Pathogenicity, Characterisation and Impact of Ambient Bio-Aerosols on the Climatic Processes: With a Special Emphasis on the Indian Subcontinent

Abstract

Airborne particulate matter contains biological entities from various anthropogenic/biogenic activities. Within 1 nm–100 μm size, these are carried to long distances through various external agents. Identified as potential pathogens, they bring forth substantial economic losses in many parts of the world. Despite these shortcomings, bio-aerosols play a vital role in cloud condensation, ice nucleation, precipitation and various atmospheric processes affecting the hydrological cycle in general. Furthermore, bio-aerosols play a decisive role in the dispersal of reproductive plant parts and fungal spores, which play important roles in the evolution and sustenance of ecosystems. However, there remains substantial knowledge on air micro-biome with respect to their occurrence, transformation, role in climate change, interaction and impact on living organisms, agriculture and ecosystem. The current COVID-19 pandemic is a wakeup call for retrospective analysis of airborne particles to reduce their emission, transmission and health risk hazards while understanding their impact on various atmospheric processes. This chapter identifies the various types of bio-aerosols and systematically includes their prime role in the climatic processes, pathogenicity to the exposed flora and fauna along with an exclusive interrogation into their types and characterisation over the Indian subcontinent with a hugely diverging population and pollution panorama.

1. Introduction

Recently, the changes in lifestyle have tremendously changed the surrounding, as the freshwater is wastewater, soil is contaminated with xenobiotic as well as quality of air is deteriorating day by day. The air we breathe not only contains oxygen, nitrogen and other trace gases but also contains fine and ultra-fine particulate matter (PM), which has aerosolised organic and inorganic components in association with live and dead cells of organism that may be microbes, plant parts or skin shedding of animal and human, termed as bio-aerosols. These are released from biosphere to the atmosphere and become a significant component to the already existing myriad air pollutants. Their size varies from 1 nm to 100 μm, whereas they are a diverse group of biological materials like living and dead organisms (bacteria, fungus, archaea, protozoa, virus and their by-products), dispersal units (fungal spore and pollinium) and fragments and defecations (plant debris/leaf litter and brochosomes) [1].

1.1 Sources of bio-aerosols

In ambient air, bio-aerosols can exist as particular entities or in aggregates with organic matter, particulate matter, water droplets and chemical constituents of aerosols, which provide an amiable condition for maintaining the metabolism of these organic components that can reproduce even under stressful conditions [2, 3, 4]. Hence, for this reason, it is crucial to know the sources of bio-aerosols, their metamorphosis during the course of movement to atmosphere and recycling to biosphere, their impact on public health, agriculture as well as ecosystem including their role in global climate change. The initial step towards becoming airborne is to be aerosolized first from contaminated sites [5]. Bio-aerosols are ubiquitous in nature and present in troposphere at an altitude of 10–20 km and even 20–40 km above sea level within the stratosphere [6]. Major sources of airborne microorganisms are municipal dumping areas, waste streams and discharge points, shabby water-soaked buildings, soil, degenerated and fermented plant and animal parts, sewage sludge dumping sites, animal husbandry, fermentation process, agriculturally active areas, animal houses, breeding farms, feedstuff-factory outlets and various other anthropogenic activities [7, 8, 9]. Some bio-aerosols are released to the environment by naturally active processes such as through fungal spores, which are emitted through osmotic stress or floor pressure impact whereas some via passive process, equivalent to thallus debris and dried fungal spores mainly due to wind [1, 10].

2. Importance of bio-aerosol and the existing knowledge gap

Bio-aerosols play a significant part in cloud condensation, ice nucleation and precipitation influencing hydrological cycle in addition to local weather patterns. Pratt et al. [11] reported that the ice-crystal residues in cloud condensation nuclei and ice nuclei were of organic origin, which accounted for about 33%. Moreover, bio-aerosols play a vital role in the dispersion of reproductive plant parts and fungal spores. Owing to their small dimension, wind movements transport these through any geo-graphical obstacles and are, therefore, the important aspects of genetic alterations between habitats and geographic changes of biomes. Hence, these are key components within the development, evolution and dynamics of ecosystems. However, a majority of bio-aerosols have a negative impact on agriculture, animals and public health. More importantly, some of these hold the potential to penetrate the deeper components of the respiratory system being of respirable sizes, distinctively 0.003 μm for viruses [12], 0.25–20 μm for bacteria [13], 17–58 μm for plant pollen [14] and 1–30 μm for fungi [15]. In addition, bio-aerosols of 1–5 μm size typically stay in the air, whereas bigger particles get deposited on surfaces sooner [16, 17]. Wet or dry deposition on the earth surface facilitates the removal of bio-aerosols from the atmosphere. Wet deposition courses are the main sink of atmospheric aerosols, while dry deposition has a much smaller global meaningful impact on native air quality and public well-being.

Recently, Haddrell and Thomas [18] have identified that it is time to merge the present and novel methods of multidisciplinary research in atmospheric chemistry, aerobiology and molecular biology to grasp the long-sought mechanisms of bio-aerosol transport and disintegration as this discipline has huge lack of expertise and enough hypothesis. Still, there is very less information on air micro-biome in comparison with their counterparts in water and soil. In addition, the lack of standard methods, environmental guidelines and databases makes it difficult to interpret and compare results. In a widely diversified country like India, there has been a significant lack of knowledge of bio-aerosols with most of the related work done in metropolitan cities, mainly Delhi (mega city), Mumbai, Chennai, Kolkata, Pune and few in central and eastern parts of India [2, 19–42]. Despite being a very important area, no field campaign has been reported yet. The methodology followed for the sampling of bio-aerosols and their identification and quantification has to be improvised over the Indian continent, as there are immense gap areas in comparison with other countries. In addition, an in-depth study is required to know more about the biological entities of the atmosphere to reduce their emission to atmosphere as well as to reduce the health risk hazards. The recent pandemic has been a wakeup call. Rise in epidemics is the most important threat to humankind across the globe since the inception of the civilisation [43]. In late 2019, a novel coronavirus illness, later named as COVID-19 outbreak, was initially announced in Wuhan, China. With the increase in the number of confirmed cases throughout the world, the World Health Organisation (WHO) soon declared it as a global pandemic. Since then, researchers have highlighted different aspects of this situation specifically on the detection, transmission, pathogenicity and epidemiology of the virus (WHO, 2020). COVID-19 attributable to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is thought of being transmitted from human to human [44]; however, the pathways of the airborne transmission of these virus are still unclear. All these potential questions need a serious attention in order to establish an organised approach to deal with such grave challenges in the future.

This chapter includes extensive information pertaining to types of bio-aerosol and their impact on the atmosphere and bio-sphere as a whole. Types and impact, monitoring, sampling methods of bio-aerosol research from the purview of the Indian subcontinent and a brief comparison with international prospectus are the crucial aspects of this critical review, which also include a special section on SARS-COV-2 and its potential transmission pathways within aerosol particles.

3. Types and impact of bio-aerosols

3.1 Bacteria

Myriad bacterial species are present in indoor and outdoor environments up to 1.5-km altitude of the surface boundary layer and typically up to 12-km altitude within the higher troposphere and even within the stratosphere at the altitudes of 20–41 km above the ocean floor. These bacteria have the potential to adapt to harsh environmental conditions and are one of the most successfully thriving life forms on the Earth [1, 6, 16, 45, 46]. The presence of bacteria in atmosphere is strongly influenced by meteorological conditions, seasonal variability, human activities as well as their natural and anthropogenic sources [2, 8, 20, 23, 36]. Bacteria may exist in the form of individual units or in aggregation with organic structures or could also be dispersed into the air through crops, organismal fragments, soil particles, pollen or spores [45, 46, 47, 48].

