Air pollution is a major threat to human health. Biological air pollution is predominantly caused by the pollen of plants, fungi, bacteria and viruses. The main sources of microorganisms in the air include soil, water and the decomposition of organic matter, while anthropogenic sources are represented by landfills, wastewater treatment plants, composting facilities and traffic. Microorganism populations in the air can be seasonal or relatively constant, but the most frequent increase in their occurrence is recorded in the summer and autumn. Studies show that humidity, the presence of carbon monoxide and ozone concentrations are the main factors affecting the diversity of bacteria and the percentage of pathogenic bacteria present in outdoor air. Microorganisms in the air inside residential buildings are primarily concentrated on dust particles. Approximately 60% of dust microbiota are spores of mould fungi. The key emitters of microorganisms into the atmosphere are municipal wastewater treatment plants. The bacteria and pathogens released are potentially resistant to antibiotics, rendering the bioaerosols of wastewater treatment plants a possible hazard to human health. There is a need for further research aimed at explaining the magnitude of impacts of air microorganisms on human health.
1. Introduction. 2. Sources, transport and factors affecting the presence of microorganisms in the outdoor air. 3. Microorganisms in the air inside residential buildings. 4. Microorganisms in indoor air in offices and public spaces. 5. Microorganisms in the air of industrial facilities. 6. Bioaerosols within sewage treatment plants. 7. Air microorganisms as an important factor influencing human health. 8. Conclusions
Atmospheric air, including indoor air, is a basic factor affecting the proper functioning of the human body. Air pollution constitutes one of the main threats to the environments in which people live. World Health Organization (WHO) and European Environment Agency (EEA) report that environmental risks such as air or water pollution have a significant impact on human health [26, 108]. Air pollution includes all substances in the Earth’s atmosphere that are not natural components, as well as natural substances in significantly increased quantities. The Earth’s atmosphere is composed of gases and vapours of chemical compounds, acid rain, airborne ashes, dust, trace elements and biological contaminants .
Biological air pollutants, also known as bioaerosols, include pollen, fungi, bacteria and viruses . Most are microorganisms that colonise the soil, water bodies, plant surfaces, rocks and buildings . The components of the bioaerosol that make up the dispersed phase are particles ranging in size from 1 to 200 μm. For example, single bacterial cells with dimensions of 0.5–2.0 μm can constitute
Microorganisms are found in virtually every environment, including at extreme temperatures, pressures, salinity and acidity. The atmosphere has been described as one of the last biological limits on Earth. The composition and biodiversity of the microbial community in the atmosphere is still poorly researched . Interestingly, bacteria and fungi have been detected in various atmospheric layers, such as the boundary layer (up to 1.5 km high), the upper troposphere (up to 12 km high) and even the stratosphere at an altitude of over 20 km above sea level [35, 105]. In addition, the fungi
As we know, one of the main factors influencing human health is air quality. Therefore, the presence of microorganisms, in particular those causing infectious diseases, in both ambient air and indoor environments can be particularly dangerous. Scientists have demonstrated that most often, microorganisms living in the air are responsible for irregularities in the immune system, such as allergies and infections [23, 31, 41, 50]. WHO alerts that the exposure to mould and other dampness-related microbial agents increases the risks of hypersensitivity pneumonitis, allergic alveolitis or chronic rhinosinusitis which is supported by
Information on the presence of microorganisms in the air remains incomplete and numerous questions still need to be answered. The recent report of European Environment Agency (EEA)  describes sources and types of air pollution along with the potential health and ecosystem impacts, however it does not refer to biological agents in the air. Research on microorganisms in air have emerged only recently as documented by number of scientific publication. Figure 1 presents number of scientific papers in the Scopus database having ‘microorganism’ and ‘air’ as keywords. They are scattered within various research domains with environmental science, medicine, immunology and biochemistry subjects most often represented (Fig. 2).
The goal of this article was to provide recent information on presence of microorganisms in outdoor and indoor air, across residential, industrial and public spaces, with implications to human health.
