All in the air: Dust and the aerobiome

Wait long enough and it will appear; dust that is, perhaps settling out in a recess in the room you are sitting in. Dust is an amalgam of sloughed-off skin cells, hair, clothing fibers, bacteria, dust mites, bits of dead bugs, soil particles, pollen, and microscopic specks of plastic. In homes and offices, human skin detritus is the largest contributor. In addition, paraffins, nonylphenol ethoxylates, and azo dyes, including 2-bromo-4,6-dinitroaniline, are major chemical components of household dust (1).

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Dust particles are also adept, especially where there is organic matter from human skin, of absorbing moisture and chemicals (including some chemicals that can impart antimicrobial resistance). There are also a lot of microorganisms – natural history all around us.

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In this week’s article, I take a look at dust and microorganisms and what we can learn from aerobiology and the aerobiome.

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Floating particles

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Dust includes particulate matter (PM), ranging from 0.001 to 100 μm in aerodynamic diameter. which is divided into various fractions. What is of interest here is the microbial association.

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As most cleanroom microbiologists know, the majority of bacteria in the air are attached to particles. Where bacteria are attached to particles, these are of detectable sizes of 1 μm or larger, whereas free-floating bacteria are smaller than 1 μm (2). Proportions will vary according to the?environment; a review conducted in Asia found the ratio to be 4 attached bacteria to every 1 free-floating organism, as an example (3). The viabilities of dust-attached bacteria were lower (~65% to ~87%) compared with free-floating organisms (4). Viabilities of bacteria in air are typically lower than bacteria in soil due to air turbulence and harsh atmospheric stressors (5).

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The landing of dust-attached bacteria on surfaces was, unsurprisingly, found to be the main way that contaminants are spread. There are some differences based on the height at which dust settles; at floor level there is an additional contribution from shoes (6).

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Types of microorganisms

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Our understanding of dust microbial composition has been advanced through metagenomic and metatranscriptomic sequencing (7). Of cultivable bacteria, typical dust in a building is dominated by Gram-positive genera, such as Staphylococcus, Corynebacterium, and Lactococcus (and to a lesser extent Streptococcus; plus, fecal related organisms Bacteroides and Faecalibacterium). Fungal profiles can be diverse, although Penicillium, Aspergillus, and Cladosporium are common (and, to a lesser extent, Alternaria and Fusarium). These are predominant species; however, with fungi and bacteria there is an estimated 500-1000 different species present in house dust (8). There are differences in the composition between households with lost of males compared with households with lots of females.

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In terms or origins, while indoor air communities closely track outdoor air communities, human-associated bacterial genera are at least twice as abundant in indoor air compared with outdoor air (9).

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Assessing ‘how many?’ is impossible. The numbers range from nondetectable to 10(9) cells g(-1) dust, depending on the dust type, detection method, type of the indoor environment; plus there are seasonal and diurnal variations (10, 11). We also need to factor in aeration habits (ventilation and air exchange rate), presence of pets, smoking, signals of dampness, temperature and relative humidity (12, 13). Even local tap water can influence the diversity and size of indoor air microbes (14).

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A proportion of bacteria come from local soil; and soil can exchange a significant number of bacteria with aerobiomes. A further influencing factor is human activity upon the climate; functional genes profiles suggest anthropogenic impact on the atmospheric microbiome by way of pollution-associated bacteria. Consistent from what we know about fungi as a contaminant in cleanrooms, most fungal taxa found indoors come from outside the home (and with a high-degree of endemism) (15).

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While room occupancy rate, ventilation conditions, and the extent of indoor furnishing influence overall microbial types; the structure of pathogenic bacterial communities is especially influenced by the number of individuals and room spatial dimensions (16). Sampling methods will also be influential (17).

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Some microorganisms have adapted to survive atmospheric transport; these functions include resistance to UV radiation, oxidative stress, and sporulation.

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Growth

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Most bacteria do not grow in the air. Unattached, growth is improbable; when attached to dust particles, a few microbial species are actually able to grow and proliferate in dust but only if enough moisture is provided.

