Bioaerosol Sources, Sampling Methods, and Major Categories: A Comprehensive Overview

Abstract

Bioaerosols are aerosols with biological traces. Recently, based on analytical methods for obtaining biological information, it has been shown that bioaerosols play an important role in the climate change as well as the reproduction and spread of organisms across various ecosystems, they also have biological effects in organisms, including humans. In particular, the nature of bioaerosols has been determined by next-generation sequencing and scanning electron microscopy. Here, we provide a comprehensive overview of the sources of bioaerosols, the sampling methods and the technologies used for their investigation. Furthermore, this review discusses the preexisting knowledge about the major categories of biological aerosol particles. This review will provide information on bioaerosols for the beneficial application of this information in future works.

1. Introduction

The Earth’s atmosphere is essential for almost every organism, having a significant impact, either directly or indirectly. It comprises of varying concentrations of gases, including nitrogen, oxygen, inert gases such as argon, and traces amounts of gases such as carbon dioxide (Salby, 1996; Jacobson, 2005; Vimal, 2017). Aerosols are tiny particles, including liquids and solids, suspended in the atmosphere and typically ranging in size between 10^-4 and 100 µm (Table 1) (Hinds, 1999; Lohmann, 2015). These microscopic particles consist of various types, including soil particles, salt from ocean sprays, and smoke from power generation, and/or human activities (Hinds, 1999). Clouds are the most evident example of aerosols in the atmosphere (Pöschl, 2005). Particulate matter (PM) including PM10 and PM2.5 are globally known as harmful air pollutants and are included in aerosols. Bioaerosols are aerosols of a biological origin (Msa et al., 2005), including viruses, pathogenic/non-pathogenic bacteria, fungi, algae, and organic compounds derived from microorganisms (endotoxins, metabolites, mycotoxins, peptidoglycans, and β (1→3)−glucans), others include pollen, plant debris, and some insect excretions (Douwes et al., 2003; Heikkienen et al., 2005). Table 1 lists the estimated size range, concentration, and global emissions of atmospheric biological particles (Coz et al., 2010; Després et al., 2012). Penner (1994) showed that global fluxes in plant emissions and submicron microorganisms are up to 50 Tg·yr^-1, whereas the global emission rate of fungal spores in the 1–10 μm size range was shown to be up to 50 Tg·yr^-1 (Elbert et al., 2007; Pachauri and Reisinger, 2008).

Aerobiology encompasses the study of biological particles present in the air, in both outdoors and indoor environments (Lacey and West, 2006), and their effects in terms of airborne human and plant pathogens (Stetzenbach, 2007). Studies on biological particles have primarily focused on their chemical composition and have recently focused on biological composition. The latter demonstrates that bioaerosols can adversely affect humans, animals, and plants (Wittmaack et al., 2005). A study by Huffman et al. (2013) detected bacteria and fungi in the air including groups containing human and plant pathogens (mildew, smut, rust, fungi, molds, and bacteria belong to Enterobacteriaceae, and Pseudomonadaceae). Moreover, aerosol particles have both a direct and indirect effect on climate change, as they directly interact with sunlight and terrestrial radiation through scattering and absorption and indirectly through their impact on cloud properties and the radiative properties of surfaces covered by snow and ice. Thus, they have an influence on the hydrological cycle and climate (Boucher et al., 2013; Fröhlich-Nowoisky, 2016; Haywood, 2016). Perturbations to atmospheric distributions of aerosols are known as perturbations of the Earth’s radiative budget, which can enhance or counteract the radiative disruption caused by changes in the concentrations of greenhouse gases such as carbon dioxide and methane (Bellouin, 2015).

This review provides an overview of bioaerosols from the following viewpoints: (i) understanding the origin of bioaerosols; (ii) providing an overview of the primary bioaerosol collection and detection methods based on nonmolecular and molecular approaches; (iii) providing an overview of the state of bioaerosol research based on their composition; and (iv) highlighting the applications and outlook of bioaerosol studies.

Table 1: Size-range, concentration, and global emissions of atmospheric biological particles. Adapted from Coz et al. (2010) and Després et al. (2012)

Content	Size range (μm)	Concentration (m-3)	Global emissions (Tg yr-1)
Virus	0.02–0.3	-	-
Bacteria	0.3–10	0.5–1,000	0.4–28
Fungal spore	0.5–30	0–10,000	8–190
Pollen	10–100	1–1,000	47–84

2. Sources of aerosol and bioaerosol

Figure 1 shows an overview of bioaerosol cycling and its effects on the Earth’s systems. Atmospheric aerosol particles originate from both natural and anthropogenic sources.