Viruses, cells or fragments of bacterial cells, as well as fragments of mycelium and fungal spores and protozoa are found in atmospheric air [5, 34]. The composition of atmospheric air in microbiological terms has become the subject of considerable research. The main sources of microbiological air pollution include natural sources such as the soil, water, and plant surface contamination, including microorganisms transmitted by gusts of wind, dust in rain drops or insects. In turn, anthropogenic sources include various types of landfill, sewage treatment plants, composting plants and traffic. It should be mentioned that the microbiological composition of the atmosphere is significantly influenced not only by the source, but also by the volume of emissions, the distance from places of increased emissions, the type and survival of microorganisms, their spread and climatic conditions [11, 14, 31, 38, 90].
In addition, it is important to note that the main source of air pollution is human activity, including the transport. Humans also contribute to the production of biological aerosols through sneezing, coughing or physical exertion by introducing significant amounts of microorganisms into the atmosphere . Further potential sources of microorganisms include domestic activities such as cooking or vacuuming as well as domestic objects like carpets. They can release spores of fungi of the genera
Recently, biological air pollution inside buildings has attracted growing attention of researchers. According to the literature, biological matter scattered in the air, such as microorganisms, their products, fragments of plants and animals directly connected with fine solid particles or droplets of liquid can constitute from 5 to approximately 34% of all contaminants present in the internal air [71, 30]. Overall assessment of the concentration of bioaerosols is very difficult, depending on numerous indicators that have a direct or indirect impact on general airborne microorganisms [15, 61].
Scientific research conducted over the years has rendered to link exposure to bioaerosols with the occurrence of adverse health effects, including infectious diseases, acute toxic effects, allergies and cancers [7–9, 23, 32, 97, 106].
According to literature data, in addition to harmful effects on health, bioaerosols can play an important role in the development, evolution and dynamics of ecosystems and atmospheric processes. Airborne bacteria, fungal spores and other bio-particles play a key role in the reproduction and spread of organisms, such as the long-range transportation of pollen over long distances and across geographical barriers. Furthermore, the dispersion of plant, animal and human pathogens can have serious consequences for agriculture and public health. Bioaerosols are therefore very important for the spread of organisms, allowing the exchange of genetic material in different habitats and latitudes . Desert sandstorms have been shown to constitute an important source and the most efficient transport mechanism for bioaerosols, enabling the spread of microorganisms over distances greater than 5,000 km . Therefore, an increasing number of studies are seeking to characterise the temporal and spatial variability of bioaerosols and their interactions with human health and air pollution.
As mentioned above, high concentrations of airborne particulate pollutants are one of the main environmental factors that have a harmful impact on the health of the population. Hwang
As shown by Zhen
The movement of dust particles, microorganisms and other contaminants over long distances is caused by the movement of the air. Research on the quantity and species composition of airborne microorganisms is a key factor in explaining the ways in which they spread and interact [10, 54, 88]. Previous literature has confirmed that some bacteria (e.g. from the genus
Rajasekar and Balasubramanian  focused their research on the occurrence and structure of microbial populations in different places, depending on the prevailing conditions: temperature, relative humidity, population density, location (indoor/outdoor) and the presence of air conditioning. They showed that bacteria constituted 50.5% and fungi 49.5% of bioaerosol composition in the internal environment, while in the external environment they constituted 20.6% on average, while the proportion of fungi was 79.4%. Moreover, they noted that population density is a factor that significantly affects the concentration of bacteria in the air. It should be noted that in the case of indoor air samples, measurements were taken at two points in each location, i.e. the bedroom, kitchen, toilet or work area. At these points, air samples were taken from a height of about 1 m above floor level. However, in the case of outdoor air, samples were taken from outside the place of residence at a height of 1.5 m. The authors summarised that the type of ventilation and activities performed in different places in the household may affect the level of biological pollutants in the air. Furthermore, the results of this study suggest that the cycle of replacement of used air with fresh air from outside and the types of human activities carried out in the outside environment represent important factors. The ventilation system should be continuous and efficient, reflected in its impacts on levels of endotoxins in the air and airborne microorganisms.