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Health

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Since we typically spend more time indoors than outdoors the fact that many studies have suggested that indoor dust is a more important factor in health and disease than outdoor dust means that understanding the composition of dust in homes, offices and factories is important. A contributor health is the microbial composition and a common contributor to allergens is lipopolysaccharide (LPS or endotoxin); LPS is a causative factor in respiratory disease (18).

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When we breathe in dust, it will reach different parts of the body: particles with a diameter of 5 to 10 μm are mainly deposited in nasopharyngeal areas, and particles larger than 0.5 μm in diameter are deposited in bronchi, bronchioles and alveoli.

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Letting in the Sun

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Research from the University of Oregon suggests that natural sunlight can, eventually, reduce the population of many bacteria that live in dust. This is around a 50% reduction when compared with equivalent populations in dark rooms. This is from open windows (with a closed window the effects are not as great, given that most windows block most ultraviolet light).

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Researchers at the University of Oregon found that in dark rooms 12% of bacteria on average were alive and able to reproduce (viable) (19). In comparison only 6.8% of bacteria exposed to daylight and 6.1% of bacteria exposed to UV light were viable. The organisms that are more vulnerable to sunlight are human skin-derived bacteria; whereas those more resistant are outdoor air-derived bacteria.

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The research was based on replicating the light dosages similar to those found in most buildings. However, there are many architectural and geographical features that produce lower or higher dosages of light.

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Distance

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Airborne bacteria make up aerobiomes. These can travel thousands of kilometers and, if the settle, the deposits can alter environmental chemistry and affect human and animal health.

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Cleanrooms

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The cleanroom was designed by engineers for the single purpose of lowering the particulate dust content of the cleanroom air (20). While a well-designed environment achieves this, dust will still be present. This leaves the objectives as reducing concentration; maintaining air movement to reduce the risk of gravitational settling; and extracting the air regularly to dilute particulates in combination with the supply of fresh, filtered air.

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With settling, Whyte and colleagues demonstrated that over 80% of airborne particles greater than or equal to 10μm are deposited in a cleanroom by gravitational settling, and gravitational settling continues to be the predominant mechanism down to about 5μm. It also remains an important one down to about 0.5μm, although other factors at play are turbulent deposition, Brownian diffusion, and electrostatic attraction (21).

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For good cleanroom control, design is important especially with the positioning of lower-side return air outlets. The model of air outlets and the number affect the degree of air movement (to keep particles in suspension) and particles generated closer to these return air outlets can be more effectively expelled (22). The number and positioning of air outlets becomes more important as the air exchange rate decreases.

Moreover, increasing the air exchange rate, assessed by air supply volume, remains the most effective method to control indoor particle concentration (23). Predicting likely particle movement can be tricky, although the more particles there are the more predictable and uniform, they become (albeit that high levels of particles are generally unwanted).

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Summary

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This article has examined dust, its composition and microbial content. The discussion has extended to microorganisms in the air in general. The aim was to outline the diversity and the different contributory factors.

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References

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1.????? van Bronswijk, J. E. M. H. (1981). House Dust Biology for Allergists, Acarologists and Mycologists. J. Bronswijk. p. 37

2.????? Zhang, D., Iwasaka, Y., Shi, G., et al. Mixture state and size of Asian dust particles collected at southwestern Japan in spring 2000, J. Geophys. Res.-Atmos., 108, 4760

3.????? Zhang, D., Murata, K., Hu, W., et al.: Concentration and viability of bacterial aerosols associated with weather in Asian continental outflow: current understanding, Aerosol Sci. Eng., 1, 66–77

4.????? Hu, W., Murata, K., Fan, C., et al: Abundance and viability of particle-attached and free-floating bacteria in dusty and nondusty air, Biogeosciences, 17, 4477–4487

5.????? Hara, K., Zhang, D., Yamada, M., et al: ?A detection of airborne particles carrying viable bacteria in an urban atmosphere of Japan, Asian J. Atmos. Environ., 5, 152–156