3.1.1 Impact on atmospheric processes

Microorganisms play an active role in ice nucleation, cloud condensation, distribution and precipitation in global scale. Besides health hazards, bio-aerosols in the 0.43–0.65 and < 0.43 μm scale ranges are unavoidable due to the cloud condensation nuclei-forming effects of aerosols [20]. Some bacteria possess the distinctive potential of synthesising ice-nucleation-active (INA) proteins at low temperature as well as under stress condition. The synthesised INA proteins, which are anchored and highly conserved in a gene [49], get attached to the outer membrane of the cell wall forming oligomers. Several researchers [50, 51, 52, 53, 54, 55] proposed that these INA protein oligomers contribute a template for the formation of ice crystal and concluded that higher temperature is likely to be one of the factors to extend INA proteins in oligomers. In addition, it was reported that cell fragments carrying INA proteins are enough to induce freezing. There have been reports [53, 56, 57, 58] that the gram-negative bacteria found in leaf surfaces, such as Pseudomonas syringae, Pseudomonas fluorescence, Erwinia herbicola, Xanthomas campestris and Sphingomonas spp. play a crucial role in ice nucleation. In addition, Akila et al. [59] suggested that the bacteria present in rainwater are significant candidates for ice nucleation. These microorganisms belong to the known ice-nucleating genera Pseudomonas, Pantoea and Bacillus. Furthermore, Bowers et al. [60] also suggested that bacteria in atmospheric samples are sometimes present in soil environments or on leaf surfaces.

3.1.2 Impact on flora, fauna and human beings

Airborne bacteria own deadly impact not only on public well-being but also on agriculture and livestock. Legionella pneumophila, a rod-shaped, gram-negative non-spore-forming bacterium, is the leading causative agent of legionellosis in humans and livestock. Inhalation is the primary route of an infection; however, the foremost sources are freshwater, portable water, cooling towers or soil. Moreover, the unprotected livestock or contaminated animal products of India are one of the serious causes for legionellosis [61, 62]. Bacillus anthracis, which is well known as anthrax, is a gram-positive, rod-formed and spore-forming bacterium causing bio-terror attack. It can have effect on both human and animal over disclosure to infected livestock or contaminated animal products [63, 64]. However, anthrax has been almost eradicated from Western countries, but still it is a cause of concern in the endemic region of India [65, 66, 67]. The inhalation of bacterium Coxiella burnetii can cause mild-flu-like condition to severe diseases such as pneumonia or hepatitis leading to increasing death rates [68, 69, 70]. Moreover, the most predominantly distributed airborne bacteria belong to genus Bacillus, Acinetobacter, Micrococcus spp. and Staphylococcus [2, 20, 21, 31, 71].

3.2 Fungi

Ubiquitous fungi are capable to bring forth serious health hazards [7, 24, 38, 39, 72, 73, 74, 75, 76, 77]. They proliferate on moist or hygroscopic materials that may be natural or synthetic in both indoors and outdoors. In addition, numerous groups of fungi represent a significant fraction (by mass and number) of the atmospheric aerosol particles. The aerosolized fungi are components of the fungal bodies that are sufficiently small to develop into airborne particles and mainly contain the spores, hyphae and mycelia. The atmospheric fungi are discharged both actively or passively from their parent bodies. Furthermore, fungal aerosols originate primarily from the fungi rising in plant/tree floor or from the fungi thriving within the soil as well as in human beings.

3.2.1 Impact of fungi on atmospheric processes

Certain fungal spores possess the potential to induce ice nuclei formation in deep convective clouds at relatively warmer conditions for homogeneous ice nucleation. This process is believed to have an influence on the hydrological cycle [76, 78, 79].

3.2.2 Impact of fungi on flora, fauna and human beings

The spores can adversely have an effect on the plants, animals and human beings as a result of their ample and widespread dispersal in environment [45, 76, 80, 81]. A lot of fungal genera are the foremost reason behind respiratory ailments primarily causing allergies, asthma and pathogenic infections to the respiratory tract. In addition, they are vital promoters for the degradation of cellulosic and non-cellulosic materials in outdoor atmosphere [82]. However, more than 80 genera of fungi have been reported to cause people susceptible to type 1 allergies [83], among which Alternaria is one of the vital allergenic fungi that cause bronchial asthma in chindren [84]. In addition, many common airborne fungal species of Aspergillus, Penicillium and Cladosporium also cause allergy [85]. The prevalence of childhood asthma is increased in developing countries due to rapid industrialisation [86] as well as distinct seasonal variation, which has resulted in declining air quality. In India, about 15–20 million individuals have bronchial asthma induced by ambient air exposure, according to the World Health Organisation. Apart from that, fungal spores exposure can cause allergic reactions such as allergic sinusitis [87, 88], atopic dermatitis [89] and diseases like sick building syndrome (SBS) [90], and their mycotoxins, especially, can adversely have an effect on human as well as animal well-being. Moreover, most of the fungal species present in ambient air topple within the range of 3.3–2.1 μm. This is in consistent with the extent to which the secondary bronchi of the lungs penetrate the human body. Most of the immuno-toxic and allergic fungi can have an effect on the secondary bronchi in human lungs [91]. Other than that, a lot of fungal plant pathogens trigger considerable losses in agricultural crops in each part of the world, which may be approximately 10,000 different types of fungal-infected plant diseases that may not necessarily be host specific [34].

3.3 Viruses

Viruses are the smallest bio-aerosols present in the atmosphere (size approximately 0.003 μm) and are considered as major environmental risk factors to human, animal and plant species [92, 93]. However, their occurrence in the atmosphere is mostly in association with suspended particles [94]. Viruses are dispersed to the atmosphere from water, soil surface and from infected animals, plants, birds and human beings [45]. The detection and identification of these viruses is still a challenge. However, the recent development of molecular tools is proving to be quite promising for their identification. There is little information on viruses in the atmosphere; however, most of the studies are focused on single viral aerosol [93, 94, 95, 96].

3.3.1 Impact of viruses on atmospheric processes

Viruses get dispersed easily into the atmosphere pertaining to their sizes. Some virus particles, such as influenza A virus (IAV), upon transmission experience rapid evaporation and shrinkage followed by their encounter with the unsaturated ambient atmosphere. The size of the virus determines the aerodynamic behaviour and its sustainability in the atmosphere. Some settle on the ground quickly with a potential to cause secondary infections. There is little information about ice-nucleating viruses and other environmental factors, such as temperature, humidity and solar radiation, which are affecting the survival of virus in atmosphere [97].

3.3.2 Impact of viruses on flora, fauna and human beings

Airborne viruses are potential environmental risk factors due to their complex disease pathogenesis in humans, plants and animals. Readily transmitted by air, they have a potential to incorporate severe acute respiratory syndrome (SARS). Airborne intestinal virus due to sewage contamination, respiratory syncytial virus (RSV), hantavirus from rodent faeces, varicella-zoster virus, measles, mumps and rubella viruses cause viral diseases in humans and animals [16].

3.3.3 Viral aerosol transmission in the recent pandemic

Viral infection causes severe-pneumonia-like symptoms, such as heavy breathing, coughing, sneezing, which, in turn, help overcome the surface tension of the fluid lining present in the respiratory tract causing virus-loaded aerosol formation [98]. Immediately after the expiration of any virus-loaded droplets from an infected person, the larger droplets (>100 μm diameter) settle down on the nearby surface due to gravity and also remain in the air for few hours [99]. It may be re-suspended in air through various human activities as well as meteorological factors, such as local turbulence, differential adiabatic lapse rate, wind velocity, hence becoming more prone to cause infection [100]. However, the smaller (a few nanometres to 100 μm) remain suspended in the air for longer time due to its circulatory flow, ambient relative humidity and the ability to disperse, thereby travelling to a longer distance based on the expiratory action [101]. These smaller droplets are of great concern as in an outdoor environment; these can travel to some kilometre depending on the wind condition and can penetrate deep into the alveolar sac causing severe health issues. It was evident that larger droplets exhaled from sneezing can travel more than 6 m, while due to coughing, these can travel 2 m and 1 m for breathing [102]. The symptoms of SARS-CoV-2 have been observed to appear within 6 days of infection and reach its peak 4 days later. It is evident that COVID-19 virus remains viable for 4 hours on copper surface whereas for 2–3 days on plastic or steel [103].

