Review Papers
Microbial Aerosols: Sources, Properties, Health Effects, Exposure Assessment—A Review
https://ror.org/03x0yya69 
2020 Volume 37 Pages 64-84
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2020 Volume 37 Pages 64-84
Microorganisms are ubiquitous in the environment. Wherever their sources are present, the particles can be released into the air forming microbiological aerosols. Although most of their particles cause no harm to the exposed individuals, some of their propagules may have infectious or allergenic potential and may carry toxic or irritant substances and components. Their inhalation usually poses a significant health risk and is responsible for numerous adverse outcomes, from allergic reactions, infections and toxic responses to various nonspecific symptoms. This review article provides fundamental background information on the role of microorganisms in the environment, defines and characterizes environmental sources of microbial aerosols, describes microbial abilities for airborne transport and comments on their role in atmospheric processes, discusses their physical and biological characteristics which result in adverse health outcomes observed in exposed individuals. The paper characterizes comprehensively numerous sampling and analysis techniques involved in the quantitative and qualitative evaluation of microbial aerosols together with their practical applications, presents strategies applied in the assessment of harmful microbial agents formed by bioaerosols, explains the ways of creating hygienic standards (understood here as reference/threshold limits) for microbiological aerosols conditioned by both medical and environmental determinants, and comments on their usefulness in the control and protection of environment and health.
From among different organisms, microbes are those that dominate earth habitats. Microorganisms are not only omnipresent but they are essential to all other life forms. They are a primary source for nutrients and the major recyclers of dead matter back to available organic form. Along with all other animals and plants, the human condition is deeply affected by microbes. Compared to the human population (~6 × 109), the population of, e.g. terrestrial ecosystem bacteria is significantly higher (> 1030). The human body is inhabited by more bacterial cells (1014) than its own cells (1013). Being aware of these population differences and using the most advanced analytical techniques, it is possible to recognize and characterize not more than 9 % of the total microbial species only (i.e. ~158,760 out of ~1,830,000 including viruses, bacteria, algae, fungi, lichen, and protozoa). Microorganisms are essential to human survival, health and disease, and hence their environmental abundance and diversity are of great practical importance (Bisen et al., 2012). Since the 1970s, enormous analytical progress has moved us from the age of phylogenetic analyses (that take into account physical and metabolic characteristics of organisms) to the molecular future, which also determines evolutionary relationships. A look at the world of organisms through their genomes has reformulated our perception of the natural system. Life’s diversity seen as comprising 3 domains, i.e. bacteria, archaea and eucarya, has been dramatically broadened taking into account both community and ecosystem interrelations (Hug et al., 2016; Whose and Fox, 1977; Woese et al., 1990). The genome-based approach allowed building the tree of life that today includes 92 named bacterial and 26 archaeal phyla as well as 5 eukaryotic supergroups. Despite these impressive numbers, however, the functional diversities of classified organisms and, what is even more important, their influence on the immunological system of exposed individuals in the form of an ‘airborne cocktail of particles’ remain so far unknown.
Microbial aerosols (i.e. airborne particles of microbiological origin) are usually naturally present in the environment. They are ubiquitous both indoors and outdoors. Their environmental presence is associated with different geographic regions, climate zones, continents or populations of plants and animals. Their major outdoor sources are located on the earth surfaces and are formed by continental (soils, plants including crops and forests, wetlands, deserts, land ice, urban, etc.) as well as natural and anthropogenic water reservoirs. As an example, Table 1 reveals approximate bacterial concentrations in near-surface air of various ecosystems (Burrows et al., 2009). Terrestrial ecosystems (e.g. cultivated soils, plant surfaces, mineral including desert dusts) are usually the most productive in this context.
Total bacterial concentrations in near-surface air of various ecosystems.
| Ecosystem | The highest estimate [# m−3] |
|---|---|
| Coastal | 1.3 × 105 |
| Crops | 1.7 × 105 |
| Deserts | 3.8 × 104 |
| Forests | 8.8 × 104 |
| Grasslands | 8.4 × 105 |
| Land ice | 1 × 104 |
| Seas | 8 × 104 |
| Shrubs | 8.4 × 105 |
| Tundra | 5.6 × 104 |
| Wetlands | 8 × 105 |
| Urban | 9.2 × 105 |
The comparison of the global bioaerosol emission between terrestrial and water ecosystems is difficult due to limited knowledge of the latter. There are several processes that favor microbial particle release from the water including eruption of rising bubbles through the sea-surface microlayer or wind ejection of spume drops from the breaking wave crest. Nevertheless, terrestrial emission processes are usually much more productive than those of aquatic sources (Aller et al., 2005; Blanchard et al., 1981; Elbert et al., 2007; Hultin et al., 2011; Veron, 2015).
Microbial aerosol sources are also widespread in indoor environments. They can derive from industrial and non-industrial settings and differ significantly in terms of their emission efficiency. In the first case, the most effective occupational aerosolization processes (being responsible for microbial aerosol concentrations up to 1012 cfu m−3) are: silo loading/unloading, animal feeding in broiler houses, piggeries as well as different dust-releasing tasks in composting plants, granaries, animal food stores, malt-houses, and reloading of stored moldy raw materials (Dutkiewicz and Jabłoński, 1989). Against this background, non-industrial indoor sources are less productive and usually closely connected with the presence and physical activity of humans (including numerous physiological processes such as breathing, talking, sneezing, coughing or scratching as well as movement and dust, including microbial dust residues, resuspension). Such types of emissions are usually able to create microbial concentrations of about 103 cfu m−3; however, some chamber bioaerosol studies revealed that even one person under seated conditions is able to release up to 106 biological aerosol particulates per hour into the air and the origin of such a microbial cloud can be assigned to the individual that emits it (Bhangar et al., 2016; Meadow et al., 2015). Also, indoor water reservoirs such as aquariums, toilets, sinks or even washing machines may load the air with high numbers of both saprophytic and pathogenic microorganisms. Such emissions (reaching usually 103–104 cfu m−3) may result not only in contamination of surrounding surfaces but pose a real threat to exposed individuals through inhalation of different pathogens (including Bacillus, Aeromonas, Campylobacter, Clostridium, Escherichia, Klebsiella, Staphylococcus, Salmonella, Pseudomonas, Serratia, Shigella bacterial genera and molds) (Barker and Jones, 2005; Best et al., 2012; Getto et al., 2011; Lai et al., 2018; O’Toole et al., 2009; Pastuszka et al., 1996; Stapleton et al., 2013).
As the locations of microbial aerosol sources are (in an obvious way) not spatially uniformly distributed and their productivity differs, both temporal (including diurnal, seasonal, and annual) and environmental changes in bioaerosol structure are observed. Moreover, microbial sources do not release bioaerosol particles continuously as numerous physical (temperature, relative humidity, availability of water and nutrients, radiation, environmental oxygen presence, etc.) and biological (individual species properties, growth in microbial consortium) factors influence such a process. Hence, all of the above results in fluctuations of quantitative and qualitative composition of airborne microorganisms (Burge, 1995; Lighthart and Stetzenbach, 1994; Macher, 1999).