2.1 Natural sources

Fine soil and sand particles are the major sources of aerosols that contribute by wind worldwide, especially in the subtropical and tropical regions (Vimal, 2017). Asian dust events are a well-known phenomenon of soil-derived dust carried over long distances that frequently occur over wide areas of East Asia and have the potential to reach America and even the Arctic regions. (Tang et al., 2018). Bioaerosol present in raindrop can be generated by the latter falling onto the soil; Joung et al. (2017) demonstrated that bacteria can be transported to different locations in the environment through precipitation. Approximately 70% of the Earth is covered by oceans, where sea-salt and sulfate aerosols are the two dominant inorganic aerosol components that are present in the marine atmosphere (Xu et al., 2013; Xu and Gao, 2015). Sea-spray aerosols also help to transmit marine microorganisms to the atmosphere (Michaud et al., 2018). Biogenic aerosols are biological materials emitted as aerosols, for example, cells/spores from microorganisms, pollen, and plant fragments. The oxidative production by vegetative emissions, including biogenic volatile organic compounds such as terpenes (Cn5Hn8) (e.g., isoprene, monoterpenes, and sesquiterpenes), act as an effective link between the Earth’s surface, the atmosphere, and the climate (Laothawornkitkul et al., 2009; Acosta Navarro et al., 2014).

2.2 Anthropogenic sources

Anthropogenic sources arise from man-made pollution released into the atmosphere (Popescu and Ionel, 2010). Increased industrialization has resulted in an increased exposure to a diverse range of aerosols/bioaerosols, such as sulfate and nitrate emissions, pollutants from industries, livestock, food processing, dust mobilized in areas with agricultural activity, fossil fuel combustion, waste sorting, and composting (Ghosh et al., 2015; Tomasi and Lupi, 2016; Humbal et al., 2018; Viegas et al., 2020). Bioaerosols in indoor environments are strongly related to human activities, such as sneezing, coughing, washing, cleaning, walking, talking (Chen and Hildemann, 2009; Mandal and Brandl, 2011) and microorganisms that dwell on human skin, feces, and hair (Jiayu et al., 2019).

Figure 1: Bioaerosol cycling in the Earth’s systems. After emission, bioaerosol particles interact with other atmospheric particles and various atmospheric gases and could also be involved in cloud formation and precipitation. After dry/wet deposition on the Earth's surface, viable bioparticles could enable biological reproduction and further emission again. Adapted from Pöschl (2005) and Fröhlich-Nowoisky et al. (2016)

3. Bioaerosol collection and analysis

3.1 Bioaerosol collection

Existing bioaerosol research principally focuses on monitoring and aims to assess the impact on human health and the environment including analyzing microbial community composition (Jeon et al., 2011), abundance, and viability (Hara and Zhang, 2012). A wide range of methods have been applied for bioaerosol sampling (Muilenberg, 2003; Reponen et al., 2011). In practice, the sampling efficiency depends on the physical characteristics of the atmospheric particles, such as size and density, as well as the environmental condition such as wind and weather (Grinshpun et al., 1994). Figure 2 shows the key experimental outline for bioaerosol studies using nonmolecular and molecular approaches. Sample collection methods, which are most frequently used in research based on bioaerosols, include impaction, impingement, and filtration (Hurst et al., 2007). However, alternative methods, including the gravity technique, electrostatics, thermal precipitation, and cyclones have been utilized in various studies (Després et al., 2012; Ghosh et al., 2015). Recently, Ferguson et al. (2019) reported that air filtration with polycarbonate filters gave the highest DNA recovery. Moreover, Parker et al. (2019) established a novel method for collecting milligram amounts of indoor PM using an electrostatic precipitator.

Figure 2: The key experimental outline for bioaerosol studies

3.2 Component analysis of bioaerosols

Bioaerosol analysis has been divided into two primary methods: culture-dependent (traditional) and culture-independent (molecular). These two methods will be explained in the following sections.

