan P. L., Yu P. H. F., Cheng Y. W., Chan C. Y., Wong P. K. Comprehensive Characterisation of Indoor Airborne Bacterial Profile. Journal of Environmental Sciences. 2009; 21(8): 1148-1152.
    CrossRef
20. Soleimani Z., Goudarzi G., Sorooshian A., Marzouni M. B., Maleki H. Impact of Middle Eastern Dust Storms on the Indoor and Outdoor Composition of Bioaerosol. Atmospheric Environment. 2016; 138: 135–143.
    CrossRef
21. Polo A., Cappitelli F., Villa F., Pinzari F. Biological Invasion in the Indoor Environment: The Spread of Eurotium Halophilicum on Library Materials. International Biodeterioration and Biodegradation. 2017; 118: 34-44.
    CrossRef
22. Madureira J., Paciência I., Rufo J. C., Pereira C., Teixeira J. P., de Oliveira Fernandes E. Assessment and Determinants of Airborne Bacterial and Fungal Concentrations in Different Indoor Environments: Homes, Child Day-Care Centres, Primary Schools and Elderly Care Centres. Atmospheric Environment. 2015; 109: 139-146.
    CrossRef
23. Al-Hunaiti A., Arar S., Täubel M., Wraith D., Maragkidou A., Hyvärinen A., Hussein T. Floor Dust Bacteria and Fungi and Their Coexistence with Pahs in Jordanian Indoor Environments. Science of the Total Environment. 2017; 601: 940-945.
    CrossRef
24. Baurès E., Blanchard O., Mercier F., Surget E., Le Cann P., Rivier A., Gangneux J-P., Florentin A. Indoor Air Quality in Two French Hospitals: Measurement of Chemical and Microbiological Contaminants. Science of the Total Environment. 2018; 642: 168-179.
    CrossRef
25. Awad A. H., Saeed Y., Hassan Y., Fawzy Y., Osman M. Air Microbial Quality in Certain Public Buildings, Egypt: A Comparative Study. Atmospheric Pollution Research. 2018; 9(4): 617-626.
    CrossRef
26. Rasli N. B. I., Ramli N. A., Ismail M. R., Shith S., Yusof N. F. F. M., Zainordin N. S., ... & Nazir A. U. M. Effects of Hoovering Activities on Biological Contaminants and Particulate Matter Levels in Main Prayer Halls of Malaysian Mosques. Current World Environment. 2017; 14(1): 134.
    CrossRef
27. Asif A., Zeeshan M., Jahanzaib, M. Assessment of Indoor and Outdoor Microbial Air Quality of Cafeterias of an Educational Institute. Atmospheric Pollution Research. 2019; 10(2): 531-536.
    CrossRef
28. Vornanen-Winqvist C., Järvi, K., Andersson, M. A., Duchaine, C., Létourneau, V., Kedves, O., Kredics, L., Mikkola R., Kurnitski J, Salonen H. Exposure to Indoor Air Contaminants in School Buildings with and without Reported Indoor Air Quality Problems. Environment International. 2020; 141: 1-14.
    CrossRef
29. Adams R. I., Miletto M., Taylor J. W., Bruns T. D. Dispersal in Microbes: Fungi in Indoor Air are Dominated by Outdoor Air and Show Dispersal Limitation at Short Distances. The ISME Journal. 2013; 7(7): 1262-1273.
    CrossRef
30. Nazaroff W. W. Indoor Bioaerosol Dynamics. Indoor Air. 2016; 26(1): 61-78.
    CrossRef
31. Chen C., Zhao B. Review of Relationship between Indoor and Outdoor Particles: I/O Ratio, Infiltration Factor and Penetration Factor. Atmosphere Environment. 2011; 45(2): 275-288.
    CrossRef
32. Osman M. E. S., Awad A. H. A. H., Ibrahim Y. H., Ahmed Y. F., Abo-Elnasr A., Saeed Y. Air Microbial Contamination and Factors Affecting Its Occurrence in Certain Book Libraries in Egypt. Egyptian Journal of Botany. 2017; 57(1): 93-118.
    CrossRef
33. Andrade-Lima M., Pereira L. F. F., Fernandes A. L. G. Pharmaceutical Equivalence of The Combined Formulation of Budesonide and Formoterol in A Single Capsule with A Powder Inhaler Device. Brazilian Journal of Pulmonology. 2012; 38(6): 748-756.
    CrossRef
34. Yang J., Kim Y. K., Kang T. S., Jee Y. K., Kim, Y. Y. Importance of Indoor Dust Biological Ultrafine Particles in The Pathogenesis of Chronic Inflammatory Lung Diseases. Environmental Health and Toxicology. 2017; 32: 1-4.