From the above-listed physical factors, one of the major factors is temperature. It directly affects the rate of microbial metabolism, reproduction, and culturability. It also correlates with a number of important meteorological and climatic variables (e.g. air turbulence, time of day and season) that may affect concentrations of microorganisms in the air. In turn, the role of relative humidity as a factor influencing microbial aerosol is often emphasized in the literature; however, the empirical data do not adequately define the range of environmental conditions that are favorable for this parameter. The environmental results of the bacterial aerosol test are ambiguous and the relative humidity is assessed as marginal and once significant (Krzysztofik et al., 1994; Nevalainen, 1989; Pasanen et al., 2000). For both of these parameters, however, their proper environmental levels are strictly connected with specific microbial strains determining the issues of survival in the air, (if deposited) conditioning the possibilities to colonize the surfaces and as such should be individually considered (Burrows et al., 2009; Stetzenbach, 1997).
Regarding radiation, each type acts destructively on microbial particles. Sunlight has a known bactericidal effect due to the part of the spectrum that includes short, violet and ultraviolet waves. For example, the vegetative bacterial forms (including Mycobacterium tuberculosis) die after several hours of irradiation, especially in the summer. Also, artificial (xenon) radiation with the same wavelength spectrum as solar light reduces the quantity of viable microorganisms (which was shown in laboratory experiments with Escherichia coli or Serratia marcescens aerosols) (Carroll et al., 2016; Hurst, 1997). Ultraviolet rays (with wavelengths from 250 nm to 280 nm), just as other high-energy radiation types (such as gamma and X rays), penetrate into microbial cells and induce a number of reactions, including those with free radicals. Such processes damage the structure of nucleic acids, proteins, carbohydrates, lipids, and cell membranes, leading to changes in cell functioning and/or death. The influence of long-wave radiation (infrared, microwaves) carrying a lower energy potential is usually reduced to its impact on temperature and by that on water relations in the cell; however, the direct (‘microwave’) effect on nucleic acids was also observed (Cox and Wathes, 1995; Górny et al., 2007).
Microorganisms have different requirements for the atmospheric presence (or absence) of oxygen. Depending on the degree of oxygen tolerance, they are divided into four groups: absolute or relative anaerobes, microaerophiles (tolerant anaerobes), and absolute aerobes. The influence of oxygen on microbial aerosols may reveal both toxic and protective effects. The positive effect of oxygen is hypothesized based on laboratory experiments with the suppression of activity of toxic substances, whereas the toxicity is due to its enzymatic reduction inside the cell to hydrogen peroxide and toxic free radicals (O2−). Aerobes and relative anaerobes are protected against these products by the presence of superoxide dismutase or catalase enzymes. The oxygen toxicity to the microbial cell depends on the degree of cell hydration and is observed at a relative humidity below 65–70 % when the oxygen content is about 30 % (further increase in oxygen content no longer produces an additional toxic effect) (Cox and Wathes, 1995; Carroll et al., 2016).
Wherever microbial sources are present, these particles can be released into the air forming biological aerosols. They can be both actively (e.g. by breathing, coughing) and passively (through meteorological processes) emitted to the atmosphere from almost all surfaces (Jones and Harrison, 2004). When airborne, microbial particles (as their non-biological counterparts) are primarily carried by air streams. In such a state, they may be present solely as microbial particles, may form aggregates with both biological and non-biological particles or simply be attached to (usually bigger) dust particles or fibers. The sizes of the formed structures condition the aerodynamic behavior influencing their half-life times (see below). Microbial particles are not ‘ideal’ spherical and smooth structures, and in the overwhelming majority of cases are asymmetric with rough or porous surfaces. All this makes the physical dimensions of microbial particles different from their aerodynamic diameters influencing their stability in the air. Independent of the air turbulence (that are almost always present in the environment), microbial particles are constantly removed from the air due to gravitational sedimentation, interception, impaction or precipitation processes, ending in both cases in deposition on surfaces as ‘free’ microbial particles or inside water droplets or ice crystals (Górny et al., 2017; Gregory, 1973; Lacey, 1991; Madelin and Johnson, 1992; Reponen et al., 1998, 2001; Tong and Lighthart, 2000).
Dry deposition plays the major role in the removal of airborne microbial particles with diameters bigger than 10 μm; however, they rarely penetrate deeper into the respiratory tract. For microbial particles, important from the human health perspective (i.e. inhalable particulates below 10 μm in diameter), ‘wet’ processes of bioaerosol sweeping from the atmosphere are of a fundamental importance (Després et al., 2012). Due to them, such microbial particles are usually present in large numbers in the troposphere (especially in the planetary boundary layer); however, both their vertical (beyond the troposphere) and horizontal (between environments, e.g. land and water, continents or climate zones) long-range transport is possible and observed. Such transport over geographic barriers supports dissemination of reproductive structures and by that allows also the transfer of genetic material within microbiocenosis and between the microbiocenoses of different ecosystems. In many aspects, the environmental transport of microbial propagules resembles those of dust particles (e.g. Bovallius et al., 1978; Jeon et al., 2011; Prospero et al., 2005), and over the last two decades, many models have been created to precisely describe the complexity of this phenomenon (e.g. Ganio et al., 1995; Helbig et al., 2004; Jarosz et al., 2004; Skelsey et al., 2008; Sofiev et al., 2006; Wilkinson et al., 2012).
When microbial aerosols are released into the air, they can be transported between different ecosystems and are influenced during this process by climate and seasonal changes, life cycles, aging, chemical and physical interactions, and variations in microbial populations (Fröhlich-Nowoisky et al., 2016; Huffman et al., 2013; Sesartic et al., 2012). Microbial particles already in the airborne state above ground may undergo frontal uplift, convection or further turbulence, driving them even above the troposphere (Lindemann et al., 1982). The presence of microbial particles has already been confirmed at very high altitudes reaching into the stratosphere (Rogers and Meier, 1936; Shivaji et al., 2006; Wainwright et al., 2003) and mesosphere (Imshenetsky et al., 1978), and by that microbial particulates may affect numerous atmospheric processes. They may act as cloud condensation nuclei (CCN) or ice nuclei (IN) responsible for the formation of cloud droplets, ice crystals and precipitation, and thus influence the hydrological cycle and climate. Biological CCN or IN may be present as both viable and non-viable microbial cells and conidia, their structural fragments and detached macromolecules. For example, many gram-negative bacterial species from Pseudomonas, Pantoea, and Xanthomonas genera act as IN through both their cells and outer membrane proteins. The bacterium Pseudomonas syringae is now available as the commercial product for use in cloud seeding (e.g. Franc and Demott, 1998; Fröhlich-Nowoisky et al., 2016; Hill et al., 2014; Huffman et al., 2013; Kozloff et al., 1991; Lundheim and Zachariassen, 1999; Möhler et al., 2007; O’Sullivan et al., 2016; Pouleur et al., 1992; Šantl-Temkiv et al., 2015; Sattler et al., 2001).