There are numerous modes of transmission routes for COVID-19 disease, including airborne transmission, endogenous infection, common vehicle, vector, aerosol transmission [104]. The three major modes of viral transmission [105] are aerosol transmission, droplet transmission and autoinoculation of viable virus from contaminated hands to respiratory tract. Droplet transmission occurs by spraying large or small droplet nuclei directly onto the recipient’s eyelids or nasal cavity when an infected person coughs, sneezes or even talks. Droplet, airborne and contact [100] are considered to be the three modes of transmission. Large virus-loaded droplets that are released with sufficient momentum by an infected person reach the respiratory organ of the healthy person through droplet transmission. During coughing, a high intrathoracic pressure of high expiratory airflow of 12 L/s breaks up the mucous into smaller droplets. Physical contact with viral droplets deposited on a surface and transmission to its respiratory organ is known as contact transmission, whereas the inhalation of aerosolized viral droplets by a healthy person is considered airborne mode of transmission [106]. However, the significance of transmitting the disease remains unclear [107]. Samples collected from the air outlet exhaust fan of a COVID-19 positive patient’s room have reported to be positive referring to the fact that microscopic virus-loaded aerosols would have been displaced by airflows [108]. Interestingly, there are claims [44] that a person in Mangolia, China, has tested positive when he has passed several times indicating that airborne transmission is credible. The airborne transmission evidence even strengthened up by the WHO statement that in indoor environment, the virus-loaded aerosols can be transmitted up to certain distance and result in an accumulation of infection in a short period.

3.3.4 Role of air pollution in COVID-19 transmission

Though it was found that the spread of COVID-19 virus is mainly by human-to-human transmission, in 55 Italian province capitals, the number of confirmed cases was in the days exceeding the PM₁₀ threshold limit of 50 μg/m³, hence suggesting the transmission mode to be air-to-human transmission rather than the direct way [109]. It was also noticed that polluted cities with low wind pace have a soaring number of COVID-19 circumstances than the much less polluted cities [109]. It was reported that maximum COVID-19 cases were found in the highly polluted regions of northern Italy (SIMA, 2020). Particulate matter performs a vital function within the transport of viable viral droplets as it comprises solid or liquid particles that act as a carrier and allow the virus to flow in airflow for hours to days. In an analysis done by Setti et al. [110], PM₁₀ samples were collected from an industrial site of Bergamo province known to be the epicentre of COVID-19 in Italy. According to Bashir et al. [111], utilising Spearman and Kendall correlation confirmed an optimistic association of PM₁₀, PM_2.5, SO₂, NO₂ and CO ranges with COVID-19. In a signification study conducted by Wu et al. [112], it was concluded that a rise in 1-μg/m³ concentration of PM_2.5 resulted in a rise in 8% death rate of COVID-19. Similarly, Travaglio et al. [113] concluded the positive correlation between high levels of SO₂ and NO₂ and COVID-19 cases. However, Dhand et al. [106] explored the fact that by considering some completely different parameters such as air quality index (AQI), four ambient air pollution types such as PM10, PM2.5, NO₂ and CO and meteorological parameters such as temperature and sunshine duration, a rise in the concentration of these parameters can set off COVID-19 cases. Similar investigation performed by Xu et al. [114] concluded the fact that the increase in AQI can enhance the spread of COVID-19 under low relative humidity.

Accepting the inter-relationship of confirmed COVID-19 cases and air pollution, many scientists worldwide carried out studies including different perspectives of linkage to this fact. Combining three databases of NO₂ concentration, atmospheric situation and confirmed COVID-19 instances from 66 areas of France, Germany, Italian republic and Spain, two NO₂ hotspots were recognised, and surprisingly, 78% of fatality was confirmed from these two hotspot areas [115]. However, there is not much exploration on the significance of airborne transmission, which may be due to the fact that it is challenging to directly detect the virus travelling in ambient air as a number of external factors are even associated with the movement of viable particles, such as size, temperature, rainfall, humidity, wind speed, wind direction and flow physics [116].

3.3.5 Possible prevention measures

The fact of COVID-19 virus spreading through air can never be denied. Considering the expiration range of SARS-CoV-2 by an infected person, social distancing and masking by using any type of mask can control the situation to a great extent [104]. Homemade cloth mask should be promoted in the countries with the shortage of mask [117]. However, transmission is not the only factor to be assessed, whereas a risk assessment, including the virus, viability of infection and aerosol formation process, is the need of the hour. The identification of the main pollutant in ambient air causing the spread of COVID-19 virus needs a thorough investigation.

Considering the above fact, a link between COVID-19 transmissions with increasing pollution level is emphasised and needs further speculations. It is obvious from the previous SARS-CoV-1 epidemic episodes that long-run exposure to particulate matter, particularly PM_2.5 can cause over-expression of alveolar angiotensin-converting enzyme receptor-2 (ACE-2), which on exposure to air pollutants can increase the viral load-depleting ACE-2 receptors making them more prone to viral diseases [118]. The airborne transmission of the SARS-CoV-2 viral aerosol is mostly influenced by disease symptoms as well as meteorological factors. The fatality of the disease lies due to past patient history such as whether he/she has been suffering from any pulmonary disorder and chronic disease such as diabetics, obesity, hypertension, anaemia, which decreases the oxygen-carrying capacity and obstructs its supply to the lungs due to pneumonic alveolar congestion. With chronic exposure to air pollution as reported for the urbanites, COVID-19 lethality increases. Though no vaccine has been developed for SARS-CoV-2 yet, prevention is the only solution. As per WHO guidelines, social distancing (2 m) and wearing of multi-layered face mask are the best-known preventive measures to get rid of being infected from SARS-CoV-2 and its airborne transmission.

3.4 Pollen

Pollen is the reproductive unit of plant, which is the largest in size among bio-aerosols; therefore, its residence time in atmosphere is short and is prevalent in near surface of atmosphere. However, the atmospheric convection can up-draft these to high altitudes. Having long residence time, the pollens too have the potential to form ice nuclei in different environments [119]. Most of the pollen grains have hard shell, which protects them from adverse environmental conditions. The environmental elements, such as temperature, humidity, rain and wind, can have an effect on the dispersal of pollen.

3.4.1 Impact of pollen on flora, fauna and human beings

Plant pollen and their fragments are one of the first causes of allergy in city facilities. The size of airborne pollen generally ranges between 10 and 50 μm [120]. The airborne bio-pollutants, such as pollen grains, fungal spores and mud mites, include particular proteins and glycoproteins, which react with the immune entity to trigger allergy. They set off allergic irritation within the nasal, conjunctival and/or bronchial mucosa in addition to bronchial asthma [121, 122]. However, the vast majority of sufferers with allergic ailments possess medical signs within the respiratory tract [122, 123]. Grass pollens are among well-known bio-aerosols causing respiratory allergy in human beings. It was reported that some of the rice pollen also cause respiratory allergy in the field workers and the people living in the vicinity of the rice field. According to researchers in Israel and Poland, the threshold values for pollen range from 4 to 20 grains/m³ air for grass pollen [1].

3.5 Algae

Algae are microscopic or macroscopic in sizes dispersed to atmosphere actively or passively and are found in almost all the environments. In comparison with other bio-aerosols, their number is less in atmosphere. Their presence in atmosphere has been reported in several studies worldwide [124, 125, 126, 127, 128, 129, 130]. These are rich in bioactive secondary metabolites, which are beneficial in the developments of new pharmaceutical agents [127, 129].