    CrossRef
35. Leppänen H. K., Täubel M., Jayaprakash B., Vepsäläinen A., Pasanen P, Hyvärinen, A. Quantitative Assessment of Microbes from Samples of Indoor Air and Dust. Journal of Exposure Science and Environmental Epidemiology. 2018; 28(3): 231-241.
    CrossRef
36. Hyytiäinen H. K., Jayaprakash B., Kirjavainen P. V., Saari S. E., Holopainen R., Keskinen J., Hämeri K., Hyvärinen A., Boor B. E., Täubel M. Crawling-induced Floor Dust Resuspension Affects the Microbiota of the Infant Breathing Zone. Microbiome. 2018; 6(5): 1-12.
    CrossRef
37. Liu Z., Ma S., Cao G., Meng C., He, B.-J. Distribution Characteristics, Growth, Reproduction and Transmission Modes and Control Strategies for Microbial Contamination in HVAC Systems: A Literature Review. Energy and Buildings. 2018; 177: 77–95.
    CrossRef
38. Piekarska K., Trusz A., Szczęśniak S. Bacteria and Fungi in Two Air Handling Units with Recirculating Air Module. Energy and Buildings. 2018; 178: 154-164.
    CrossRef
39. Lee B. U., Yun S. H., Ji J., Bae G. N. Inactivation of S. Epidermidis, B. Subtilis, and E. Coli Bacteria Bioaerosols Deposited on A Filter Utilizing Airborne Silver Nanoparticles. Journal of Microbiology and Biotechnology. 2008; 18: 176-182.
40. Noris F., Siegel J. A., Kinney K. A. Evaluation of HVAC Filters as A Sampling Mechanism for Indoor Microbial Communities. Atmospheric Environment. 2011; 45(2): 338-346.
    CrossRef
41. Gunnbjörnsdóttir M. I., Franklin K. A., Norbäck D., Björnsson E., Gislason D., Lindberg E., Svanes C., Omenaas E., Norrman E., Jõgi R.,  Jensen E. J., Dahlman-Höglund A., Janson,  C. Prevalence and Incidence of Respiratory Symptoms in Relation to Indoor Dampness: The RHINE Study. Thorax. 2006; 61(3): 221-225.
    CrossRef
42. Mudarri D., Fisk W. J. Public Health and Economic Impact of Dampness and Mold. Indoor Air. 2007; 17(3): 226-235.
    CrossRef
43. Mendell M. J., Mirer A. G., Cheun, K., Tong M., Douwes J. Respiratory and Allergic Health Effects of Dampness, Mold, and Dampness-Related Agents: A Review of the Epidemiologic Evidence. Environmental Health Perspectives. 2011; 119(6): 748-756.
    CrossRef
44. Casas L., Tischer C., Täubel M. Pediatric Asthma and the Indoor Microbial Environment. Current Environmental Health Reports. 2016; 3(3): 238-249.
    CrossRef
45. Kanchongkittiphon W., Mendell M. J., Gaffin J. M., Wang G., Phipatanakul W. Indoor Environmental Exposures and Exacerbation of Asthma: An Update to the 2000 Review by the Institute of Medicine. Environmental Health Perspectives. 2015; 123(1): 6-20.
    CrossRef
46. Nunes I., Mesquita N., Verde S. C., Bandeira A. M. L., Carolino M. M., Portugal A., Botelho M. L. Characterization of an Airborne Microbial Community: A Case Study in the Archive of the University of Coimbra, Portugal. International Biodeterioration and Biodegradation. 2013; 79: 36-41.
    CrossRef
47. Flannigan B., Samson R. A., Miller J. D. (Eds.). Microorganisms in Home and Indoor Work Environments: Diversity, Health Impacts, Investigation, and Control. 2016; CRC Press, United States.
48. Ponizovskaya V. B., Antropova A. B., Mokeeva V. L., Bilanenko E. N., Chekunova L. N. Effect of Water Activity and Relative Air Humidity on the Growth of Penicillium Chrysogenum Thom, Aspergillus Repens (Corda) Sacc., and Trichoderma Viride Pers. Isolated from Living Spaces. Microbiology. 2011; 80(3): 378-385.
    CrossRef
49. Madsen A. M. Effects of Airflow and Changing Humidity on the Aerosolisation of Respirable Fungal Fragments and Conidia of Botrytis Cinerea. Applied and Environmental Microbiology. 2012; 78(11): 3999-4007.