For both viable and non-viable microbial particles, the atmosphere in not only the environment allows their transport but influences and modifies their basic physical, chemical, and biological features as well. For example, the atmospheric fragmentation of microbial particles may affect their transport and abilities to serve as CCN or IN (Diehl et al., 2001; Morris et al., 2004). Also, coating formation on microbial particles by organic and inorganic materials may influence their basic biological features and reactivity. Certain chemical reactions that influence primary structural and functional cell compounds may reorganize both the molecular arrangement and immunological reactivity of microbial aerosol particles (e.g. ozone and nitric oxide reveal the ability to increase the allergenicity of airborne mold conidia proteins) (Gruijthuijsen et al., 2006; Lang-Yona et al., 2016). Although the air is the main environment for the spread of microorganisms, it is a biotope unfavorable to their survival. A lack of sufficient nutrients, numerous physical and chemical stressors often result in sudden changes in microclimate, radiation, and osmotic parameters. Together with the imbalance between oxidants and antioxidants in the surroundings, they may influence the metabolic activity and even viability of microbial cells. Atmospheric conditions are not conducive to maintain the metabolic activity of microbial particles (except for a few experiments with cloud water droplets and microorganisms being airborne for an extended period of time) (Amato et al., 2005, 2007; Dimmick et al., 1975; Vaïtilingom et al., 2013). On the other hand, these unfavorable atmospheric conditions may (to some extent) provoke a substantial shift among microbial particles leading to microbiome reformulation through relative selection and/or evolutionary changes. However, it should be clearly stated that the influence of the airborne state on microorganism behavior is so far not well understood and still requires more studies.
In the physical sense, microbiological agents can be treated as solid particles. Several physical (size, shape, density, electrical charge), chemical (composition, hygroscopicity), and biological (breathing pattern, route of breathing, anatomy of the airways) factors condition their behavior both in the air and, if inhaled, in the lungs (WHO, 2002). The basic and one of the most important characteristics of microbial particles is their diameter. The sizes of microbial aerosol particles range from ~0.02 μm up to tens of micrometers. Bioaerosol particles are rarely spherical and, when studied, their usually irregular shapes are characterized using certain equivalents of particle diameter (in aerobiology, the most frequently used are aerodynamic and optical approximations). It is worth mentioning here that data from remote measurements (see below) suggest that microbial particles usually tend to form aggregates larger than 2.5 μm in diameter (Ho J., 2014). Both equivalent diameters may be significantly distinct from their physical dimensions. As examples, one can mention here the dimensions of Bacillus subtilis endospore-forming gram-positive rods and Cladosporium cladosporioides fungal conidia, whose physical sizes of 0.7–0.8 × 1.5–1.8 μm and 3–7 × 2–4 μm correspond with aerodynamic diameters of 0.9 μm and 1.8 μm, respectively (Kulkarni et al., 2011; Macher, 1999; Reponen 1994).
To be inhaled, microbial particles must be airborne for a sufficient period of time. In the vast majority of environmental situations, when the air is almost permanently turbulent, such a time frame is characterized by the ‘half-life’ term. In brief: as rarely spherical, the sedimentation velocity of airborne microbial particles depends inter alia on the square of the particle diameter. Hence, according to Stokes’ law, for microbial particles of 100, 10, 3, 1, and 0.5 μm, the expected half-life times are 5.8 sec, 8.2 min, 1.5 h, 12 h, and 41 h, respectively (Kulkarni et al., 2011; Martinez, 2002).
As already mentioned, the air – being deprived of nutrient sources, with constantly changing moisture content, and a wide range of different stress factors—on the whole disfavors microbial survival. Despite these disadvantages, microbial particles are capable of preserving their viability and, with it, also all related biological properties (including infectivity, toxicity, allergenicity, etc.) much longer than bigger organisms. Table 2 presents a few examples of survival times for microorganisms representing different groups of biological agents that are important from the human health perspective (Burge, 1995; Flannigan, 1994; Kramer et al., 2006; Mandrioli et al., 2003; Neira et al., 2016; Yang, 1994). If deposited on surfaces, the microbial particles may also maintain their viability for a long period of time and, when resuspended in the air, may still pose a serious threat to the exposed individuals. As the inhalation of immunologically active particles can be responsible for numerous adverse health conditions, the relationship between their stability on surfaces and viability in the air is of great importance from the exposure assessment point of view.
Viability of microbial particles in the air and on surfaces.
| Biological agent | Viability/persistence | |
|---|---|---|
| In the air | On inanimate surfaces | |
| Influenza virus | Up to 21 days | Up to 11 days |
| Staphylococci | About 3 days | From 7 days to 7 months |
| Streptococci | Up to 48 hours | From 1 day to 6.5 months |
| Escherichia coli | Up to 60 minutes | From 90 minutes to 16 months |
| Legionella pneumophila | Up to 15 minutes | Not detected |
| Aspergillus and Penicillium conidia | Up to 22 years | Up to 22 years |
| Dust mite allergens | Up to few months | Up to few months |
A microbial aerosol has a dual nature, i.e. physical like other particulates or fibrous aerosols, and biological, i.e. it possesses specific features characteristic for these types of airborne particulates only. As mentioned earlier, microorganisms may be present in the air as viable (i.e. having the ability to reproduce/replicate or possessing metabolic activity under given conditions) or non-viable particles (deprived of the capacity to form progenies, being, e.g. dead or unable to reproduce neither from whole cells nor from their fragments). Conditio sine qua non for microorganisms to be infectious is their viability (which in contrast to allergenicity and toxicity can also be maintained by both non-viable cells and their fragments). Moreover, in the environment, viable microorganisms are under the constant influence of numerous disruptive and/or stress factors. Their interference may end in genotype (mutations) and phenotype changes, evolution and relative selection, which can equip microbes with features and properties (e.g. bacterial mutants resistant to antibiotics or disinfectants, fungi resistant to fungicides, etc.) hitherto not possessed (Després et al., 2012; Dutkiewicz and Jabłoński, 1989).
Microbiological aerosols are of the greatest epidemiological importance. Although most of the bioaerosols are harmless constituents of normal environments, some of their particles may be infectious agents or allergens, may carry or produce toxic and/or irritant substances or metabolites. Taking into account the health effects provoked by airborne microbial particulates, the difference between outdoor and indoor environments seems to play a significant role. In an outdoor environment, except in periods of snow cover, microorganisms are always present. Except for intensively emitting sources, their concentrations are not usually high, as spatial effect result in their more or less uniform dispersion. Once disseminated in the ambient air, microbial aerosols may find their way into the indoor environment. Here, a lack of ventilation and other dispersal or elimination mechanisms, together with the substantial amount of time people spend indoors, create harmful conditions from the health perspective. Global estimates reveal that in an occupational environment only, each year several hundred million workers have been exposed to microbial agents in concentrations harmful to their health. The social costs of such exposure are significant, reaching billions of dollars (Cox and Wathes, 1995; Crook, 2007).