The speed of multiplication, resistance to adverse environmental conditions and differentiation of preferences as to environmental conditions allow for the spread of these bacteria in the environment. Hara and Zhang  conducted a study on the presence of airborne Asian dustborne bacteria in southwest Japan. Aerosol particles were collected in the form of dust on the campus of the University of Kumamoto in both high and low concentrations. The results of the study indicated that the total concentrations of bacterial cells in the dust were higher in the dusty air. In addition, it was found that dead bacterial cells were superior to dead bacterial cells in dust, but the concentrations of living bacterial cells were comparable or higher than in non-dusty air. Bacterial cells transported with airborne dust can be exposed to many adverse factors, including changes in the weather such as temperature and humidity, atmospheric pressure, wind speed and direction as well as solar radiation intensity. It should be noted that the presence of living bacterial cells may be dispersed over long distances .
In residential buildings, biological factors in direct contact with humans may affect the human health. Due to the time spent in residential or office buildings (approximately 90 percent of time is spent by human in closed rooms) , indoor air quality has a greater impact on people’s well-being and health than does outdoor air quality. The construction techniques of the 21st century, such as the installation of air-tight windows or ventilation systems, are causes of increased biological pollution. In addition, these pollutants can come both from inside residential buildings and from outside, flowing directly through chimneys, open windows, cracks or other leaks .
Nabrdalik i Latała  undertook research aimed at the quantitative and qualitative characterisation of fungi present in building structures. They demonstrated that it is necessary to develop test methods and criteria to assess the degree of air pollution in residential buildings due to the presence of potentially harmful fungi in the analysed objects. Excessive humidity, poor ventilation and insufficient insulation are reasons for the excessive development of filamentous fungi .
Microorganisms in the air inside residential buildings are mainly found in dust, which has become an increasingly frequent subject of research aimed at assessing the quality of indoor air. Dust is primarily characterised by the presence of organic compounds, trace elements and metabolites of mites, responsible for many hypersensitivity reactions in the form of allergies . It has been shown that about 60% of dust microbiota are spores of mould fungi of the genera:
Staszowska  conducted research on dust samples from dwellings and educational institutions from a microbiological perspective. Samples were collected from various objects in Lublin (Poland). Dust samples were collected from places such as the floor (carpet) and from shelves with books from a height of 1 m. The most common bacteria were Gram-positive
Other authors have also demonstrated the presence of
In enclosed spaces with a fairly high density of people, the number of microorganisms is much higher than in open spaces. The air inside buildings is richer in various microorganisms, including pathogenic microorganisms. Humans constitute a source of indoor bacteria. Bacteria are emitted from the human respiratory airways through talking, coughing or sneezing. They also get into the air with skin scales. Therefore the level of air contamination with bacteria is dependent on the number of people inside a room and efficiency of ventilation system . Kubera
In the course of studies carried out in offices and laboratories, 20 different species of bacteria have been detected, belonging to genera such as
The results of biological research of air pollution in office buildings in Warsaw (Poland) equipped with or without air conditioning systems were presented by the team of the Central Institute for Labour Protection – National Research Institute . Two genus of fungi predominated:
Xu and Hao  reviewed the available literature on air quality in public metro spaces. Their data show that air quality in subways is particularly alarming. Airborne particles, aromatic hydrocarbons and airborne bacteria are classified as major air pollutants within metro systems. Despite the fact that metro passengers usually spend only a dozen or so minutes there, they are exposed to pollution emitted from various sources as well as directly from metro passengers. The harmfulness of the underground internal microenvironment is associated with the exposure time resulting from its frequent and even daily use .