3.5.1 Impact of algae on flora, fauna and human beings

Despite their usefulness, microflora cause several adverse health consequences. They are capable of contaminating drinking water, act as allergen to humans [129] and deteriorate old-age architectural heritage [131], and some of the taxa produce toxins posing threat to humans [132]. The genera Chlorella, Scenedesmus, Chlorococcum, Klebsormidium (Hormidium) and Lyngbya are most commonly associated with health results [133]. However, algae and cyanobacteria can make a significant contribution to the total load of suspended air particle making a pleasing contribution when inhaled and causing many environmental problems. In addition, these are everlasting constituents of indoor and outdoor environments [134]. They also cause inflammation of pores and skin, hay fever, allergies, headaches, sinusitis, rhinitis, sclerosis and various respiratory illnesses.

4. Indoor versus outdoor environment

The risk of hazards is higher in indoor surroundings than outdoor as folks spend most of their time indoors. In addition, the lack of proper ventilation and dispersal mechanism has posed a high risk to human health [135]. The World Health Organisation has recently reported [136] that the inhabitants of damp or mouldy building both in houses and public buildings are at enhanced threat of experiencing respiratory symptoms, respiratory infections and exacerbation of bronchial asthma. In India, approximately 10 million people suffer from Asthma, whereas 15 million from frequent/intermittent Allergic Rhinitis due to the inhalation of air containing various allergens [137]. The foremost sources of indoor bio-aerosols are home dust, fungal spores and hyphae, pollen grains, shedding of human pores and skin cells and actions such as talking, coughing and sneezing, fragments of insects, meals crumbs, pure fibres. However, outdoor bio-aerosols have always been a source of bio-aerosols in a variety of indoor environments [16, 31]. People belonging to open area were more prone to seasonal attack, whereas those living in congested areas were at high risk to perennial attack [138].

Several studies on monitoring and impact of indoor/outdoor bio-aerosols have been conducted to address health risk issues [1, 16, 20, 75, 76, 96, 135, 139, 140, 141, 142]. Bio-aerosols are reported for 5–34% of indoor air pollution [67]. In addition, the global burden of diseases was 2.7%, while 2 million deaths occurred per year due to the indoor bio-aerosols that exceeded the annual mortality attributed to malaria [43, 143, 144]. One of the best examples of indoor bio-aerosols impact is sick building syndrome (SBS) [31, 123]. Moreover, the bio-terror attack (2001), outbreak of SARS (2003) and H1N1 influenza virus (2009) are some of the life-threatening issues due to bio-aerosols. However, it is estimated that the fraction of outdoor particulate matter (PM) below 2.5 in aerodynamic diameters has attributed to 3.1 million deaths in 2010 worldwide [144]. It has been predicted that in near future, 85% of global population of the developing world, in particular children, would grow up exposed to unsafe air both in indoor and outdoor environments due to the lack of major policy for clean air quality [145, 146]. Morey [147] suggested that concentrations above 1000 CFU/m³ indicate potential microbial contamination, and further investigation in indoor environment is recommended.

5. Bio-aerosol characterisation in India

India with 1.2 billion population (2011, Census) is the second most populated nation on the Earth and likewise accounts for 17% of world’s population. Besides, it is a rapidly growing nation and is among the top 10 most industrialised nations of the globe. According to the Central Pollution Control Board (CPCB) of India, there are 17 classes of drastically polluting industries (large and medium scale). Higher population density, rapid urbanisation, industrialisation and improper control policy lead to high environmental burden and a significant health risk. In 2012, the WHO reported that about 7 million folks died prematurely from both indoor and outdoor air pollutions. It accounts for one-eighth of all the premature deaths worldwide [144, 148, 149, 150]. In 2013, Global Burden of Disease Study (GBD) concluded that 19% of the world’s untimely loss of life occurred in India, while air pollution accounts for 620,000 premature death in each year [151]. These reports signify that air pollution is a global environment burden as well as an important risk factor for morbidity and mortality, especially in the developing countries [143], and is also the world’s largest single environmental health hazard [144, 150].

Based on massive developmental activities along with topographical/climatic diversity, the Indian subcontinent has become a breeding ground for quite some complex air pollution issues that is taking a toll on the environment in general, impacting the plant animal and human health in particular.

Biological components in the aerosols are important to understand from several perspectives as these are an integral part of the aerosolisation process apart from having a strong impact on various actively spreading airborne diseases across the country and the world. In spite of the predominance of the biological components in ambient air, bio-aerosol research is still in its infancy in a tropical climatic zone like India. In this study, the authors have made a conscious attempt to compile the entire microbial composition detected in the atmospheric particles across eastern, western, northern and southern parts of India. A vivid description of the bacterial and fungal types and their spatial-temporal variation has been summarised along with the detection techniques. This will be a very first report of its kind from the Indian subcontinent.

5.1 Northern India

The northern part of India comprises megacity Delhi, tourist places like Rajasthan, Uttar Pradesh, predominantly biomass-burning states such as Punjab, Chandigarh and Industrial state Haryana as well as the pristine Jammu and Kashmir.

5.1.1 Delhi

The megacity Delhi is the nationwide capital and second largest metropolitan in India having ~16 million inhabitants (2011 Census, India). The city experiences mainly three seasons: monsoon (subtropical climate, average amount of rainfall of ~611.8 mm), excessive chilly winter (dense foggy climate and low atmospheric boundary layer top conditions) and hot summers (temperature ~ 46–49°C). Occasionally, Delhi also experiences dust storms event from the Thar Desert or the Middle East [2]. According to the recent air pollution monitoring data of 2011 and 2015, Delhi is the second most polluted major city in the world [152]. Therefore, we observed that most of the bio-aerosol-monitoring reports were made from Delhi.

Most of the studies have been reported on viable culturable bacterial and fungal concentration in both the outdoor and indoor environments of the city premises of Delhi. The research group of [41] has been actively monitoring the bio-aerosol concentration in both the indoor and outdoor environments of Jawaharlal Nehru University (JNU) and nearby areas. This research group mainly focuses on the concentration of fungi and bacteria (both gram positive and gram negative). The highest concentrations of both gram-positive and gram-negative bacteria were reported near a garbage dump site where gram-negative bacteria were predominant [41]. However, most of the fungal bio-aerosols identified were of respirable sizes and were associated with immune-toxic diseases and allergies. Higher concentrations of viable airborne microbes were detected in post-monsoon than in monsoon season in indoor sites in comparison with the outdoor ones, which may be due to seasonal rain wash [153]. However, Ghosh et al. [31] reported an enchanting connection between the concentration of fungal spores and microorganism in relation to each environmental and human factor. They analysed ambient ranges of viable bio-aerosol (fungi, gram constructive and destructive bacteria) in the Central Library of JNU (five indoors and one outdoor samples) and suggested that most of the fungal species were Rhizopu soryzae, Aspergillus nidulans and Aspergillus flavus, whereas predominant bacteria were of Bacillus and Coccus types based on gram staining and morphology observation. They noticed the highest fungal concentration in indoor surroundings, but bacterial concentration for both gram negative and positive has been the highest within the reading corridor. Furthermore, Kumar et al. [8] reported the abundance of culturable airborne bacteria during monsoon and winter season at the building of the School of Environmental Science in JNU. Their observation revealed that the mean concentrations of bacteria were higher throughout the rainy season than in winters, which deviated from the reports from Delhi as well as from data of temperate as well as tropical climate zones. In addition, the percentage of gram-positive bacteria dominated over gram-negative bacteria during both the seasons, and among gram-positive bacteria, cocci dominated over rod-shaped ones. The ambient level of viable fungal bio-aerosols of a sewage treatment plant (from six different segments) of Vasant Kunj, a posh area in Delhi city, depicted eight genera of fungi, and among them, four, for example Mucor, Rhizopus, Aspergillus and Penicillium, were found in larger number throughout all the seasons [73]. Moreover, Lal et al. [91] identified and characterised size-segregated bio-aerosols at four different sites in JNU campus, Delhi, during all the four seasons. They suggested the highest bio-aerosol concentration (fungi, gram-positive bacteria and gram-negative bacteria) during post-monsoon and the lowest in monsoon season in all the four sites. Fungal bio-aerosols, such as Penicillium sp., Alternaria sp. and Aspergillus sp., were predominant.