    CrossRef
50. Dannemiller K. C., Weschler C.J., Peccia, J. Fungal and Bacterial Growth in Floor Dust at Elevated Relative Humidity Levels. Indoor Air. 2017; 27(2): 354-363.
    CrossRef
51. Haines S. R., Adams R. I., Boor B. E., Bruton T. A., Downey J., Ferro A. R., Gall E., Green B. J., Hegarty B., Hornerj E., Jacobs D. E., Lemieux P., Misztal P. K., Morrison G., Perzanowski M., Reponen T., Rush R. E., Virgo T., Alkhayri C., Bope A., Cochran S., Cox J., Donohue A., May A. A., Nastasi N., Nishioka M., Renninger N., Tian Y., Uebel-Niemeier C., Wilkinson D., Wu T., Zambrana J., Dannemiller K. C. Ten Questions Concerning the Implications of Carpet on Indoor Chemistry and Microbiology. Building and Environment. 2020; 170: 1-16.
    CrossRef
52. Environmental Protection Agency EPA:  A Brief Guide to Mold, Moisture, and Your Home. United States, ISO. 2020; https://www.epa.gov/mold/brief-guide-mold-moisture-and-your-home (Accessed 1 October 2020).
53. Verdier T., Coutand M., Bertron A., Roques C. A review of Indoor Microbial Growth across Building Materials and Sampling and Analysis Methods. Building and Environment. 2014; 80: 136–149.
    CrossRef
54. Huttunen K., Tirkkonen J., Täubel M., Krop E., Mikkonen S., Pekkanen J., Heederik D., Zock J. P., Hyvärinen A., Hirvonen M. R. Inflammatory Potential in Relation to the Microbial Content of Settled Dust Samples Collected from Moisture‐Damaged and Reference Schools: Results of HITEA Study. Indoor Air. 2016; 26(3): 380-390.
    CrossRef
55. Wang J., Pindus M., Janson C., Sigsgaard T., Kim J. L., Holm M., Sommar J., Orru H., Gislason T., Johannessen A., Bertelsen R. J. Dampness, Mould, Onset and Remission of Adult Respiratory Symptoms, Asthma And Rhinitis. European Respiratory Journal. 2019; 53(5): 1-23.
    CrossRef
56. Adams R. I., Miletto M., Lindow S. E., Taylor J. W., Bruns T.D. Airborne Bacterial Communities in Residences: Similarities and Differences with Fungi. PLoS One. 2014; 9(3): 1-7.
    CrossRef
57. Bhangar S., Huffman J.A., Nazaroff W.W. Size‐resolved Fluorescent Biological Aerosol Particle Concentrations and Occupant Emissions in a University Classroom. Indoor Air. 2014; 24(6): 604-617.
    CrossRef
58. Adams R. I., Bhangar S., Pasut W., Arens E. A., Taylor J. W., Lindow S. E., Nazaroff W. W., Bruns T. D. Chamber Bioaerosol Study: Outdoor Air and Human Occupants as Sources of Indoor Airborne Microbes. PLoS One. 2015; 10(5): 1-18.
    CrossRef
59. Dedesko S., Stephens B., Gilbert J. A., Siegel J. A. Methods to Assess Human Occupancy and Occupant Activity in Hospital Patient Rooms. Building and Environment. 2015; 90: 136-145.
    CrossRef
60. Tringe S. G., Zhang T., Liu X., Yu Y., Lee W. H., Yap J., Yao F., Suan S. T., Ing S. K., Haynes M., Rohwer F. Wei C. L., Tan P., Bristow J., Rubin E. M., Ruan Y. The Airborne Metagenome in an Indoor Urban Environment. PloS One. 2008; 3(4): 1-10.
    CrossRef
61. Fujimura K. E., Demoor T., Rauch M., Faruqi A. A., Jang S., Johnson C. C., Boushey H. A., Zoratti E., Ownby D., Lukacs N. W., Lynch S. V. House Dust Exposure Mediates Gut Microbiome Lactobacillus Enrichment and Airway Immune Defence Against Allergens and Virus Infection. Proceedings of the National Academy of Sciences. 2014; 111(2): 805-810.
    CrossRef
62. Qian J., Peccia J., Ferro, A. R. Walking-induced particle resuspension in indoor environments. Atmospheric Environment. 2014; 89: 464-481.
    CrossRef
63. Cao L., Yang L., Swanson C. S., Li S., He Q. Comparative analysis of the impact of human occupancy on indoor microbiomes. Frontiers of Environmental Science & Engineering. 2021; 15(5): 1-10.
    CrossRef