Among microbial aerosol particles, one can find: infectious agents, allergens, toxins and other biological compounds that are able to provoke similar toxic effects (e.g. endotoxins, β-glucans), carcinogens (e.g. mycotoxins), and biologically active (submicrometric and nanometric) fragments of microorganisms. All these particles may perform a causative role of many adverse health conditions from allergic reactions, infections and toxic reactions to other non-specific symptoms known in scientific literature as ‘sick or tight building syndrome’ (SBS or TBS), ‘building-related illnesses’ (BRIs) or ‘mucous membrane syndrome’ (MMS). Regarding those non-specific reactions, accompanied usually by a dry cough, eye, nose and throat irritations, they may not necessarily involve immune responses or inflammatory mediators because in most cases, the observed adverse effects are a result of mixed exposure to both toxins and allergens. Aerosols composed of infectious microorganisms may attack the respiratory tract and, in more generalized cases, other body organs as well. When among airborne microbial contaminants, non-viable ones—usually together with their fragments—are present, such exposure may lead to chronic or acute illnesses. Such non-infectious microbial particulates, if abundantly present in the environment, may also sensitize exposed individuals (Bunger et al., 2004; Douwes et al., 2003; Falkinham, 2003; Fogelmark et al., 1991; Roy and Milton, 2004; Roy and Reed, 2012; Shahan et al., 1994).
During the inhalation of microbial aerosols, both the upper and lower airways are exposed. The diseases (if they occur) most often appear within the part of respiratory tract into which the particular microbial agent penetrates and deposits. For example, the influenza virus (representing the Orthomyxoviridae family) together with other rhino-, adeno-, and coronaviruses that are responsible for the common cold usually infect the upper respiratory tract. Also, sensitization diseases (such as allergic rhinitis or sinusitis) are located in the upper airways (Lopardo et al., 2012; Louie et al., 2005; May S. et al., 2012). In turn, lower respiratory tract diseases (such as bronchitis or pneumonia) are mainly provoked by aerodynamically bigger structures such as bacteria (e.g. Legionella spp., Streptococcus spp., Haemophilus influenzae); however, nanometric particles such as avian influenza (H5N1; virion of 80–120 nm in diameter) or parainfluenza viruses can also infect the lower respiratory tract (Dasaraju and Liu, 1996; Hall, 2001). The lower airways (especially the bronchi and alveoli) are also a target for numerous viable bacterial cells and spores, fungal conidia, and non-viable microbial allergenic and toxic particulates responsible for chronic diseases such as asthma, chronic obstructive pulmonary diseases, and hypersensitivity pneumonitis (Bettoncelli et al., 2014; O’Connor et al., 2013; Reynolds et al., 2013; Sferrazza Papa et al., 2014; Takemura et al., 2008).
Microbial aerosols are ubiquitous and, as such, should also gain special attention indoors. Such closed spaces, quite often harboring a wide spectrum of microbiota, characterize high concentrations of bioaerosols and as such pose a real risk to human health. The observed diseases, however, are often hard to attribute to specific microbial agents as they usually constitute only a part of a particulate cocktail together with other allergens derived from organic chemistry products (Blais-Lecours et al., 2015; May S. et al., 2012).
Microbial aerosols may affect human health in several ways, i.e. due to their inhalation or direct contact with the skin and mucous membrane of the eye. Inhalation is a major route of entry for microbial pathogens and the possible adverse reactions can affect both upper and lower respiratory tracts. The type of interactions between microbial aerosol particles and human cells depends on their place of deposition and is conditioned by their retention time in the airways.
Microbial aerosol particles can be deposited due to several mechanisms (Fig. 1): larger particles—by inertial impaction, sedimentation, and if resembling fibers (like chains of fungal conidia)—by interception; smaller particles—by diffusion. Electrostatic effects may enhance or modify the deposition of charged particles of any size.
Schematic view of major mechanisms of particle deposition in the respiratory tract.
As already mentioned, the settling velocity of particles with diameters above 10 μm is big enough to prevent their inhalation (except the cases when exposed individuals are in the immediate vicinity of the active emission source). Microbial particles with smaller diameters (represented usually by vegetative cells, spores, conidia, their fragments, aggregates entirely formed by microbial particles or consisting of microbial and other non-biological aerosols) may penetrate into the respiratory tract. From them, about 80 % of particles between 5–10 μm in diameter are stopped by the nasopharynx. Here, the air entering the airways has the highest velocity and their anatomic formation supports and facilitates both the inertial impaction and centrifugal condensation processes. For particles resembling fibers (such as chains of fungal conidia), their capture is realized due to interception. When inspired air leaves the nasopharynx and then turns downward into the lower stages of the respiratory tract, its velocity decreases and directional changes are less abrupt, allowing the separation of smaller particles between 0.5–5 μm in diameter. Particles within that size range are separated from the air stream by impaction and sedimentation, and their deposition probability is directly proportional to the residence time. Finally, the finest particles with diameters below 0.1–0.5 μm are removed from the air and subsequently deposited almost entirely by Brownian motion and diffusion. This process is inversely proportional to the diameters of driven particles and is usually bolstered by electrostatic forces. Very small aerosol particles with diameters of about 0.01 μm have negligible inertia and they may start to behave as highly reactive gases, diffusing dynamically through the walls of the respiratory tracts. It is worth mentioning here that some of the microbial aerosol particles are highly hygroscopic. During their passage through the airways, they face significant changes of both the air stream temperature and humidity (the latter parameter may reach almost 100 %). In such circumstances, the aerodynamic diameters of microbial particles may expand, resulting in their deposition. These hygroscopic changes are most pronounced for particles between 0.5–2 μm in diameter and its increase may reach 20 %. This fact can have a significant impact on both the magnitude of the actual exposure and on the dose of inhaled particles and should always be considered when microbial exposure assessment is to be performed (Clarke, 1990; Cohen et al. 1998, 2000; Lippmann, 1986; Reponen et al., 2001; Spengler and Wilson, 1996; Utell and Samet, 1996; Walton, 1977; Wang, 2005).
In contrast to the deposition of particles, which is a purely physical phenomenon, lung clearance is a biological process. There are four major mechanisms by which deposited particles are removed from the airways: transport by mucociliary escalators to the pharynx followed by entry into the gastrointestinal tract, incorporation by alveolar macrophages followed by mucociliary escalator transport or lymph nodes, entry into lymph nodes via lymphatic vessels, and dissolution followed by absorption into the blood circulation. The particle clearance efficiency is different in specific airway regions. Particles stopped in extrathoracic regions are removed by wiping, sneezing or blowing, and these processes usually take minutes; those trapped in tracheobronchial parts are removed with the mucus flow and the average time necessary for that ranges from less than an hour to two days; finally, removal of insoluble particles caught in the lower respiratory tract (alveolar region) is much slower taking hundreds of days; highly soluble particles may dissolve almost immediately after their deposition (Foster and Costa, 2011; Gehr and Heyder, 2000; Spengler and Wilson, 1996; Utell and Samet, 1996; Wang, 2005).
The place of particle deposition and their retention in the respiratory tract determine the type and severity of interactions between the inhaled microorganisms and exposed tissues. Particles of: 10 μm in diameter or bigger (usually microbial aggregates with other non-biological particulates) may be responsible for eye and/or nose irritations; 5–10 μm in diameter (e.g. bigger fungal conidia or multiparticle microbial aggregates) may elicit asthmatic reactions; 5 μm or smaller (e.g. individual fungal conidia, bacterial cells or spores, their fragments as well as small aggregates of microbial and dust particles) may induce allergic alveolitis-type reactions (Horner et al., 1995; Owen et al., 1992). Lessons learned from both experimental and field studies revealed that the physiologic impact of ‘fine’ (including microbial) particles of 2.5 μm in diameter or smaller (e.g. actinomycetal spores, majority of indoor mold conidia) has been associated with negative health results. Adverse respiratory effects in exposed human populations include an increase in asthmatic episodes, a rise in the prevalence of chronic bronchitis and chronic obstructive pulmonary disease. These particles may also negatively impact the cardiovascular system; however, their deleterious role here is still poorly understood (Cohen et al. 1998, 2000; Foster and Costa, 2011; Gehr and Heyder, 2000; Spengler and Wilson, 1996; Walton, 1977; Wang, 2005).
