Ambient air serves as a medium for the dispersal of various microorganisms, including dangerous bacteria and fungi, which pose significant risks to human health. These airborne pathogens may contribute to a range of respiratory and systemic diseases, particularly among individuals with compromised immune systems. This review explores the types of harmful bacteria and fungi commonly found in the air, their modes of transmission, associated health risks, and the factors influencing their presence in the atmosphere. Special emphasis is placed on the epidemiology of airborne infections and the global importance and need of controlling microbial contamination in indoor and outdoor environments.

The air we breathe is not just a mixture of gases. It also harbors various microorganisms, like bacteria, fungi, viruses, and pollen. While many of these microorganisms are harmless or maybe even beneficial, others can cause serious health issues. Airborne bacteria and fungi are of particular concern owing to their ability to trigger infections in humans. This leads to a range of diseases like respiratory infections, allergies, and in some cases, systemic conditions. Environmental factors like temperature, humidity, and pollution can influence the proliferation and dispersion of these harmful microorganisms in the air. This article reviews the dangerous bacteria and fungi commonly found in the ambient air, focusing on their pathogenesis, the diseases they cause, and their environmental and human-related impacts. Particles suspended in air are termed aerosols. They pose a threat to human health mainly through respiratory intake and deposition in nasal and bronchial airways. In addition, soil or dust particles can act as a “raft” for biological entities known as bioaerosols. Biological contaminants include whole entities like bacterial and viral human pathogens. They also include airborne toxins, which can be parts or components of whole cells (1).

Airborne bacteria are a significant concern due to their potential to cause various diseases, especially when inhaled in large quantities or by vulnerable individuals. Conditions like sick building syndrome and Allergic bronchopulmonary Aspergillosis (ABPA) can be attributed directly to microbial toxins and fungal conidia remaining dispersed in ambient air, respectively. Sick building syndrome produces symptoms like running nose, reddish eyes with discharge and headache. ABPA is a restrictive chest disease characterised by production of granular material by cough. The most common dangerous airborne bacteria include.

Mycobacterium tuberculosis

Overview: M. tuberculosis is the causative agent of tuberculosis (TB), a chronic infectious disease that primarily affects the lungs but can spread to other parts of the body.

Transmission: TB is transmitted through the air when an infected person coughs, sneezes, or talks, releasing bacteria into the surrounding environment.

Health Risks: Inhalation of airborne M. tuberculosis can lead to active TB infections, which can be life-threatening if left untreated.

Streptococcus pneumoniae

Overview: S. pneumoniae is a bacterium that causes pneumococcal infections, including pneumonia, meningitis, and otitis media.

Transmission: This bacterium spreads through respiratory droplets from infected individuals.

Health Risks: Pneumonia caused by S. pneumoniae can be severe, especially in young children, elderly individuals, and those with underlying health conditions like asthma or diabetes.

Legionella pneumophila

Overview: The bacterium Legionella pneumophila is responsible for Legionnaires’ disease, a severe form of pneumonia. It was first recognized in 1976 as a cause of Legionnaire’s disease.

Transmission: Legionella is commonly found in water systems but can become airborne in mist or aerosols, especially in poorly maintained cooling systems, hot tubs, and air-conditioning units.

Health Risks: Inhalation of contaminated aerosols can cause a severe respiratory illness, with symptoms including fever, cough, and muscle pain.

Escherichia coli and Salmonella spp., Staphylococcus aureus 

Overview: While not typically airborne, E. coli and Salmonella can be aerosolized in environments with contaminated water or waste, and cause gastrointestinal and systemic infections when ingested or inhaled.

Transmission: These bacteria spread through fecal contamination, particularly in areas of poor sanitation, and can become airborne under certain conditions.

Health Risks: Inhalation or ingestion can lead to gastrointestinal distress, fever, and in severe cases, septicemia or kidney failure.

Staphylococcus aureus, especially MRSA can spread in ambient air and cause infections like lung and skin infections.

Bacillus cerues

Bacillus cereus is especially Important and dangerous in air, in places like Ophthalmology operation theatres, and food preparation areas in hospitals.

Dangerous Airborne Fungi

Airborne fungi are also significant pathogens, contributing to respiratory and systemic infections, particularly in immunocompromised individuals. The most dangerous airborne fungi include:

Aspergillus spp.

Overview: The genus Aspergillus contains several species that can be harmful to humans, including Aspergillus fumigatus, A. flavus, and A. niger. These fungi are common in the air, especially in environments with organic matter like compost heaps, decaying vegetation, and stored grains. ABPA is caused commonly by A. fumigatus and also by A. flavus. 

Transmission: Aspergillus spores are easily dispersed into the air and inhaled, especially in occupational settings, construction sites or poorly ventilated areas.

Health Risks: Exposure to Aspergillus spores can lead to conditions such as allergic bronchopulmonary aspergillosis (ABPA), allergic rhinitis, and invasive aspergillosis, a life-threatening disease that often affects immunocompromised patients.

Penicillium spp.

Overview: Penicillium species are widespread in the environment and can be found in indoor and outdoor air. While some strains are beneficial in the production of antibiotics, others can cause health problems.

Transmission: These fungi release airborne conidia (spores) that can be inhaled.

Health Risks: Penicillium spores are linked to respiratory issues such as asthma, rhinitis, and in severe cases, fungal infections in individuals with weakened immune systems.

Histoplasma capsulatum

Overview: Histoplasma capsulatum is a dimorphic fungus that causes histoplasmosis, an infection that primarily affects the lungs.

Transmission: The fungus is typically found in soil contaminated with bird or bat droppings. Its spores become airborne when the soil is disturbed.

Health Risks: Inhalation of Histoplasma spores can lead to flu-like symptoms, respiratory distress, and chronic lung disease in severe cases. In immunocompromised individuals, the infection can disseminate to other organs.