3.2.1 Culture-dependent methods

Traditional methods in aerobiological studies have focused on collecting bacteria cells and analyzing samples by total count and culture-based techniques (Yoo et al., 2017). The plate count method is a conventional method for analyzing airborne microbes using both liquid and solid media, depending on the aim of the study. Colony forming units on agar media are counted as viable bacteria or fungal cells after incubation (Buttner, 1997; Mandal and Brandl, 2011; Kallawicha et al., 2019). Note that colonies growing on the plates might also be counted. However, cultivation methods have some limitations in environmental research, for example, only a small number of species found in nature could be grown in a laboratory environment (Fröhlich-Nowoisky et al., 2016). The most probable number counting assay (MPN) has been used for quantifying microorganisms present in liquid media post-cultivation, and the results depend mostly on statistical calculations (Makkar and Casida, 1987). However, this serial dilution method has limited suitability, for example, cell aggregation could affect the bioaerosol analysis (Ghosh et al., 2015). Identifying microorganisms by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDITOF-MS) is a technical revolution, which is both culture-dependent and independent and has been applied previously for the identification of unidentified colonies in clinical microbiology laboratories (Seng et al., 2010; Cox et al., 2020). This method was later applied in bioaerosol studies (Liang et al., 2013; Madsen et al., 2015; Azhar et al., 2017; Duquenne, 2018; Garcia-Alcega et al., 2020) and has the potential to replace many genetic identification methods due to its low cost, excellent performance, and suitable results. Light microscopy and fluorescence microscopy, along with scanning and transmission electron microscopy (SEM and TEM, respectively) are mostly used for the morphological characterization of atmospheric aerosol particles and bioaerosols (Wittmaack et al., 2005; Kutchko and Kim, 2006; Campos-Ramos et al., 2009; Kang et al., 2012; Valsan et al., 2016; Li et al., 2019b).

3.2.2. Culture-independent methods

Lately, culture-independent-based analyses have been applied to various areas of airborne microbiology research (Fahlgren et al., 2010; Jeon et al., 2011; Adams et al., 2013; de Carvalho Ferreira et al., 2013; Buters et al., 2015; Kraaijeveld et al., 2015; Woo et al., 2018). Bioaerosols have been determined using DNA and RNA base methods and the methods most widely used in bioaerosol studies are summarized as follows. Polymerase chain reaction (PCR) is commonly used for detecting microorganisms in environmental samples. The DNA region of a genome is copied and amplified by a million-fold in less than an hour and is suitable for further analysis. The most common gene used for community analysis is the gene of the 16S rRNA subunit of ribosomes (coded by16S rDNA) of bacteria and archaea, and its analog, the gene of the 18S rRNA subunit of ribosomes (coded by 18S rDNA), in eukaryotic microorganisms, animals, and plants (Georgakopoulos et al., 2009). Real-time PCR has been used previously to measure the total microbial concentration in environmental samples (Ghosh et al., 2015) by comparing them with the standard curve of known microbial concentrations (An et al., 2006). Some studies also used quantitative PCR (qPCR) to evaluate the diversity and taxon-specifics of fungi in environmental aerosol samples (Yamamoto et al., 2014). Denaturing gradient gel electrophoresis (DGGE) is a culture-independent technique used to analyze the genetic diversity of complex microbial populations; it is based on separating PCR-amplified fragments of different sequences while maintaining the same fragment length (Muyzer et al., 1993). DGGE has been used to analyze airborne communities in many studies (Nehme et al., 2008; Kim et al., 2011; Xu and Yao, 2013; Escalante et al., 2014; Tanaka et al., 2015). PCR is used to amplify the DNA region of interest prior to the traditional Sanger sequencing. Many bioaerosol studies use Sanger sequencing to identify an organism in terms of its individual genera/species by comparing its DNA with sequences available on online databases, such as the National Center for Biotechnology Information (NCBI) (Després et al., 2007; Fang et al., 2019; Fröhlich-Nowoisky et al., 2014; Gonzalez-Martin et al., 2018). Sanger sequencing-based bioaerosol analysis is being gradually replaced by the modern next-generation sequencing (NGS) (Fröhlich-Nowoisky et al., 2016) due to its high efficiency and lower cost. NGS is a highly sensitive technology that is faster and more cost-effective in sequencing DNA and RNA than the traditional Sanger sequencing (Ghosh et al., 2015), which established genome reference sequences for various model organisms and humans (Mardis, 2013). Over the last decade, NGS has been used to characterize microbial communities in various environmental samples (Sogin et al., 2006; Deagle et al., 2009; Zhang et al., 2009; Hajibabaei et al., 2011; Shokralla et al., 2012). NGS platforms have also been used in atmospheric biological studies (Yamamoto et al., 2014; Madsen et al., 2015; Yan et al., 2016; Tang et al., 2018). As shown in Table 2, many studies used "Illumina Miseq" to analyze the bioaerosol samples.