Subway air quality was also the subject of research by Robertson
Tsai and Liu  conducted a study on workers’ exposure to bioaerosols at a pasta plant in central Taiwan. The biological risks associated with pasta production had not previously been studied. The aim of the study was to characterise the biological composition and concentrations of bioaerosols at different locations in the pasta factory over a period of one year, as well as to investigate the impact of various environmental factors on the concentration of bioaerosols. Air samples were collected twice daily. Nine species of fungi were identified in the air and strains from the genera
Czerwińska and Piotrowski  conducted a risk assessment of microbiological contamination in a bakery in Koszalin (Poland). Microbiological analyses of the air were carried out in the warehouse, production hall and the room where the bread was stored in order to cool it down. Their results demonstrated that air pollution in the bakery area was negligible, with the most frequently appearing microorganisms being bacteria of
In 2016, a paper on bacterial and fungal aerosol concentration in the air in the production halls of a meat plant was published . In the examined air the following indicators were determined: total number of bacteria and fungi, as well as
As previous research has shown, municipal sewage treatment plants are significant emitters of microorganisms to atmospheric air. The presence of microorganisms in wastewater entering the air together with the droplets of wastewater that are formed during mixing and aeration creates a certain pathogenic risk. Breza-Boruta
Of particular concern are pharmaceutical wastewater treatment plants, which have been identified as important reservoirs and sources of emissions to the natural environment of bacteria resistant to antibiotics . Moreover, antibiotic-resistant microorganisms migrate within and around wastewater treatment plants in the form of biological aerosols, mainly produced by aeration equipment, which is the main reason for the release of bioaerosols in wastewater treatment plants [42, 57, 91].
According to Zhang
The presence of released antibiotic-resistant bacteria and pathogens in sewage treatment plants renders the bioaerosols of such plants a potential health risk. Despite new reports, there remains little systematic research regarding the exact composition, possible health consequences and resistance to antibiotics of pathogenic bacteria found in bioaerosols from sewage treatment plants. Existing studies primarily focus on the antibiotic resistance of pathogenic Gram-negative bacteria [4, 21, 37, 58, 59].
Gram-negative bacteria usually constitute a small proportion of the bacteria present in bioaerosols. It should also be remembered that pathogenic bacteria living in bioaerosols function in conditions that are not always favourable to them, affecting their survival. The air represents a challenging environment for such microorganisms. Variable meteorological conditions, such as sunlight and humidity, can inhibit their growth and even kill them [2, 59].
Seasonal changes in the number of bacteria occurring in the air within sewage treatment plants are mainly observed in the summer, when higher emissions are recorded due in particular to lower precipitation. Szyłak-Szydłowski
At present, awareness is growing with regard to external and internal air pollution, as evidenced among others by recent studies on the influence of air quality on the prenatal and postnatal development of children [17, 19, 98, 101]. One example is a study by Shehab and Pope , which demonstrates the relationship between air quality and human cognitive performance. Air quality was assessed through annual average levels of nitrogen dioxide and indoor air by means of data on humidity and exposure to secondhand smoke. In terms of biological air quality, studies show that exposure to microorganisms and their derivatives (endotoxins, mycotoxins) in early childhood has a significant impact on the development of the immune system and the emergence of allergies, asthma and atopy [64, 77, 92]. Based on epidemiological data, it has been found that children living in rural areas on a farm are much less likely to develop allergies and asthma than are children raised in the same rural area but without exposure to farming environment .
Air is often considered an important carrier of bacterial pathogens such as
Numerous studies have shown that bioaerosols in the air may be potential causes of bodily dysfunctions. Airborne microorganisms, especially bacteria and fungi, can cause inter alia asthma, hay fever, bronchitis, chronic lung failure, lung cancer, cardiovascular diseases, gastrointestinal tract infections, tuberculosis, allergic reactions, sinus and conjunctivitis and acute viral infections [45, 62, 69]. As many as 80 types of fungi can cause respiratory allergies, and as many as 100 types of fungi can cause severe infections not only in humans and animals, but also in plants. These include fungi-representing genera such as
Aside from pathogenic microorganisms in the air, attention should be paid to their biologically active products, such as endotoxins (toxins occurring in the outer membrane of Gram-negative bacteria) or mycotoxins (secondary metabolic products of mould fungi, which can also have a significant impact on human health). Airborne biological agents are becoming important issues for human safety and health [24, 68].