To properly evaluate the health hazards, an appropriate conceptual framework must be built and applied covering both the physical (how many microbial particles are in a particular volume of the air; how many microbial agents are in or on other non-biological, i.e. particulate and/or fibrous aerosol, particles; what is the particle size distribution) and biological characteristics (which microbial agent is present, how many agents are viable and non-viable, how many agents are immunologically reactive, how many of them are necessary to cause adverse reactions) of aerosolized microbial agents. To reach that goal, both the identification and characterization of microbial aerosol particles should be performed precisely and accurately. Despite the constantly expanding knowledge within this field, a clear link between the dose of microbial pollutants and the subsequently provoked response for the majority of these agents cannot be conclusively confirmed. Among the rationales behind this is the inappropriateness of analytical methods related to and used in assessment of the exposure to microbial aerosols.
Compared to other aerosols, microbial aerosol sampling requires special handling procedures. Traditional sampling methods (i.e. filtration and impaction, including impingement) and analysis (cultivation) focus on the evaluation of viable microbial propagules including conidia, spores and vegetative cells. Such an approach excludes the meaning of both non-viable particles and/or their fragments and consequently underestimates the real exposure, measuring only a small part of available particles. The quantitative and qualitative results of these methods (regardless of whether they are carried out as stationary or personal measurements) are additionally biased by the sampling time (relatively short in the majority of cases) and numerous environmental, spatial or temporal alterations. Even characterization of the microbial source, performed usually by surface sampling (using transparent sticky tape, swabs or contact plates) or, much less often, by source strength evaluation (based on aerosolization techniques that use a perpendicular or swirling air stream for the release of microbial propagules) cannot precisely estimate the magnitude of microbial particle emission into the air (Després et al., 2012; Górny and Ławniczek-Wałczyk, 2012; Hung et al., 2005; Kulkarni et al., 2011; Macher, 1999; Yang and Heinsohn, 2007).
Samplers utilizing inertial forces have been broadly applied for microbial aerosol measurements. Among them, the most common are: single-stage and cascade impactors as well as slit samplers (if the impaction stage consists of slits instead of circular holes). Especially cascade impactors in their six-, seven-, eight- or ten-stage forms have often been utilized as a reference device for the collection of culturable microorganisms, providing a precise separation of sampled particles due to the well-established cutoff sizes of particular impactor stages. These impactors are available in both stationery (e.g. Andersen type) and personal (e.g. Marple type) models. Particle separation from the air by centrifugal force as a special case of inertia but with a radial geometry has also been successfully applied for microbial aerosol particle collection. On the other hand, impingers (such as AGI-30 or BioSampler) combine the collection of airborne particles by impaction into a liquid with particle diffusion within the liquid bubbles. Compared to impactors, impingers offer a significant extension of sampling times and increase of collection efficiency, preserving at the same time a vast majority of biological properties of the microbial aerosol particles (Crook, 1995a, b; Kulkarni et al., 2011). Non-inertial collection techniques represented by sedimentation or filtration have also been frequently applied for microbial aerosol sampling. More sophisticated devices such as electrostatic samplers and thermal precipitators have been developed which enable the ‘gentle’ collection of microbial aerosol particles while preserving their viability during the sampling process and at the same time enhancing their collection efficiency (Kethley et al., 1952; Mainelis et al., 1999, 2001, 2002; Orr C. et al., 1956; Tan M. et al., 2011).
Traditional microbiological methodology utilizes cultivation methods and microscopic analysis to describe both the diversity and concentration of microorganisms in collected airborne samples. Although these are already ‘older’ methods, they are still to this day successfully used in aerobiological studies. On the other hand, however, cultivation methods have serious limitations as they are dedicated to the detection of viable (understood usually as culturable) microbial agents, neglecting the remaining load of particulates of microbial origin including viable but non-culturable and dead microbes as well as their fragments. It should be clearly stated here that the majority of airborne particles of microbial origin, even when viable, is non-culturable and unable to form new colonies on an appropriate medium (Amann et al., 1995; Colwell, 2000; Cox and Wathes, 1995; Hung et al., 2005; Rappé and Giovannoni, 2003; Roszak and Colwell, 1987; Staley and Konopka, 1985; Wainwright et al., 2004; Yang and Heinsohn, 2007). Hence, by definition, the inference about air pollution based on cultivation methods significantly reduces the value of real exposure. The hitherto obtained results show that the percentage of culturable microbial particulates in the total microbiota did not exceed ~25 % for bacteria and ~17 % for fungi, being usually between 0.03 % and 1 % (Bridge and Spooner, 2001; Chi and Li, 2007; Gołofit-Szymczak and Górny, 2010; Heidelberg et al., 1997; Lighthart, 2000). Among the factors influencing these numbers are those of technical (collection method, sampling time, collection/growth medium, incubation conditions), biological (microbial strain; type of sampled propagules, i.e. vegetative cells, spores, conidia), and environmental origin (e.g. temperature; relative humidity; time of day; season; climate zone; geographic region; type of natural reservoirs such as water, soil, forest, desert, etc.; presence of plant and animal populations, etc.) (Amato et al., 2007; Dutkiewicz and Jabłoński, 1989; Griffin et al., 2006; Shahamat et al., 1997; Stewart et al., 1995; Tong and Lighthart, 1999; Wang Z. et al., 2001; Wang C.-C. et al., 2007, 2008).