Culturable airborne bacterial and fungal concentrations were detected near a busy roadside restaurant cluster in New Delhi. A big variation in bacterial and fungal focus was noticed in numerous seasons starting from 1.7 × 10⁴–9.8 × 10⁴ (averaged 6.3 × 10⁴ ± 2.6 × 10⁴ cfu m⁻³) and 3.5 × 10²–9.5 × 10³ (3.9 × 10³ ± 3.1 × 10³ cfu m⁻³) cfu m⁻³, respectively. Based mostly on 16S rDNA sequencing, Bacillus and Acinetobacter were predominant microorganisms, whereas predominant fungal genera were Aspergillus, Cladosporium, Alternaria and Fusarium, which are popular for bad health effects resulting in quite a few allergic and pathogenic irritation [21]. In another study, Agarwal et al. [2] reported size-segregated bio-aerosols at Okhla landfill of Delhi, known to be a municipal dumping station, and stated that both bacteria and fungi had major peak in winter than summer. Larger focus of bio-aerosols could be related to low atmospheric boundary layer height and beneficial meteorological situations in New Delhi. However, the low concentration in summer may be due to microbial lethal consequences of critical meteorological situations (high temperature and photovoltaic radiation) that are more pronounced than the release of microbial flux due to the effect of summer photovoltaic underfloor heating. In addition, size distribution evaluation confirmed that microorganisms were largely ample in fine particle sizes, that is <0.43–2.1 μm, but few peaks were additionally seen in dimension ranges between 5.8 and >9.0 μm, whereas fungal spores largely peaked in coarse sizes (2.1–5.8 μm). Bacillus, Staphylococcus, Streptococcus, Klebseilla and Escherichia genera were predominant bacterial strains. However, a lot of identified fungal spores possess negative health effects resulting in quite a few pathogenic inflammations. In addition, in various places of Delhi, that is at a landfill site, agricultural facilities, highway orientation and restaurant cluster sites report bio-aerosol size disparate monitoring in winter, spring and summer seasons [20]. In spring, bacterial particles were enhanced in size ranging between 5.8 and >9.0 μm, but concentrations were higher during winter. More importantly, ample strains were Bacillus cereus (16%), Bacillus licheniformis (11%), Bacillus thuringiensis (9%), Micrococcus sp. (7%) and Acinetobacter sp. (9%). The variability in bio-aerosol concentration was reported at three different sites, such as land use configuration commercial complex, dumping site and vegetated ridge with respective microclimatic variation in Urban Delhi. It was observed that the dumping site had the maximum bacterial count, whereas the vegetated ridge site had the highest fungal count [23]. Although spatiotemporal variation was quite distinct, the meteorological parameter independently failed to show a uniform and conclusive relationship. From this study, it was interpreted that the land use pattern and human activity seem to play an important role in determining aerosolized microbial diversity than meteorological variables.

5.1.2 Kanpur

Kanpur is an industrialised metropolis in the state of Uttar Pradesh in India and is positioned with the central part of highly polluted Indo-Gangetic Plain (IGP). Many studies on the physical and chemical characterisation of aerosols and their climate impact have been reported from Kanpur. Viable aerosols and particle concentration in ambient atmosphere of Kanpur were analysed during southwest monsoon and post-monsoon (June-October 2015). Gram-negative bacteria were reported to be predominant over gram-positive bacteria and fungi [154]. However, in a separate study, the same group of researchers reported the maximum concentration of bio-aerosols during wintertime (gram-positive bacteria, gram-negative bacteria and fungi) followed by post-monsoon, monsoon and pre-monsoon during June 2015-May 2016 at the same location. In addition, they reported the correlation between meteorological parameters and bio-aerosol concentration and suggested that ambient temperature affects the bacterial concentration, whereas wet precipitation relates to higher abundance of fungi [155].

The northern part of India is the heavily polluted part where life is quite worse. Still, most of the researchers focus on chemical constituents of particulate matter. However, both the chemical and biological constituents have adverse impact on living beings causing numerous respiratory problems. Many of the researchers represented an excellent correlation between the concentrations of culturable bacteria and fungi with meteorological parameters, and some correlate with organic carbon (OC), elemental carbon (EC) or EC/OC, volatile organic carbon (VOC) and total suspended particulate matter (TSPM) [8, 21, 155]. Moreover, some of them used low-volume handy sampler, single stage and size-segregated impactor to enumerate the culturable bio-aerosol concentration [2, 8, 20, 21]. However, most researchers identified bacterial species based on phenotype characteristics and gram staining [2, 8, 23, 31], whereas very few followed PCR amplification and 16S identification to identify bacteria up to species level [20, 21]. In addition, fungal bio-aerosols were identified based on morphology and lacto-phenol cotton blue staining (microscopic observation). Still, there is a huge gap as culturable bio-aerosols are only a fraction of the bacterial and fungal diversity as most of them were unculturable and have potential impact on living beings [46].

5.2 Eastern India

The states Bihar, Jharkhand, West Bengal and Odisha are located in the eastern part of the Indian subcontinent. Kolkata, the capital city of West Bengal, is also one of the largest metropolitan cities in the region, while Bihar and West Bengal are in Indo-Gangetic Plain (IGP); Jharkhand, rich in mineral wealth and dense forest, lies in the Chota Nagpur Plateau. Odisha is also rich in mineral wealth and lies along the Eastern Ghats and Deccan Plateau. Most research activities on bio-aerosols were reported in Kolkata compared to other cities in eastern India.

5.2.1 West Bengal

Kolkata, the capital of West Bengal, is the third most populous city in India having a population of about ~4.5 million according to a 2011 census. In spite of that, this metropolis is the economic, cultural and academic hub of eastern India and is below traffic congestion, poverty, overpopulation and logistic issues associated with the inhabitant bust [156]. However, Sundarbans mangrove forest, the world’s biggest mangrove eco-region on the land-ocean boundary of the Ganges delta, is located at approximately 100 km from Kolkata city [157]. Studies on bio-aerosol from the ambient air and both from outdoor and indoor environments have been reported in many parts of urban and sub-urban Kolkata. Mostly, the studies focused on the identification of fungal species using various sampling techniques. Few reports were on the pollen and bacteria as well; mostly, the effect of meteorological parameters was discussed along with the health impact of the identified microorganisms.

Initial studies on airborne fungal load in agricultural environment throughout threshing operation of paddy and wheat crop within the neighbourhood of Barrackpore, West Bengal, were published way back in 1994 [158]. During wheat threshing, a relatively better frequency of Alternaria tenuissima was detected followed by Drechslera sp. and Cladosporium herbarum, while during the threshing of paddy, a high count of A. humicola and A. tenuissima seen was detected in Boro variety, but in Aman variety, Helminthosporium oryzae was dominated followed by C. herbarum and a few unidentified yeasts. However, the frequency of Aspergillus sp. was observed before and after threshing but was quite negligible during the threshing period. Adhikari et al. [19] attempted quantitative and qualitative strategies to estimate the extent and kinds of indoor airborne fungal spores in two unique cowsheds: one in a rural region of Konnagar (December 1994-September 1995), and the other in a sub-urban space located at Kestapur (during December 1995-September 1996) of West Bengal. In addition, they attempted to emphasise the fact that these species can cause respiratory allergies in sensitive people. In addition, they monitored the spread of spores in the air. This helps predict the spread of fungi that contaminate milk and dairy products rising out of these cowsheds. Moreover, they attempted for searching out the link between the categories and the amount of fungal spores and culturable moulds produced under completely different meteorological parameters because it is believed that the climate plays a completely crucial feature in fungal growth, sporulation and dispersal. Their investigation revealed that 29 airborne spores were from the rural cowshed, whereas 24 were from the sub-urban area. At every location, a comparatively higher frequency of Cladosporium sp., Aspergillus/Penicillium group, Periconia sp., Nigrospora sp., and some unidentified ascospores and basidiospores were reported. However, the occurrences of fungal spore were completely exceptional in several meteorological components in each cowshed.