Metagenomics is an effective method for studying entire microbial species in an environment without necessarily obtaining pure cultures; it involves various stages, such as the extraction of DNA from an environmental sample, so that each genome of organisms in the environment is pooled. Thereafter, these genomes are usually fragmented and cloned into an organism that can be cultured to create “metagenomic libraries”, which are then analyzed based on DNA sequences (Sabree et al., 2009). The commonly used methods for high-throughput data include amplicon-based methods and whole metagenomic shotgun sequencing, which are frequently used for pathogen identification (Ghosh et al., 2019). Many metagenomic studies of airborne microbes have been published recently (Whon et al., 2012; Nicholas et al., 2015; Núñez et al., 2016; Cha et al., 2017; Hu et al., 2018; Yang et al., 2018). Recent bioaerosol studies have applied various methods with different scopes and abilities; as described above. Various factors affect the choice of air sampling and detection approach, for example, target organisms, type of air sampler, environmental factors (such as volume of air sampled or air-flow rate), number and types of replicates, DNA extraction protocol, genomic targets to be amplified, and sequencing technology. The bioinformatics pipeline should also be chosen carefully and must be specifically adapted to meet project requirements (Mbareche et al., 2017). The following tools have a free and open NGS bioinformatic workflow for bioaerosol sequence analysis such as Mothur (Schloss, 2020), QIIME2 (Bolyen et al., 2019), USEARCH (Edgar, 2010), and Claident (Tanabe, 2018).

4. Bioaerosol components

The presence of bioaerosols has been suspected to significantly impact human health as a biological pollutant (Douwes et al., 2003; Kim et al., 2018; Yao, 2018). Bioaerosols are not self-contained pollutants, but a mixture of various compounds, for example, bacteria, fungi, viruses, and pollen (Jiayu et al., 2019). The next section considers the predominant with bioaerosol origins and components, listing these aspects for bioaerosols located worldwide, such as in Asia, the USA, and Europe (Table 2).

Bacteria reside in the atmosphere for a relatively long amount of time (over several days) and can be transported over long distances (up to thousands of kilometers) because of their smaller size (Després et al., 2012). The mean concentrations of airborne bacteria were found to lie between 10⁴ and 10⁷ cells·m^-3 over most continental regions (Xia et al., 2013; Spracklen and Heald, 2014; Liu et al., 2018), 10⁴ and 10⁵ cells·m^-3 over coastal regions (Gong et al., 2020), and 10⁶ and 10⁷ cells·m^-3 in dust (Hara and Zhang, 2012). The most abundant bacterial genera found in the indoor atmosphere include Bacillus, Micrococcus, Kocuria, and Staphylococcus (Mandal and Brandl, 2011). Recently, research on bacteria in air has been attracting attention. In Japan, over Tokyo city, bioaerosols were collected by eight sets of 2-m-long conductive silicon tubes (filtration), and genomic DNA in bioaerosols was extracted using the FastDNA™ SPIN Kit for Soil (MP Biomedicals, Santa Ana, CA), following partial 16S rRNA gene sequences. The V3 and V4 regions were amplified using the primers Bakt_341F and Bakt_805R, and PCR amplicons were performed following Illumina methods for 16S metagenomic sequencing. Proteobacteria (51.4%) was the most abundant bacterial phylum, followed by Firmicutes (13.6%), Cyanobacteria (7.9%), Actinobacteria (7.7%), Bacteroidetes (5.2%), and Acidobacteria (3.0%) (Uetake et al., 2019). Similar results have been reported for bacterial communities in Xinxiang, Central China, where air samples were collected here using a high-volume air sampler (TE-6070VFC; Tisch Environmental, Inc., USA). Total genomic DNA was extracted using the cetyltrimethylammonium ammonium bromide (CTAB) method. For sequencing analyses, the V4 region of the 16s rRNA gene was amplified using 515F and 806R primers through PCR, before the high-throughput sequencing of 16s rRNA was conducted on the Ion S5™ XL platform. The bacterial communities were found to be dominated by Proteobacteria (35.5%), Firmicutes (23.0%), and Actinobacteria (16.2%) (Li et al., 2019a). After dust invasions in Shanghai, Cyanobacteria was found to have increased significantly in dust samples and is known to promote hepatotoxicity and tumors in humans (Rao et al., 2020). This finding could be used to monitor pathogens in the air in the future.