Since construction of the first optical microscope at the end of the 16th century, microscopic techniques still to this day play an important role in microbiological studies, including those related to biological aerosols. They allow the observation and assessment of the size of both large micrometric and smaller objects with submicro- and nanometric dimensions. They also enable observation of their internal structure. At the same time, they provide the opportunity not only to qualitatively identify the viable and non-viable microorganisms, but also to assess the number of such agents from vegetative cells, spores or conidia through the particles that are their structural elements, to small infectious agents such as viruses. Bright field or light microscopes are usually used for simple observations of particulate shapes and sizes as well as to count microorganisms. The particles in the samples can be observed in the transmitted light (for transparent specimens) or in the reflected light (for opaque ones), in a light or dark field or in polarized light to increase their contrast and make their details visible in more precise way. They can be also stained using, e.g. methylene blue, crystal violet, safranin, fuchsine or other differential stains to allow classification of microbial particles into groups of species. On the other hand, a phase-contrast microscope is used when the microbial particulates are nearly invisible and an alternative mounting medium is not possible or permissible. Such microscopes do not require sample staining but offer detailed examination of the internal structure of observed particulates. A variety of light microscopy is fluorescence microscopy which uses an ultraviolet or near-ultraviolet source of illumination that causes fluorescent particle compounds to emit light. Direct count methods, applied also to aerobiological studies to, e.g. count the number of ‘total’ microbial particles (i.e. viable and non-viable together), today constitute one of the most wide-spread analytical techniques. The application of a wide range of fluorochromes (e.g. acridine orange, 4′,6-diamidino-2-phenylindole, 5-cyano-2,3-ditolyl tetrazolium chloride or fluorescein isothiocyanate), selectively bounded to the studied cell structure or coupled with specific antibodies or molecular probes, allows the location of individual structures in a cell and observation of selected physiological processes. Currently, the majority of the fluorescent microscopes are the ‘epi’ ones (i.e. where the light source is mounted above the specimen) equipped with digital cameras which allow recording the image directly in a digital form. The automatic acquisition of images together with their analysis makes the examination time shorter and the use of several fluorochromes enables the simultaneous measurement of multiple parameters (Bartoszek and Rosowski, 2017; Després et al., 2012; Francisco et al., 1973; Harrison et al., 2005; Hernandez et al., 1999; Hobbie et al., 1977; Jensen et al., 1994; Karlsson and Malmberg, 1989; Kepner and Pratt, 1994; Macher, 1999; Pöhlker et al., 2011). Electron microscopy uses a beam of electrons instead of light. In a scanning electron microscope (SEM), the sample surface is scanned with a collimated electron beam. Electron signals, obtained as a result of the electron stream interaction with the surface of the examined object, are collected by appropriate detectors and transformed into images. The high-vacuum mode is fundamental for each SEM mode of work. It allows obtaining the magnification of sample images up to 10,000,000 × . On the other hand, use of the low-vacuum mode prevents the accumulation of a charge on the surface of the examined sample, which allows good quality images for non-conductive samples to be obtained. In turn, the environmental mode enables the imaging of samples of solids, suspensions and non-dehydrated biological preparations and carrying out experiments under dynamic change conditions. SEM in aerobiological studies is usually used to characterize the size and shapes of individual microbial particulates, describe their ability to form aggregates, and bind to other aerosol particles or fibers. Also, taxonomical characterization of microbial aerosols is possible to a certain degree based on morphological criteria. On the other hand, in transmission electron microscope (TEM), the stream of high-energy electrons produced by the electron gun is directed to the thin (i.e. several dozen to several hundred nanometers) sample where it may be reflected, absorbed or penetrate through. In the transmission mode, electrons are used to create the image of the sample structure. Reflected electrons in scanning mode are utilized to characterize the object surface. TEM allows high-resolution images to be obtained. With the limit of about 0.1 nm, TEM enables observation of the arrangement of atoms in the examined sample. Finally, atomic force microscopes (AFM) use a scanning probe (cantilever) to examine surfaces, registering the force affecting it in the function of its position. The forces (mainly van der Waals ones) between the cantilever tip and the sample cause deflection of the measuring lever, which leads to imaging of the topography of the examined material. In the tapping mode, the cantilever tip is in contact with the sample surface for a short time and, in this way, is able to visually reproduce the image of soft and delicate samples like microbial ones (Bartoszek and Rosowski, 2017; Jensen et al., 1994; Jonson et al., 2014; Karlsson and Malmberg, 1989; Macher, 1999).
In many cases, the above-mentioned laborious and time-consuming analyses, often requiring the involvement of highly qualified analysts and expensive research instruments, are simply not possible. In such cases, numerous constituents or metabolites of microorganisms can be measured as a surrogate of environmental microbial exposure. So far, several chemical tracers have been introduced as surrogates of different microbial contaminants (e.g. endotoxins for gram-negative bacteria; muramic acid from peptidoglycans for gram-positive bacteria; ergosterol, N-acetylhexosaminidase, and (1→3)-β-D-glucan for fungal biomass). Also, numerous instrumental and bioanalytical techniques corresponding to the selected microbial constituents have been practically tested. For example: for the qualitative and quantitative assessment of endotoxins in environmental samples, a wide palette of in vitro analyses are proposed including a few modifications of the Limulus test, recombinant Factor C assay as well as liquid or gas chromatography (used alone or coupled with mass spectrometry, GCMS). In turn, the measurements of muramic or diaminopimelic acids as markers of peptidoglycans that are structural components of bacterial cell wall can be done using GCMS. Also, detection of β-glucans can be realized by applying four different assays: modified Limulus test, inhibition enzyme immunoassay, enzyme-linked immunosorbent assay, and monoclonal antibody-based two-site enzyme immunoassay. Markers for the assessment of fungal biomass include also ergosterol analysis by GCMS, extracellular polysaccharides measured with specific enzyme immunoassays, and N-acetylhexosaminidase activity assessment (method based on a fluorescence labeled substrate which can be cleaved by the enzyme found in fungi). Moreover, secondary products of fungal metabolism, i.e. mycotoxins, can be measured using different variants of thin-layer chromatography, gas chromatography with or without mass spectrometry or high-pressure liquid chromatography. These two latest techniques can be also successfully applied in quantitative analyses of volatile organic compounds (VOCs), which can be suitable markers of fungal growth (Demirev and Fenselau, 2008; Després et al., 2012; Douwes et al., 1999; Griffith and DeCosemo, 1994; Hung et al., 2005; IOM, 2004; Macher, 1999; Miller and Young, 1997; Pöschl, 2005; Reeslev et al., 2003; Reponen et al., 1995; Rylander et al., 2010; WHO, 2009). Evaluation of the pros and cons of the above-listed methods clearly shows that although the analysis of chemical markers provides quite precise quantitative information, it does not supply us with data regarding the biodiversity of microbial particles. Nowadays, such a gap in knowledge can be fortunately fulfilled by a spectrum of different molecular techniques enabling precise identification of microbial strains that form bioaerosols (see below).
As none of the above-described methods offers real-time detection of microbial aerosols, numerous optical techniques were introduced to overcome this limitation. Among them are those utilizing light scattering, condensation, and fluorescence of microbial particles. Some of these methods offer both the measurement and separation of microbial particulates beginning from nanometer sizes. Amid the instruments operating on the above-described principles are optical (utilizing light scattering) particle counters (e.g. Grimm 11A; allowing the measurements of particle sizes, mops, within the range of 0.25–32 μm) and condensation (alcohol vapor) nuclei counters (e.g. P-TRAK; mops of 0.01–2 μm) as well as aerodynamic (combining analyses of the particle aerodynamic diameters and their light-scattering intensities; e.g. DSP Aerosizer; mops of 0.2–200 μm) and scanning mobility particle sizer spectrometers (e.g. SMPS; mops of 1–1000 nm based on differential mobility analysis). In turn, the electrical low-pressure impactor (ELPI) allows combining the quantitative control of microbial propagules with rapid real-time description of their size distribution (mops of 0.006–10 μm). In ELPI, like in many other volumetric devices, the sampling process is inseparably connected with certain measurement limitations. The impaction of particles onto hard surfaces, their bounce and subsequent reaerosolization, friction during passage through the instrument, desiccation can significantly affect the precision of the assessment and control of microbial aerosols. These facts should always be kept in mind when sampling biological particles (Hung et al., 2005; Jonsson et al., 2014; Kulkarni et al., 2011; Macher, 1999).