6.????? Shen F, Wang M, Ma J, et al: Height-Resolved Analysis of Indoor Airborne Microbiome: Comparison with Floor Dust-Borne Microbiome and the Significance of Shoe Sole Dust. Environ Sci Technol. 2024 Oct 1;58(39):17364-17375

7.????? Tignat-Perrier, R., Dommergue, A., Thollot, A., et al (2020). Microbial functional signature in the atmospheric boundary layer. Biogeosciences, 17(23), 6081–6095

8.????? Rintala H, Pitk?ranta M, T?ubel M. Microbial communities associated with house dust. Adv Appl Microbiol. 2012;78:75-120

9.????? Meadow JF, Altrichter AE, Kembel SW, et al. Indoor airborne bacterial communities are influenced by ventilation, occupancy, and outdoor air source. Indoor Air. 2014 Feb;24(1):41-8

10.? Mentese S, Rad AY, Arisoy M, Güllü G. Multiple comparisons of organic, microbial, and fine particulate pollutants in typical indoor environments: diurnal and seasonal variations. J Air Waste Manag Assoc. 2012;62(12):1380-93

11.? Lepp?nen HK, T?ubel M, Jayaprakash B, et al. Quantitative assessment of microbes from samples of indoor air and dust. J Expo Sci Environ Epidemiol. 2018 May;28(3):231-241. doi: 10.1038/jes.2017.24

12.? Kettleson EM, Adhikari A, Vesper S, et al. 2015 Key determinants of the fungal and bacterial microbiomes in homes. Environ. Res. 138, 130–135

13.? Dannemiller KC, Gent JF, Leaderer BP, Peccia J. Influence of housing characteristics on bacterial and fungal communities in homes of asthmatic children. Indoor Air. (doi:10.1111/ina.12205)

14.? Miletto M, Lindow SE. Relative and contextual contribution of different sources to the composition and abundance of indoor air bacteria in residences. Microbiome. 2015 Dec 10;3:61. doi: 10.1186/s40168-015-0128-z

15.? Adams RI, Miletto M, Lindow SE, et al. 2014 Airborne bacterial communities in residences: similarities and differences with fungi. PLoS ONE 9, e91283

16.? Zhang T, Liu M, Zhou D, et al. Environmental factors and particle size shape the community structure of airborne total and pathogenic bacteria in a university campus. Front Public Health. 2024 Apr 8;12:1371656

17.? Sajjad B, Hussain S, Rasool K, et al. Comprehensive insights into advances in ambient bioaerosols sampling, analysis and factors influencing bioaerosols composition. Environ Pollut. 2023 Nov 1;336:122473

18.? Shan Y, Wu W, Fan W, et al.. House dust microbiome and human health risks. Int Microbiol. 2019 Sep;22(3):297-304. doi: 10.1007/s10123-019-00057-5

19.? Fahimipour, A., Hartmann, E., Siemens, A. et al. Daylight exposure modulates bacterial communities associated with household dust. Microbiome 6, 175 (2018). https://doi.org/10.1186/s40168-018-0559-4

20.? Tenenbaum, D. J. The cleanroom: how clean? Environews Focus). Environmental Health Perspectives, vol. 111, no. 5, May 2003, pp. A282+. Gale OneFile: Health and Medicine, link.gale.com/apps/doc/A103563616/HRCA?u=anon~cb349d7&sid=googleScholar&xid=dcb99c57

21.? Whyte W, Agricola K and Derks M (2015). Airborne particle deposition in cleanrooms: Deposition mechanisms. Clean Air and Containment Review, Issue 24, pp4-9.

22.? Hu SC, Tung Y.C. Performance assessment for locally balanced and wall-return turbulent clean rooms by the stochastic particle tracking. Int J Archit Sci 3. 2002:146–62

23.? Li C, Li H, Zhang M, Wang X, Huang C (2024) The research on the particle concentration distribution of directed airflow in cleanrooms for operators. PLoS ONE 19(3): e0296803