Fungi represent a unique group of organisms found everywhere, in outdoor and indoor environments. The most common organisms frequently found in the atmosphere include spores and fragments (Bauer et al., 2008; Crawford et al., 2009). High fungal emissions into the atmosphere (Després et al., 2012) (Table 1), easy dispersal of spores by wind, and environmental stress resistance help fungi survive during atmospheric transport (Madelin, 1994; Griffin, 2004; Griffin and Kellogg, 2004). The mean concentrations of fungal spores were found to be less than 2.5 × 10⁴ m⁻³ over continental midlatitudes (Spracklen and Heald, 2014). Ascomycota (sac fungi) and Basidiomycota (club fungi) were the most abundant phyla of fungi detected in PM by DNA analysis in Mainz, Germany (Fröhlich-Nowoisky et al., 2009). In recent observations carried out in urban Syracuse, New York, USA., air samples were collected using Andersen N6-type single-stage impactors, which collect the sample onto a culture media. Then, the cultures were transferred to slides for the microscopic identification of genera and species. The predominant fungi were Cladosporium, Penicillium, Aspergillus, and Alternaria (Crawford et al., 2015). However, a study by Rocchi et al. (2017) detected Penicillium, Aspergillus, and Cladosporium in the Franche-Comté region of eastern France by using an electrostatic dust collector to perform DNA extraction from the washing liquid using mechanical and thermal lysis (Scherer et al., 2014). Amplification of the internal transcribed spacer 2 (ITS2) region was performed and the amplicon were sequenced on an Illumina MiSeq. In Asia, metabarcoding studies on airborne fungal communities in urban areas were conducted by Fang et al. (2019). The air samples were collected using a FA-1 sampler (imitation Andersen sampler). After incubation and counting, the fungal colonies growing on each dish were identified microscopically to the genus level, each pure isolate was homogenized in a liquid culture medium, and the DNA was extracted using the CTAB method. The ITS region of the fungal rRNA genes was amplified using the following universal primer set: ITS1 and ITS4. This study found that Penicillium, Cladosporium, Aspergillus, Alternaria, and Trichoderma were the most predominant fungal genera in the Hangzhou area of southeastern China. These findings may be influenced by regional differences, measurement methods, meteorology, and the biogeographic distribution of fungi in the atmosphere. Most fungal species reported here belong to Ascomycota and Basidiomycota.

Viruses are among the smallest bioaerosols, with a diameter up to 20 nm (Duan, 2006). Although they may be more easily suspended in the atmosphere due to their smaller size and potential attachment to finer sized PM (Gonzalez-Martin et al., 2018), they are also quite sensitive to adverse conditions during transport, e.g., UV radiation (Tseng and Li, 2005; Walker and Ko, 2007) and ozone (Tseng and Li, 2006), which could inactivate the viral particles. It has been reported that virus-containing aerosols are formed in sprays from water surfaces (Aller et al., 2005). However, Michaud et al. (2018) reported that viruses do not transfer to sea-spray aerosols as efficiently as bacteria, and the enrichment of lipid-enveloped viruses suggests that hydrophobic properties increase viral transport to the sea surface and sea-spray aerosols, as shown in the aerosols which were collected using a SpinCon^®PAS 450-10A Portable Air Sampler (Scepter Industries) (Michaud et al., 2018). A large number of viruses have been detected in atmospheric samples, as shown in Table2 (Hietala et al., 2005; Blachere et al., 2009; Corzo et al., 2013; Gonzalez-Martin et al., 2018; Scoizec et al., 2018; Wei et al., 2018). Numerous diseases present in humans, animals, and plants are caused by viruses associated with aerosols. The influenza virus A strain H5N1 in Asia that was previously thought to infect only wild birds and poultry, has now infects humans, cats, pigs, and other mammals (Lewis, 2006), and can be transmitted by direct/indirect contact and by droplet/droplet nuclei (i.e., airborne) transmission (Bridges et al., 2006). Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a major pathogen that primarily targeting the human respiratory system. Several outbreaks of these types of viruses have occurred over the past decades. Namely, the severe acute respiratory syndrome coronavirus (SARS-CoV) in 2003, the Middle East respiratory syndrome coronavirus (MERS-CoV) in 2012 (Rothan and Byrareddy, 2020). In late December the emergence of the coronavirus disease 2019 (COVID-19) caused by the novel SARS-CoV-2 which represents a global public health concern, that caused the WHO to declare a public health emergency in China (Rothan and Byrareddy, 2020; Lauer et al., 2020). These examples show that air is an important intermediary for the spread of infection.