The instruments utilizing fluorescence as a method of detection are based on the phenomenon that all biological particles containing fluorophores originate from aromatic amino acid residues (intrinsic constituents of almost all proteins). Tryptophan, tyrosine, and phenylalanine are the amino acids capable of emitting fluorescence induced by UV light, and this phenomenon can be utilized for the detection of microbial aerosols. Techniques based on the fluorescence phenomenon enable recognition of microbial particles both on a laboratory scale and over areas of tens of square kilometers. Such devices can successfully form the core of monitoring systems to be used over, e.g. crowded spaces (Eng J. et al., 1989; Harrison and Chance, 1970; Iwami et al., 2001; Jonsson et al., 2014; Kell et al., 1991; Li J.K. et al., 1991).
Among the first commercial applications utilizing induced fluorescence were the fluorescence aerosol particle sensor (FLAPS) and the ultraviolet aerodynamic particle sizer, UV-APS, the instruments dedicated to real-time analysis of biological aerosols. FLAPS technology was developed in the early 1990s and was based on an aerodynamic particle sizer with laser added to excite fluorescence of the sized particles. In its latest version, it uses a CW laser diode for both excitation and optical sizing. The system analyses individual particles in the size range of 0.8–10 μm, offering exceptional discrimination of microbial aerosol particulates including bacteria, fungi, viruses, and toxins. Its performance was broadly confirmed in laboratory, military, working and non-occupational applications. On the other hand, the UV-APS (that is technically a newer version of FLAPS II) measures the real-time fluorescence of airborne particles (after excitation by a pulsed UV laser) and simultaneously provides a number size distribution and scatter light intensities for them. The instrument is able to discriminate particles within 0.8–15 μm aerodynamic size range and has so far been applied to investigate bioaerosol emission sources, microbial particulate properties in laboratories, clean rooms, hospitals and other indoor (including industrial and built) and outdoor settings in both urban and rural environments (Brosseau et al., 2000; Delort and Amato, 2018; Després et al., 2012; Hairston et al., 1997; Jonsson and Kullander, 2014). The wideband integrated bioaerosol sensor (formerly named wide-issue bioaerosol sensor, WIBS) combines the optical detection and sizing of total aerosol particulates using a continuous wave CW laser and fluorescence detection resulting from UV pulsed excitation from two xenon flash lamps. During measurement, the instrument collects five different types of information including optical particle size, particle asymmetry, and fluorescence from three channels. WIBS is capable of processing up to 125 particles/s and its performance was already successfully tested in urban and tropical locations (Delort and Amato, 2018; Després et al., 2012; Gabey et al., 2011; Jonsson and Kullander, 2014; Kaye et al., 2005).
Like epifluorescence microscopy, flow cytometry has been successfully applied to the real-time control of microbial particles in the air. The evaluated particles can fluoresce naturally or may gain this feature after specific staining. Being transported in a particle-free medium (e.g. deionized water) at laminar flow, they are excited by a laser beam. The fluorescence emitted by the flowing objects and the light dispersed on them are measured by photon detectors, which regarding microorganisms, offer e.g. total, viable and non-viable particle quantification as well as delivering data about their sizes and taxonomical classification (Chen and Li, 2005, 2007; Delort and Amato, 2018; Ho J. and Fisher, 1993; Lange et al., 1997; Prigione et al., 2004)
Mass spectrometry (MS) techniques have also been adapted for microbial aerosol analysis; however, such particles must be converted into a vapor state before entering the mass spectrometer. One of the successfully employed MS techniques is matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF). MS can analyse both bulk amounts and single microbial aerosol particles. MALDI-TOF identifies microorganisms, determining their unique proteomic fingerprints. Their characteristic spectrum patterns are used to reliably and accurately identify a particular microorganism by matching thousands of reference spectra of microbial strains. Both rapid detection and accurate identification have decided on the usefulness of MS techniques in microbial aerosol studies including both bacterial and fungal pollutants (Jonsson and Kullander, 2014; Kim et al., 2005; Kleefsman et al., 2008; Madsen et al., 2015; van Wuijckhuijse et al., 2005).
A recently observed tremendous development of analytical techniques enables a comprehensive characterization of the biological, chemical and physical features of microbial aerosol particles. This is particularly evident in the case of deoxyribonucleic acid (DNA)-based and ribonucleic acid (RNA)-based methods including taxon-specific quantitative polymerase chain reaction (qPCR) or droplet digital PCR (ddPCR). The use of these methods in both Sanger sequencing-based and Next Generation Sequencing (NGS) analyses enables not only the identification of individual genera or species but also the characterization of diversity and metabolic potential of airborne microbiota (Fig. 2). Independent of the method used, the efficient extraction of DNA is a basic condition of successful analysis. Polymerase chain reaction technologies can be applied for detection and quantification of either groups of microorganisms or specific strains regardless of whether they are viable or culturable (Alvarez et al., 1994; Blais-Lecours et al., 2012; Mukoda et al., 1994). In aerobiological practice, the usefulness of this technique was widely confirmed for analysis of airborne bacterial and fungal contaminants (Peccia and Hernandez, 2006; Prussin et al., 2014). Regarding viruses, no comprehensive PCR assay so far exists and profile screening is necessary each time when these agents are considered to be a major analytical target (Carducci et al., 2013; Masclaux et al., 2014; Prussin et al., 2014). The most commonly applied laboratory protocols in microbial aerosol investigations include 16S rRNA gene (for total bacteria, total archaea, influenza, and bacteriophages) as well as 18S and 28S rDNA or fragments located between them, called internal transcribed spacers (ITS) for fungi. Because of their structure, these genes have been termed molecular chronometers. They are characterized by the presence of both conservative (i.e. slowly evolving) fragments as well as those that are characterized by interspecific variability (Blais-Lecours et al., 2012; Nehme et al., 2008; Oppliger et al., 2008; Perrott et al., 2009; Verreault et al., 2011; Woese, 1987).
Procedure in the assessment of exposure to harmful biological agents.
Various techniques utilizing 16S rRNA or ribosomal RNA gene spacers that can be used to characterize the biodiversity in aerosol samples and assess the presence of etiological agents of bioaerosol-related diseases comprise cloning, fingerprinting, pyrosequencing, and NGS. With cloning or sequencing, one can unequivocally confirm the presence of specific microbial strain in the analysed sample. In turn, using fingerprinting methods including denaturing or temperature gradient gel electrophoresis (DGGE or TGGE, respectively), terminal restriction fragment length polymorphism (T-RFLP), and ribosomal intergenic spacer analysis (RISA), both the biodiversity and temporal changes in microbial communities can be studied (Gandolfi et al., 2013; Madsen et al. 2015; Maron et al., 2005). When bioaerosol samples are studied, analysts almost always must have to deal with a cocktail of microorganisms. During their analysis, different microbial agents can generate many species-specific signals, which must be properly recognized and interpreted to accurately describe the collected microbiota. To do it in a holistic and univocal way, the applied molecular methods should allow the performance of such an analysis, depriving the possibility of confounding factors influencing its outcome. Today, among such methods are pyrosequencing and NGS (Blais-Lecours et al., 2012; Delort and Amato, 2018; Nonnenmann et al., 2010).