A comparative research of airborne fungal spores was undertaken in five indoor and outdoor environments in Burdwan, West Bengal, for an interval of 2 years utilising Rotorod samplers and sedimentation plates (culture plate). They revealed the lowest count in summer season and the highest in the wet season. However, Aspergillus was fairly predominant in all of the environments investigated [26].

The vertical profile of major airborne pollen and spore concentration at totally different heights in an agricultural farm situated in West Bengal, India, revealed that the frequency of tree pollen confirmed roughly good correlation with the increase in heights, whereas pollen from herb/shrub is dominant at decreasing heights throughout all of the three seasons (winter, summer season and rains). However, the smaller fungal spores were dominant at increasing heights, and larger spores and conidia were more often at decrease levels. In addition, Aspergilli group, Cladosporium and Nigrospora were predominant throughout the investigation, whereas Aspergillus japonicus and Drechslera oryzae were discovered to be potential inflicting respiratory allergy in agricultural workers. The examination was performed utilising Rotorod samplers [25].

Airborne fungal spore concentration was studied in five indoor environments in Santiniketan, West Bengal, India, for an interval of 2 years utilising the Astir 1-day personal volumetric sampler in addition to a Rotorod sampler and sedimentation plates. Aspergillus sp. was the very best aerospora contributor within the environment, followed by Curvularia and Cladosporium. However, during winter, the focus of Cladosporium was highest attributable to the presence of a giant variety of saprophytic kinds [159].

Aeromycoflora studies over a paddy field near Barrackpore, West Bengal, were carried out in two consecutive crop seasons (Rabi) using the gravimetric culture plate exposure technique. Cladosporium was dominated at the early stage of seeding, which was declined in the later stage with the onset of summer, while Penicillium showed a reverse pattern. However, Curvularia showed no seasonal variation and was observed throughout the investigation period. In addition, Alternaria, Fusarium, Helminthosporium and Nigrospora also observed in air were potent phytopathogenic fungi of which Alternaria was predominant. However, a species of Alternaria was detected as an epidemic-causing agent in this variety of rice, which causes leaf injury to the host plant [160]. In another study, Uddin [161] investigated the character and number of pathogenic as well as non-pathogenic aeromycoflora in some of the jute fields close to Barrackpore, West Bengal, India. They revealed that Penicillium and Aspergillus were probably the most dominant saprophytes adopted by Curvularia and Cladosporium in the initial season, whereas in 1991, Aspergillus and Curvularia were essentially the most dominant, followed by Cephlalosporium, Penicillium and Cladosporium, together with Pullularia within the mid-season. However, Macrophomina phaseolina, a pathogenic fungus, appeared irregularly in the initial season but grew to become extra predominant in later phases within the second season. In addition, different pathogens Helminthosporium occurred twice and thrice in the sooner part of the two-crop season, whereas Alternaria, Nigrospora and Fusarium occurred very often.

An aerobiological survey revealed seasonal variation in Trichoderma harzianum within the outside surroundings of an agricultural farm and its vicinity within the northern fringe of Kolkata city, India, by an Andersen air sampler. It was noticed that the colony rely was elevated throughout February in around 28 days, and the relative humidity was discovered to be a major (P < 0.05) factor predicting the incidence of T. harzianum in the air [27]. A scientific quantification of the indoor fungal flora of the Shyambazar Metro-Railway Station, Kolkata, was carried out for an interval of 4 months by utilising gravitational settling methodology via Petri dishes with potato dextrose agar (PDA) culture media. In this study, it was revealed that Aspergillus niger was probably the most prevalent fungal genera followed by Aspergillus flavus and Penicillium sp. [162]. The fungal spore concentration within the air of Madhyamgram, a sub-urban space close to Kolkata, was carried out for five consecutive years to find out the effect of various meteorological parameters on the frequency of airborne fungal spores and their well-being on native inhabitants with regard to respiratory allergy. Greater than 50 taxa were detected, out of which 15 were allergenic. In addition, the concentration of spore increased throughout early-winter and rainy season and diminished throughout late-winter and mid-summer. Moreover, a positive correlation was discovered between the respiratory allergy instances and the air-spora concentrations [24]. The fungal spectrum of Konnagar city was evaluated using the Burkard personal sampler, which showed 39 types of fungal spores such as Cladosporium sp., Aspegilli/Penicilli, Nigrospora sp., Periconiasp., Chaetomium sp., Drechslera sp. and Alternaria sp. in a 1-year calendar [30]. One more study monitored the connection of each day bronchial asthma hospital admission of school-age youngsters and each day concentration of outdoor Alternaria conidia, ozone, PM10 and climate parameters within the ambiance of Kolkata, utilising nonparametric generalised additive model [25]. Ghosh et al. [32] evaluated the airborne fungal flora of the Nationwide Library, Kolkata, for an interval of 3 months (February-April, 2010) via gravitational settling technique utilising Petri dishes with malt extract agar (MEA) media. Aspergillus niger, A. tenuissima, C. herbarum and Penicillium sp. were accounted highest in indoor surroundings, whereas outside atmosphere confirmed clear dominance of Alternaria alternata, Asperillus niger, A. tenussima, C. herbarum, C. cladosporioides, Curvularia lunata and Fusarium oxysporum. For the enumeration of bacteria and fungi, a qualitative and quantitative research of indoor air in a hospital at Kalyani, West Bengal, India, was carried out using the settle plate method. The aim of this research was to evaluate microbial inhabitants of indoor air of various wards of the hospital and in several sampling time. The highest microbial inhabitants were recorded during evenings in comparison with the morning hours. Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa and Klebsiella sp. were predominant among bacteria; however, Aspergillus sp., Fusariumsp, Penicillium sp. and Candida sp. were prevalent among fungi [39]. The aeromycoflora of the Institute of Agriculture library, Visva-Bharati, Santiniketan, India, was monitored with the help of Burkard personal 1-day volumetric sampler to suggest preventive measures towards the collections of library and the library personnel’s. Mainly, Drechslera sp., Aspergilli/penicillin sp., Bispora sp., Basidiospore sp., Ascospore sp., Nigropora sp., Pericornia sp., Fusarium sp., Cladosporium sp. and Trichornis sp. were monitored during the investigation. In addition, the concentration of fungal spores was maximum in September and minimum in November [163]. The quantitative and qualitative analyses of fungal spores in the atmosphere of a sub-urban area of West Bengal, India, near India-Bangladesh border were carried by Chakraborty et al. [164], by using the Burkard personal volumetric sampler (for over all spore count) and the Andersen two-stage sampler (for viable spore). The fungal spore concentration has shown two peak seasons (February-March and September-October) with the maximum contribution of Aspergillus-Penicillin group. In addition, from the antigenic extract of 15 spore members, Aspergillus fumigatus showed the highest allergic sensitivity followed by A. niger. Kashinath et al. [165] carried out a survey on viable airborne fungal spore in two outdoor environments in Farakka, West Bengal, India, for a period of 2 years, using Andersen two-stage volumetric air sampler. They revealed that monthly total viable fungal spore load (CFU/m³) has shown significant negative correlation with monthly average temperature. However, it was depicted that antigenic extracts from dominant culturable fungi showed high allergic potentiality by skin prick tests (SPTs) among local allergic patients, suggesting health issues in sensitised people.