Biological factors are one of the main causes of the so-called “sick building syndrome” (SBS). The term is used to describe a situation in which residents of a given building experience health problems and a range of symptoms such as e.g. skin rashes, headache, itchy eyes, throat irritation, dry skin, rashes, dry cough, hoarseness of voice, asthma attacks and other. It is assumed that biological pollutants such as viruses, bacteria, dust, pollen, toxic black mold, insect body parts contribute to the SBS along with chemical pollutants, ozone produced by printers, electromagnetic radiation, inadequate ventilation . A low level of fresh air in a room, a concentration of living organisms, poor ventilation, elements of building equipment and human physiology all affect the quality of life in buildings. In general, factors responsible for so called thermal comfort in buildings are also decisive for biological quality of indoor air. They include ambient temperature and its fluctuations, relative humidity, air movement, average radiant temperature and personal parameters, describing metabolic activity or clothing . The optimal humidity is defined in a range between 40–60%. In too dry conditions harmful substances enter human organism faster whereas in high humidity reproduction of microorganisms is accelerated. Ambient temperature most recommended for the thermal comfort is in the range 20–24°C, which is also conducive to good air quality . Mniszek i Rogiński  reviewed the literature on structural defects in buildings as the causes of fungal infections in rooms. They concluded that moulds attack not only old, unheated houses but also new or recently renovated buildings. This is due to numerous factors appearing even in the initial stages of construction (incorrect construction of walls, roofs with inadequately protected wood, lack of adequate ventilation, leakage of water system, etc.). Moisture, which results in the appearance of mould, is linked to the presence of mycotoxins produced by certain species of mould from the genera
Currently there are no binding legal instruments regulating the permissible content of microorganisms in Poland in both atmospheric and indoor air. Previous standards, such as PN-89/Z-04008/01; PN-89/Z-04008/08; PN-89/Z-04111/02; PN-89/Z-04111/03 have been waived and they have not been replaced by new instruments until now. However the waived standards are still occasionally used to interpret the results . For example, the standard PN-89/Z-04111/02 specified the strong air contamination with bacteria as state when their total content exceeded 3000 CFU m–3 or the content of
There are no safe level values for exposure to microbiological contamination on the air specified by WHO. The WHO guidelines  specify the conditions that contribute to the health risk, emphasizing that microbial growth may result in greater numbers of spores, cell fragments, allergens, mycotoxins, endotoxins, β-glucans and volatile organic compounds in indoor air. The guidelines were formulated by WHO, such as the following: persistent dampness and microbial growth on interior surfaces and in building structures should be avoided or minimized; indicators of improper conditions include the presence of condensation on surfaces, visible mould, mouldy odour and frequent water leakage or penetration. The document also states that no quantitative health-based guideline values or thresholds can be recommended for acceptable levels of microorganisms in indoor air .
The composition of atmospheric air is an extremely important factor affecting human health. This review of literature from Poland and elsewhere has shown that biological aspects of the air significantly reflect the state of the environment in which we live and function on a daily basis. Humidity is an important indicator of air quality, because in most cases it contributes to the development of microorganisms, both naturally occurring and those that operate under conditions of excessive humidity. This can in turn cause allergic reactions. The most frequently mentioned allergens in the air include fungi such as
This research was funded by the statutory program of the Institute of Soil Science and Plant Cultivation – State Research Institute (IUNG) – task 2.41: “Sources of exogenous organic matter in agriculture” and Task 1.4. “Evaluation and formation of biodiversity of soil and microbial activity of soil with regard to habitat conditions and management system,” Multi-Annual Programme IUNG – PIB (2016–2020).