Pollen has a larger diameter size range than other bioaerosol groups and represents reproductive units of plants transported via wind. Pollen grains vary in size from 10 µm to 100 µm (Table 1) (Després et al., 2012). Previous studies have reported pollen concentrations up to 10³ m^-3 in the atmosphere over vegetated regions (Fröhlich-Nowoisky et al., 2009; Sofiev et al., 2006). Aeropalynology is a branch of aerobiology that deals with the study of pollen grains and plant spores in both indoor and outdoor environments (Ukhanova and Bogomolova, 2015). Most studies have focused on pollen biodiversity and human exposure, mainly related to allergies and health problems (Bogawski et al., 2016; Korpelainen and Pietiläinen, 2017; Brennan et al., 2018; Brennan et al., 2019; Banchi et al., 2020). Pollen sampling collection follows typically the same filtration method used for bacteria and fungi collection, using an Andersen sampler or volumetric (Hirst type) (Banchi et al., 2020). DNA extraction methods mainly entail proprietary kits (usually PowerSoil DNA isolation Kit, Qiagen, formerly MoBio) (Banchi et al., 2020). The detection and analysis of pollen commonly uses microscopy, such as transmission electron microscopy (TEM) (Taylor et al., 2004), as well as DNA barcoding such as rbcL, matK, trnL, trnH-psbA, and nuclear ribosomal DNA (rDNA) ITS2) (Bell et al. 2016; Banchi et al., 2020). Common airborne pollen allergies associated with both asthma and allergic rhinitis have showed large worldwide variations in the prevalence of symptoms (Stipić-Marković et al., 2003; Asher et al., 2006; Trakultivakorn et al., 2007; Asher et al., 2010; Asher et al., 2012). The most important outdoor aeroallergens are caused by pollen from the grass family (Poaceae) (Bauchau and Durham, 2004; Bousquet et al., 2007), and artemisia pollen grains are important aeroallergens worldwide (Grewling et al., 2020; Myszkowska, 2020).

In addition to the bioaerosols described above, other varieties of bioaerosol particles exist (biological) which can usually be detected when the air sample is collected by an impactor and are characterized by electron microscopy (Wittmaack, 2005).

Target	Dominant	Enumeration technique	Country	References
Bacteria	Acinetobacter, Alcaligenes, Edwarsiella, Pseudomonas, Burklolderia, Moraxella, Sphingomonas and CDC NO-1. CDC NO-1	DGGE and Sanger sequencing	Chile	(Escalante et al., 2014)
Bacteria	Actinomycetes and Firmicutes	Sanger sequencing	China	( Yuan et al., 2017)
Bacteria	Proteobacteria, Firmicutes, Cyanobacteria, Actinobacteria, Bacteroidetes, and Acidobacteria	Illumina Miseq	Japan	(Uetake et al., 2019)
Bacteria	Rubellimicrobium, Paracoccus, Deinococcus, Chroococcidiopsis, Clostridium and Deinococcus	Illumina Miseq	China	(Rao et al., 2020)
Bacteria and Fungi	[Proteobacteria, Actinobacteria, Firmicutes andBacteroidetes] and [Basidiomycota (club fungi) and Ascomycota (sac fungi)]	Sanger sequencing	USA	(Huffman et al., 2013)
Bacteria and Viruses	[Actinobacteria, Gammaproteobacteria, Flavobacteriia and Alphaproteobacteria)]and [Caudovirales]	HiSeq 4000 Illumina	USA	(Michaud et al., 2018)
Fungi	Cladosporium, Penicillium, Aspergillus, and Alternaria	Cultivation	USA	(Crawford et al., 2015)
Fungi	Cladosporium, Alternaria, Fusarium, Penicillium, Sporisorium, and Aspergilus.	Illumina Miseq	China	(Yan et al., 2016)
Fungi	Dothideomycetes, Leotiomycetes, Sordariomycetes, Eurotiomycetes, Agaricomycetes, Exobasidiomycetes, Microbotryomycetes and Tremellomycetes	454 pyrosequencing	Netherland	(Nicolaisen et al., 2017)
Fungi	Penicillium, Aspergillus and Cladosporium	Illumina Miseq	France	(Rocchi et al., 2017)
Fungi	Penicillium, Cladosporium, Alternaria, Aspergillus, and Trichoderma	Sanger sequencing	China	(Fang et al., 2019)
Pollen	Poaceae, Artemisia spp., Betula spp., and Pinus	Microscopy	Finland	(Hugg et al., 2007)
Pollen	Gramineae and Ambrosia artemisiifolia	Microscopy	France	(Leru et al., 2018)
Pollen	Alopecurus, Festuca, Lolium, Holcus and Poa	Illumina Miseq	UK	(Brennan et al., 2019)
Pollen	Salicaceae, Cupressaceae, Ulmaceae, Moraceae, Ambrosia artemisiifolia, Artemisia vulgaris and Gramineae	Microscopy	Romania	(Leru et al., 2019)
Virus	Enteroviruses and Rotaviruses	Sanger sequencing	Spain	(Gonzalez-Martin et al., 2018)