The bio-aerosol studies in eastern India mainly focused on fungal bio-aerosols and their allergenic impact on workers by the skin prick test; very few reported on pollen and the least number of studies were focused on viable bacterial aerosols. Most of the sampling of bio-aerosols was based on the sedimentation method, the Rotorod sampler and the Andersen two-stage sampler. Still, there is scanty information on the correlation of bio-aerosols with aerosols as well as meteorological parameters. In addition, unculturable bio-aerosols were also not explored.

5.3 Western India

The Western India includes states such as Maharashtra, Gujrat and Goa as well as the union territories of Dadra Nagar Haveli and Daman and Diu. In its north, it is bounded by the Thar Desert, in east Vindhya Range and the Arabian Sea in the west. Mumbai, a metropolitan city, is the largest city as well as the financial centre of India. The climate is mostly tropical wet and dry and semi-arid. However, during summer, most of the cities have temperature about 40–42°C and in winter near about 6–7°C.

A seasonal variation of fungal propagules in a fruit marketplace environment in Nagpur (India) was analysed by utilising the Rotorod sampler and the uncovered Petri plate technique to enumerate the quantitative and qualitative estimations of fungus. Aspergillus was probably the most often and predominantly detected genus, whereas Cladosporium, Penicillium and Alternaria spores were ample that are generally known as allergenic and pathogenic fungi. The elevated focus of airborne spores was throughout December-January, whereas the utmost focus of Aspergilluswas during summer season months. The work-related exposure of Indian agricultural workers to airborne microorganisms, dirt and endotoxin, in small agricultural facilities (farms, warehouses and mills) within side Aurangabad area, India, was also studied [166]. In two farms, throughout the threshing of maize and pearl millet, there was noticeable escalation in the concentration of airborne microbes by two to four orders of magnitude. However, whilst threshing of maize, probably, most of the microorganisms were thermophilic actinomycetes and mesophilic actinomycetes of the genus Streptomyces, whereas in the course of threshing of pearl millet, probably, the most predominant were corynebacteria and gram-negative microorganism. Airborne fungal spore and pollen of Ismail Ysuf college campus at Jogeswari, Mumbai, India, were analysed by Kakde [167]. Both the qualitative (Petri plates exposed with media) and quantitative (Rotorod air sampler) estimations of the culturable fungal spore were also analysed. Aspergillus-Penicillium types were predominant, while the spores of Deuteromycotina followed by Ascomycotina and Basidiomycotina were the highest during July-September and November-December. In addition, the predominant pollen were Poaceae, Amaranthaceae, Tridax, Cassia and Ricinus. Airborne bacterial species were detected in six wastewater treatment plants in Mumbai, and the impact of endotoxin on the health of the local population was also analysed. Several clinically important bacterial species were detected in the samples, and it was reported that the workers at the treatment plant are exposed to opportunistic and infectious bacteria [29]. Post-harvest diseases through airborne fungi were investigated in a vegetable market of Nagpur city. Some widespread vegetables were screened for the isolation of fungi as marketplace pathogen to test post-harvest diseases. Virtually, 59 fungal spore types and 78 species from 33 genera belonging to completely different groups were reported. Essentially, the most dominant types of fungi were of Aspergillus followed by Cladosporium, Penicillium, Alternaria, Fusarium, Curvularia, Trichoderma and Rhizopus. However, vital mycotoxin-producing fungi similar to A. flavus, A. fumigatus and Fusarium moniliforme were also removed from the vegetables collected from the market [33]. Karne [34] analysed the pathogenic and allergenic fungal bio-aerosols over Jowar, Wheat and groundnut fields with the help of continuous volumetric Tilak air sampler for three consecutive Rabi seasons. Most of the bio-aerosols reported were allergenic as well as pathogenic. Airborne fungi were identified in the 80-year-old college library in Mumbai using viable volumetric sampler. Aspergillus spp., Penicillium spp., Alternaria spp., Cladosporium sp., Curvularia sp., Trichodermaspp and Chaetomium were the predominant species [168]. Aeromycological study was carried in the closed indoor environment of various laboratories of Mahatma Jyotiba Phule Campus, RTM Nagpur University, by using the culture plate exposure method. Altogether, 3368 fungal colonies in 19 genera and 28 species were reported. Among them, Ascomycota contributed to more than half of the colonies, whereas Oomycota had least count. However, Zygomycota and Deuteromycota contributed to moderate count, while no Basidiomycota was reported. The prevalence of fungal culture on cellulosic material in the laboratory depends on the changing indoor environment [22]. Air mycoflora study was done in order to estimate the presence of bio-allergens in different locations of STRM University Campus, Nanded, India. The concentrations of fungal spores were observed to be dominant during winter compared to rainy and summer seasons. The most common fungi identified were Albugo, Mucor, Rhizopus and Aspergillus [169]. In Mumbai, the diversity of airborne bacterial species was studied using six-stage microbial impactor and bio-sampler to elucidate the role of endotoxin in particulate matter that elicits pro-inflammatory response ex vivo [28]. Gram-positive and sporogenic bacteria corresponding to Bacillus species dominated the concentration of airborne culturable bacterial. This study suggested the predominance of pathogenic/opportunistic bacteria from human or animal and sporulation present within the ambient air environment. The fungal bio-aerosol concentration across the Deonar landfill website, which is one of the major landfills in Mumbai, was monitored by the impaction method. The maximum concentration of fungal bio-aerosols was reported in monsoon season, while the minimum concentration in winter season. Moreover, environmental factors, such as relative humidity and wind speed, showed positive correlation with culturable aeromycoflora. In addition, the prevalent species reported were Aspergillus, Penicillium, Alternaria, Curvularia, Trichoderma and Rhizopus [38]. A comparative research of airborne microbial burden within the vicinity of two landfill sites in Mumbai was carried out by Patil and Kakde [37] as municipal landfills are rich in organic matter and, hence, are favourable sites for the growth of microorganisms. The heterotrophic bacterial concentration was maximum in monsoon and minimum during summer season. In addition, Staphylococci and Actinomycetes concentrations were higher in monsoon and minimum during winter season. Moreover, they reported no bacterial-free month during their investigation.

5.3.1 Rajasthan

The impact of meteorological factors on the spread of airborne bacteria and fungi at Sardar market, Jodhpur, was studied by Naruka and Gaur [170]. Gram-negative bacilli were predominant among bacteria, whereas Aspergillus sp. was dominant among fungi. In addition, both bacteria and fungi showed significant seasonal variations.

In the western part of India, the studies on bio-aerosols were also focused on viable fungal and bacterial bio-aerosols based on the sedimentation method, the Rotorod air sampler and the six-stage microbial sampler. The research group of Gangamma [29] performed many studies on the characteristics of airborne bacterial diversity and pro-inflammatory response of particulate matter. However, the unculturable part of the bio-aerosols was neglected; hence, there is also still a large gap in scientific understanding on bio-aerosols.