Coccidioides immitis

Overview: Coccidioides immitis is the causative agent of coccidioidomycosis, commonly known as Valley fever.

Transmission: This fungus thrives in arid environments, particularly in areas of the southwestern United States, and its spores are released into the air during dust storms or soil disturbance.

Health Risks: Inhalation of Coccidioides spores can cause respiratory symptoms, including fever, cough, and chest pain. Severe cases may result in disseminated coccidioidomycosis, affecting multiple organs.

Aerial microbes in special situations. The microbial population detected in the air of vaccine production units consist mainly of bacteria from the genera Staphylococcus sp, Micrococcus sp and Bacillus sp, and filamentous fungi from the genus Penicillium sp, along with some other fungal genera (2).

Factors Influencing the Presence of pathogenic Microorganisms in Ambient Air:

The prevalence of dangerous bacteria and fungi in the air is influenced by various environmental and anthropogenic factors, including:

 Climate and Weather Conditions

Temperature, relative humidity (RH), precipitation and other factors like solar radiation play a crucial role in the survival and dispersal of airborne microorganisms (3). For example, fungal spores are more likely to become airborne in dry, warm conditions. However, bacteria and fungal spores thrive in air maximally at a relative humidity of 60-90% as found in our studies. Again, as per reports, viruses with lipid envelopes tend to survive longer at a lower RH (20–30%) (4).

Urbanization and Pollution

Urban areas, with their high concentration of people, vehicles, and industrial activities, often experience elevated levels of air pollution. Polluted air can harbour a range of microorganisms, including pathogenic bacteria and fungi, which are dispersed easily in respiratory droplets or dust. Microbes may also be transported over thousands of kilometres by dust and fine sand (5).  Human Activity and Occupation. Certain demographic groups, like children and the elderly, immunocompromised persons, and various categories of workers are particularly exposed and vulnerable to the harmful effects on health of air microbial pollution (6). Certain occupations, like farming, construction, and healthcare, increase the likelihood of exposure to harmful airborne pathogenic microbes. For example, workers in agriculture may be exposed to Aspergillus and Histoplasma spores, while healthcare workers may encounter Mycobacterium tuberculosis.

How to detect bacteria and fungi in air? 

Active and passive methods are there for surveying these bacteria and fungi in air. Active methods include slit sampler and centrifugal sampler. Passive methods include settle plate method using Nutrient agar and Sabouraud’s dextrose agar. Here the plates are kept open, for 3 hours, 3 feet away from walls and 3 feet above ground, for 2-3 hours. Following this, plates are closed and incubated. Other media have also been tried by us successfully for this purpose, like Milk agar and Egg yolk agar containing mannitol and Andrade’s indicator. Some studies show that the passive method is a better monitoring tool than the active method. So, people recommend using passive air sampling method over the active method, since the passive method is easy, cheap, and no instrument is needed for sampling air (7). Sometimes microbial DNA itself can be detected in ambient air using molecular methods. An illustrative image of settle plate technique using Egg yolk agar with mannitol and Andrade’s indicator is shown below in figure1.

Figure 2 shows pink colonies of mannitol-fermenting Staphylococcus aureus on Egg yolk agar with mannitol and Andrade’s indicator. Preventive Measures and Control Strategies To reduce the risk of exposure to airborne pathogens, several strategies can be implemented:

1. Air Filtration: Installing high-efficiency particulate air (HEPA) filters in homes, offices, laboratories and hospitals can significantly reduce the concentration of airborne microorganisms, dust and pollen (8).
2. Proper Ventilation: Ensuring adequate ventilation, especially in crowded or enclosed spaces, helps reduce the build-up of pathogens in the air. At least 12 air changes are needed in 1 hour for ensuring good and safe air from microbiological viewpoint. The CDC recommends that whenever possible, one should aim for 5 or more air changes per hour (ACH) of clean air to help reduce the number of viral particles suspended in air (9).
3. Personal Protective Equipment (PPE): In occupational settings where exposure to harmful microorganisms is high, using PPE such as masks and respirators can reduce the risk of inhalation.
4. Hygiene and Sanitation: Regular cleaning of environments prone to microbial growth, such as hospitals, agricultural sites, and water systems, can limit the spread of airborne bacteria and fungi.
5. Avoiding noisy inoculation with loop, from broth and good pipetting practices. Then the count of aerial microorganisms will diminish.
6. Avoiding construction site, and old caves and mansions, where Aspergillus spp. and Histoplasma spp. are found in abundance, respectively, may help protect oneself from airborne microbes. A significant increase in the colony counts of moulds occur after demolition activities (10).

Microbial contamination of laboratories, operating theatre (OT) and other intensive care units (ICU) in hospitals have continued to have major problems leading to nosocomial infections (11). Airborne bacteria and fungi represent a significant threat to public health, especially in regions with high levels of pollution or in settings where individuals are at increased risk of infection (12). While many of these pathogens are naturally occurring, human activities like inappropriate pipetting and improper inoculation can exacerbate their presence in the air. Appropriate methods should be employed to detect these pathogens in ambient air, using techniques that suit the laboratory or the facility. Understanding the types of pathogenic microorganisms present, their transmission routes, and effective preventive measures can help mitigate their impact and reduce the incidence of respiratory and systemic diseases caused by these pathogens. By continuing to monitor and control airborne microorganisms, it is possible to reduce their impact on public health, particularly for vulnerable populations, and ensure safer living and working environments.

Microorganisms in air pose threat to the dwellers especially in indoor ambient air. They cause different allergic and invasive disorders. They should be surveyed very meticulously. Many methods like active and passive methods are there for accurate detection of these pathogens. Prevention and control methods should also be focussed upon.