Plant fragments are one of the largest mass fractions in the atmosphere (Després et al., 2012). Vegetative debris can be defined as piece of plant left over after destruction (Boucher, 2015) that consists of humic-like substances (Graber and Rudich, 2006) and cellulose (which is a major component of plant-tissue and pollen), and could be produced by some bacteria (Puxbaum and Tenze-Kunit, 2003; Winiwarter et al., 2009; Pöhlker et al., 2012).

Brochosomes are particles produced intracellularly in specialized glandular segments of the Malpighian tubules of leafhoppers (Cicadelliae) (Rakitov, 2002; Wittmaack, 2005), and they serve as an efficient water-repellent protective surface and liquid exudate (Wittmaack, 2005; Rakitov, 2009; Rakitov and Gorb, 2012; Després et al., 2012). They are usually found in the atmosphere in the form of large clusters containing up to 10,000 individuals or more (Wittmaack, 2005; Després et al., 2012). Minor biological traces have been found that include fur fibers, dandruff, and skin fragments from animals (Després et al., 2012). Some insect fragments, for example, insect scales, have also been observed in several previous studies (Wittmaack et al., 2005; Coz et al., 2010; Chow et al., 2015; Yao et al., 2019).

5. Application of bioaerosol studies

Aerobiology has a wide range of applications, for example, in analyzing clinical and environmental samples, and in food safety and industrial waste regulation. Bioaerosol exposure has been linked with to a wide range of deleterious health effects in humans, animals, and plants. Detecting microorganisms and organic/inorganic components in bioaerosols is essential for identifying the toxicological properties of bioaerosols and their possible impacts on human health (Humbal et al., 2018). Some previous studies have focused on understanding the damaging oxidative reactions (oxidative potential). A study by Samake et al. (2017) showed that airborne fungi and bacteria induce “reactive oxygen species” production and can increase the oxidative potential on the interaction with some chemical compounds in PM. This demonstrates that the presence of airborne microorganisms in PM can produce damaging oxidative reactions, which could affect human health much more than exposure to individual PM. Bioaerosol monitoring could be applied in industrial hygiene and bioaerosol control is critical during food processing. It is necessary to evaluate their presence quantitatively, by a count or determination method, and/or qualitatively, by identifying the genus and species to ensure high product quality (Goyer et al., 2001; Theisinger and Smidt, 2017).

6. Conclusion

The presence of microorganisms in the atmosphere has a long history of investigation. In the last decade, there has been an increased interest in bioaerosols based on their biodiversity, the development of new measurement techniques, and the challenge of molecular analysis, which could play an important role in climate, ecosystem, and human health studies. The methods used depends on the scope or requirement of the biological particle of interest, thus, this review presents an overview of the state of current methods in bioaerosol research, highlighting recent progress worldwide. The application of aerobiology in other areas of research could be addressed in future studies which would help to improve the understanding of their impact on humans.

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