5.4 Southern India

5.4.1 Visakhapatnam

Visakhapatnam is the 15th largest city in India and the principal commercial hub and tourist place of Andhra Pradesh [171]. Most of the research studies on air microbiome were on indoor and outdoor environments of different places of Visakhapatnam. Reddy et al. [172] carried out an analysis of air microbiome of a food warehouse using gravity Petri plate method. The maximum dominant fungal aerosols reported in the five different places of the warehouses were Penicillium, Mucor, Rhizopus, Cladosporium, Aspergillus, Alternaria, Trichoderma, Fusarium, Pseudomonas, Proteus vulgari and Enterobacter aerogenes. They reported that the strains of Aspergillus produce aflatoxin of about 1927.3 μg/kg. However, Mohan et al. [173] analysed the indoor microbiological air quality of 30 government schools using Koch’s sedimentation method. They interpreted that the sources of bacterial aerosols were mostly from classroom, toilets and canteen, whereas fungal concentration was high in libraries and classrooms. In addition, Bacillus species, Staphylococcus species, Micrococcus species, Pseudomonas species, E. coli and Serratia species were predominant, whereas among fungal species Aspergillus, Mucor, Rhizopus, Alternaria, Penicillium and Cladosporium were frequent. The indoor and outdoor bacterial concentration of AU High School of Chinna Waltair area of Visakhapatnam was evaluated using the Andersen six-stage cascade sampler [174]. The concentration of bacteria in eosin methylene blue (EMB) plates was the lowest, while that in nutrient agar media in both the indoor and outdoor environments was the highest. However, relatively high frequencies of bacteria were reported in summer season than in rainy and winter seasons. Kumari et al. [174] also investigated the indoor and outdoor environments of two private and two government schools in Chinna Waltair area by using the Andersen six-stage viable impactor. They reported higher bacterial levels in the outdoor environments of all the schools as compared to indoor environments. In addition, bacterial species, such as Bacillus, Micrococcus and Staphylococcus, were the dominant bacterial genera reported. Bomala et al. [171] evaluated both the indoor and outdoor environments of public places, such as RTC complex, government school and college buildings, by the principle of the Andersen air sampler (five stages) for microbiological contamination in air. They reported that the most significant source of fungal aerosols is outdoor environment as well as the soil, water, plant, etc. Moreover, areas affected by frequently large human traffic were significantly more microbially contaminated compared to the school principal’s room and the university laboratories, which were least contaminated as a result of the specific characteristics of the rooms. Recently, Lalitha [175] carried out an analysis of bacterial and fungal load in indoor environment of 25 areas in Visakhapatnam city by the settle plate technique. They interpreted that the areas that are rich in vegetation, such as Arilova and Simhachalam, have less number of bacterial and fungal loads and placed in green zone. However, places such as King George Hospital (KGH), Jagadamba, Daba Gardens and Old town have maximum counts and were in red zone. In addition, gram-positive and spore-forming bacteria ruled over gram-negative bacteria. Moreover, the reported fungal colonies were Cryptococcus sp. (13%), Cladosporium sp. (11%), Penicillium sp. (10%), Alternaria sp. (9%) and Rhodotorula sp.

5.4.2 Hyderabad

Hyderabad is the capital of Telangana. In 2013, there was an article in The Times of India, making it official that the ambient air of Hyderabad was not suitable for breathe (Andhra Pradesh Pollution Control Board report). Aparna et al. [176] carried out an analysis of fungal and pollen bio-aerosols in the ambient air at various major junctions of greater Hyderabad area by using the Rotorod sampler. Most of the sampling was done in the outer habitats. They reported high concentration of Alternaria, Cladosporium and Helminthosporium. However, Mahatma Gandhi bus station, Charminar, Uppal and Dilsukhnagar contributed high concentration of fungal species, while the lowest value was detected in Kukatpally.

5.4.3 Kerala

Kerala is a south-western state situated at the Malabar Coast of India, which experiences monsoon (June-November; 21–30°C), winter (December-February; 18–33°C) and summer (March-May; 23–37°C) seasons. Jothish and Nayar [177] analysed the airborne fungal spores focus in indoor and outside environments of a sawmill in Palakkad district utilising the Burkard private slide sampler. They revealed higher spore concentration in the indoor environment than the outdoor environment. The dominated airspora were Aspergillus/Penicillium, Cladosporium, Nigrospora and Ganoderma, whereas Aspergillus/Penicillium was predominant in the indoor environment and Cladosporium in the outdoor environment.

The viable bio-aerosols were well studied in the southern part of India; most of the sampling was based on the sedimentation method, the Andersen six-stage cascade sampler, the Rotorod sampler and the Burkard personal slide sampler. The research group of Gunthe also performed extensive studies on fungal and bacterial bio-aerosols, their identification based on 16S rDNA sequencing as well as the seasonal variability of allergenic fungal aerosols and the ice-nucleating bacterial species over southern tropical India.

6. Bio-aerosol sampling and characterisation

All kinds of bio-aerosol sampling methods, such as filtration, impaction, impingement, cyclone, gravity sampling and electrostatic precipitation, have been developed for the measurement of bio-aerosols. Different types of filter membrane, such as glass filter, quartz filter and polytetrafluoroethylene filter of different pore sizes, have been widely used for the collection of bio-aerosol samples. In addition, different types of cascade impactors have been implemented successfully for the measurement of size-segregated bio-aerosols [2]. The culture-based methods, such as diverse forms of Polymerase chain response (PCR), actual time PCR (RT-PCR), quantitative PCR (qPCR), denaturing gradient gel electrophoresis-PCR (DGGE-PCR), restriction fragment length polymorphism (RFLP) and 16S rDNA/rRNA sequencing, for the identification of each culturable and non-culturable bacterial and fungal bio-aerosols have put the bio-aerosol analysis a promising path to study the variety of atmospheric microflora. Moreover, ultraviolet aerodynamic particle sizer (UV-APS) [178] and ultraviolet induced fluorescent (UV-LIF) [179] were used for the real-time detection of atmospheric bio-aerosols. Recently, Nasrabadi et al. [180] characterised airborne live and dead bacteria by using LIVE/DEAD BacLight viability kit together with UV-APS.

7. Bio-aerosol characterisation in international perspectives

Considering the international perspectives, the researchers worldwide carried out rigorous work on bio-aerosols by using efficient and advanced techniques. However, in India, the published literature suggested that most of the work on bio-aerosols is based on culture-dependent methods [2, 20, 28, 75, 76, 91, 181, 182] to isolate and quantify only viable bacteria and fungus of atmospheric importance, which are a fraction of the total microbial diversity of atmosphere. Moreover, most of the identification of culturable bacterial and fungal aerosols is based on morphology, shape, size, specific selected media, staining (gram staining, endospore staining and lactophenol cotton blue staining), biochemical analysis and previously published literature [2, 31]. In addition, few researchers followed DNA extraction, PCR, RFLP and 16S rDNA/rRNA sequencing for the identification of the culturable bacterial and fungal aerosols [2, 20, 76]. However, Gangamma [28] tried to quantify endotoxin concentration (EU/m³) in ambient air of specific site, that is municipal wastewater treatment plant in greater Mumbai. Recently, Valsan et al. [181, 182] have used UV-APS and UV-LIS to characterise biological aerosols in real time in the southern region of India.

8. Future direction

In India, there is still a substantial lack of information in the field of bio-aerosols; hence, the following areas could be deeply studied to mitigate the present situation as well as the climate change.

• the diversity of both culturable and non-culturable bacterial and fungal aerosols and also endotoxin in different environment

• suitable sampling method to avoid biased data analysis

• pathways for their transmission and their sustainability in atmosphere.

9. Conclusion

In the early nineteenth century, the investigation on bio-aerosols was initiated to grasp the prevalence and dispersion of microorganisms and spores in air, which suggested that these were released from the biosphere to atmosphere. Since then, research on bio-aerosols and their impact on environment have been going on to improve the air quality and, hence, reduce the health risk. Worldwide, various research groups have deeply explored the bio-aerosols, their occurrence, transformation, role in climate change, interaction and impact on living organisms, agriculture and ecosystem from molecular to global scales until now. Recently, in East Asia, a significant number of research papers (on bio-aerosol) have been published. Furthermore, other researchers are also involved in intense work on bio-aerosols to understand the science and their influence with the meteorological parameters to mitigate the present climatic situation. Recent space is the time to merge the prevailing and novel methods of multidisciplinary fields, that is atmospheric chemistry, aerobiology and molecular biology to acknowledge the long-sought mechanisms of bio-aerosol transport and decay. Along with these, bio-aerosols can even be discovered useful as forensic fingerprints.

Acknowledgments

The authors acknowledge the Director of the CSIR-IMMT and Head of E&S Department, CSIR-IMMT, Bhubaneswar for their expression of approval and support. B.R. and T.D. are grateful to ISRO-GBP (ATCTM & ARFI) for the financial support. J.N. is thankful to DST for the financial support.