Amid the most promising techniques for microbial aerosol assessment is also droplet digital PCR (ddPCR). This technique utilizes droplets formed in a water-oil emulsion to arrange the partitions that separate the template DNA molecules. The droplets basically have the same function as individual test tubes or wells in a micro-titer plate in which the PCR reaction takes place, albeit in a much smaller format. The ddPCR system partitions nucleic acid samples into thousands of nanoliter-sized droplets, and PCR amplification is performed within each of them. Such a technique can be adapted for a variety of applications including copy number variant analysis, rare variant detection, gene expression analysis, and single-nucleotide polymorphism genotyping. Such an approach allows also smaller requirements for the sample than other digital PCR systems, thus reducing costs and preserving template molecules (Hindson et al., 2011; Mazaika and Homsy, 2014).
Other distinctive analytical protocols based on enzyme-linked immunosorbent assay (ELISA) and electrochemiluminescence (ECL) are employed to precisely localize and recognize microbial aerosol molecules as well as to confirm the immunological reactivity of microbial propagules using antibody-epitope binding as a common detection mechanism. Antibody-based immunoassays, particularly ELISA, are widely used, among other things for the measurement of aeroallergens. Methods for their measurement are not widely available, mainly due to the highly variable allergen production in different environments influenced by numerous factors, including substrate, temperature, biodiversity in microbial community, etc. Such variability makes it difficult to develop specific antibody-based immunoassays that detect the relevant fungal allergens in a specific environment; however, such a situation opens the door for innovative solutions in this area (IOM, 2004; Schmechel et al., 2003, 2004).
The threshold and/or reference limit values for some microbiological agents (e.g. bacteria, fungi or endotoxins), if recognized and accepted by the international scientific community, could be a useful tool in the interpretation of the measurement results and, as a result, in protecting the health of the exposed individuals. The documented exposure to bacteria and fungi that ended in the proposal of limit values for microbial contaminants dates back to the 19th century. In 1872, Carnelly, Haldane, and Andersen conducted an extensive study into bacteria and molds in the air of schools and public housing. As they used a volumetric air sampler for the first time (i.e. Hesse’s beef-agar-coated glass tube) and subsequently formulated, based on the airborne concentrations of microbial particles, the recommended level of 600 cfu m−3 as an indicator for excessive microbial pollution, this fact can be treated as a pioneer bioaerosol exposure control. Although almost 150 years have passed since that moment, it has not brought us much new data regarding the exposure standards for microorganisms.
As was mentioned earlier, a precise relationship between the inhaled dose of microbial propagules and a subsequent adverse health response is not possible in the vast majority of cases. Numerous environmental as well as microbial species and/or consortia-oriented biases prevent a comprehensive and holistic analytical assessment of such exposure. Contrary to appearances, and despite the time that has passed since the first volumetric measurement of bioaerosol, to this day there are not many guidelines regulating the issues of microbiological material collection, its thorough analytical elaboration and, finally, quantitative and qualitative interpretation of the obtained results.
Nowadays, on the worldwide scale, there are no widely accepted regulations standardizing microbial aerosol sampling for exposure assessment in different environments. To create them, some tips can be taken from the regulations elaborated for the control of the working environment (up to now, they are summarized in four European standards (EN) published by the European Committee for Standardization (CEN), i.e. EN 13098:2000—currently its revised version is under CEN approval, EN 14031:2003, EN 14042:2003 and EN 14583:2004). Nevertheless, even with their consideration, it is necessary to act according to a certain strategy, at the end of which an unequivocal interpretation of the concentrations of microbiological aerosols measured in the environment will be possible.
The dynamic development of analytical methods allows continuous improvement of the quantitative and qualitative description of the microbial pollution of the environment. Along with the progress in this area, there is also an urgent need for the development of hygienic standards for bioaerosols based on properly constructed strategies of their creation. In the case of microbiological aerosols, they are usually referred to medical (clinical) and environmental approaches. Such a strategy takes into account the research method as well as several environmental, source, quantitative and qualitative criteria, which play a key role in such a process. Fig. 3 illustrates their interdependencies.
Key factors in the elaboration of threshold limit values for microbial aerosols.
Regarding adverse clinical outcomes provoked by the exposure to microbial aerosols (that are probably the most crucial ones), the ideal situation would be when:
Today, as was already underlined, such a situation does not exist for any microbiological agent. Therefore, a wise solution to this situation is urgently needed. As it would seem, ‘a helping hand’ in this situation could be a so-called ‘environmental philosophy’, being a reasonable alternative for the above-described ‘clinical’ approach. According to ‘environmental philosophy’ in the situation in which ‘a solid link between the concentration of investigated parameters and resulting adverse health effects cannot be effectively established, then—based on the multiple biological agent concentration measurements—the reference values should enable an evaluation of the quality of the environment, as well as determination of ‘what is typical and/or acceptable’ and ‘what is atypical and/or unacceptable’ for a specific type of environment (or for its certain part)’.
The threshold limit values hitherto proposed in the scientific literature are usually formed for the total number of mesophilic bacteria, gram-negative bacteria, bacterial endotoxins, mesophilic actinomyces, fungi, glucans, and subtilisin for both occupational and non-occupational environments. A special emphasis is given to pathogenic microorganisms for which their environmental tolerance is at ‘zero’ level (i.e. no safety level exists and the threshold limit is usually 0 cfu m−3). For a more comprehensive overview of worldwide exposure standards for microbial aerosols, readers are referred to Brandys and Brandys (2012) and Górny et al. (2011).
According to World Health Organization guidelines, today’s societies should live in a safer environment, with exposure to contaminants hazardous to health at levels not exceeding internationally agreed standards. They also have the right to breathe healthy air, to receive adequate information about potentially harmful exposures, and to be provided with effective containment measures. Nevertheless, exposure to microbial aerosols is still common in many different environments and is often the cause of many adverse health effects. Although the protection of human health against the risks associated with this type of exposure is not only a legal requirement but also a logical consequence of today’s state of knowledge, both the control of exposure to microorganisms and the resultant assurance of safe living and working conditions are not treated with due seriousness. This situation should change quickly, and the knowledge already available in this area and the increasingly widespread use of tools for the precise quantification of exposure to airborne microorganisms should result, if not in elimination, at least in a significant reduction of microbiological hazards.
This scientific work was supported by the Polish Ministry of Family, Labour and Social Policy from the Multi-annual Program “Improvement of safety and working conditions (2017–2019)” as Research Project No. II.N.15.
Rafał L. Górny
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Rafał L. Górny is a Full Professor of medical sciences, is currently Head of the Laboratory of Biohazards at the Central Institute for Labour Protection—National Research Institute, Warsaw, Poland. In the last 25 years of professional work, he has been engaged in numerous studies on the health-related aspects of exposure to particulate, biological, and fibrous aerosols in occupational and non-occupational environments. His research efforts have been presented in more than 70 peer-reviewed publications, more than 100 conference presentations, several monographs and book chapters. Since 2002, he has been working as a World Health Organization and European Commission adviser within the field of biological agents.
