What is Bioaerosol?

• Bioaerosols (short for biological aerosols) are a subclass of particles produced into the atmosphere by terrestrial and marine ecosystems.
• They are made up of both live and nonliving components, including fungi, pollen, bacteria, and viruses. Common bioaerosol sources include soil, water, and sewage.
• Typically, bioaerosols are delivered into the atmosphere by wind turbulence over a surface. Once in the atmosphere, bioaerosols can be transported locally or globally: similar wind patterns and intensities are responsible for local dispersal, whereas tropical storms and dust plumes can transport bioaerosols between continents. Over ocean surfaces, sea spray and bubbles form bioaerosols.
• Human-sensitive infections, endotoxins, and allergens can be transmitted through bioaerosols.
• Meningococcal meningitis outbreak in sub-Saharan Africa, which was connected to dust storms during dry seasons, is a notable example. Other dust-related outbreaks include Mycoplasma pneumonia and TB.
• An rise in human respiratory issues in the Caribbean may have been caused by concentrations of heavy metals, microorganism bioaerosols, and pesticides carried over the Atlantic Ocean by dust clouds.
• Charles Darwin was the first to discover the transmission of dust particles, but Louis Pasteur was the first to study the activity of germs in the air. Prior to Pasteur’s study, several bioaerosols were grown and isolated in laboratory cultures.
• Since not all microorganisms can be cultivated, many were unidentified before DNA-based methods were developed. Pasteur also established experimental methods for measuring bioaerosols and shown that microbial activity was greater at lower altitudes and reduced with increasing altitude.
• Included among bioaerosols are fungi, bacteria, viruses, and pollen. Their highest concentrations are in the planetary boundary layer (PBL) and decrease with increasing altitude. The survival rate of bioaerosols is influenced by a variety of biotic and abiotic factors, such as climatic conditions, ultraviolet (UV) light, temperature and humidity, as well as resources contained in dust or clouds.
• Bioaerosols over maritime settings are predominantly composed of bacteria, whereas those over terrestrial areas are abundant in bacteria, fungi, and pollen.
• The predominance of specific microorganisms and their nutritional sources varies according to time and place.
• Bioaerosols can range in size from virus particles measuring 10 nanometers to pollen grains measuring 100 micrometres.
• Pollen grains are the largest bioaerosols and, due to their weight, are less likely to remain suspended in the air for an extended length of time.
• Therefore, pollen particle concentration drops more rapidly with height than the concentration of smaller bioaerosols such as bacteria, fungi, and possibly viruses, which may be able to persist in the upper troposphere.
• Little is known about the specific altitude tolerance of various bioaerosols at present. However, scientists believe that air turbulence influences the potential locations of various bioaerosols.

Types of bioaerosols

1. Fungi

• Due to the desiccating effects of greater altitudes, fungal cells typically perish during atmospheric transport.
• Nonetheless, it has been demonstrated that certain fungi bioaerosols are able to survive air transit despite exposure to intense UV light.
• Despite the fact that the concentration of fungal spores in bioaerosols rises in conditions of high humidity, these spores are also active under low humidity and in the majority of temperature ranges. Certain fungal bioaerosols increase even at relatively low humidity levels.

2. Bacteria

• In contrast to other bioaerosols, bacteria can complete full reproductive cycles during the days or weeks they survive in the atmosphere, making them a significant component of the air biota ecosystem.
• These reproductive cycles provide credence to an untested idea that bacterial bioaerosols create communities inside the atmosphere’s ecology.
• Bacteria are dependent on water droplets from fog and clouds, which provide them with nutrition and UV light protection.
• Bacillota, Actinomycetota, Pseudomonadota, and Bacteroidota are the four known bacterial groupings that are abundant in aeromicrobial settings throughout the world.

3. Viruses

• Airborne viruses and other diseases are transported. Viruses have the ability to move farther than other bioaerosols due to their smaller size.
• In one scenario, a virus and a fungal spore were simultaneously launched from the top of a building, with the spore travelling only 150 metres while the virus travelled nearly 200,000 kilometres horizontally.
• In one investigation, SARS-CoV-1 and SARS-CoV-2-containing aerosols (5 m) were produced by an atomizer and fed into a Goldberg drum to create an aerosolized atmosphere.
• The inoculum produced cycle thresholds between 20 and 22, comparable to those seen in samples from the upper and lower respiratory tracts of humans. Similar to SARS-CoV-1, SARS-CoV-2 remained alive in aerosols for 3 hours, with a drop in infection titre.
• The average half-life of both viruses in aerosols was between 1.1 and 1.2 hours. The data indicate that the transmission of both viruses by aerosols is possible, as they can remain infectious and alive in suspended aerosols for hours and on surfaces for as long as seven days.

4. Pollen

• Despite being larger and heavier than other bioaerosols, pollen can be carried thousands of kilometres, according to certain research.
• They are a key source of allergens carried by the wind, especially seasonal releases from grasses and trees.
• Pollen records can be interpreted by tracing the distance, movement, resources, and deposition of pollen in terrestrial and marine settings.

Collection Methods of Bioaerosol/Deposition of Bioaerosol

Bioaerosols are primarily collected via collection plates, electrostatic collectors, mass spectrometers, and impactors. Other experimental approaches are also utilised. In comparison to other PC filter alternatives, Polycarbonate (PC) filters have produced the most accurate bacterial samples.

1. Single-stage impactors

• Impactors can be stacked to gather bioaerosols falling within a certain size range by capturing the variance of particle matter (PM). A PM10 filter, for instance, allows tiny particles to pass through. T
• His size is comparable to that of a human hair. At the impactor’s base, particulates are deposited onto slides, agar plates, or tape.
• The Hirst spore trap samples at a rate of 10 litres per minute (LPM) and incorporates a wind vane to ensure that samples are always taken in the direction of wind flow. The collected particles are struck onto a petroleum-greased vertical glass slide.
• Variations, such as the seven-day recording volumetric spore trap, have been created for continuous sampling utilising a gently spinning drum that drops impacted material onto a coated plastic tape.
• The airborne bacteria sampler can collect samples at up to 700 LPM, enabling for rapid collection of large samples. The impact and deposition of biological material onto an agar-lined Petri dish allows colonies to form.

2. Cascade impactors

• Similar to single-stage impactors in terms of collection methods, cascade impactors contain various size cuts (PM10, PM2.5) that allow bioaerosols to segregate by size.
• Due to distinct types of organisms dominating various size ranges, separating biological material by aerodynamic diameter is beneficial (bacteria exist range from 1–20 micrometres and pollen from 10–100 micrometers).
• The Andersen line of cascade impactors is most commonly utilised to assess air particle concentrations.

3. Cyclones

• A cyclone sampler consists of a circular chamber with one or more tangential nozzles through which the aerosol stream enters.
• A cyclone sampler, like an impactor, relies on the inertia of the particle to induce it to deposit on the sampler wall as the air stream curves inside the chamber.
• Similarly to an impactor, the collection efficiency is proportional to the flow rate. Cyclones are less susceptible to particle ricochet than impactors and can capture more material.
• Additionally, they may provide a more gentle collection than impactors, hence enhancing the recovery of live microorganisms.
• However, cyclones tend to have collection efficiency curves that are less sharp than impactors, and it is simpler to design a compact cascade impactor compared to a cascade of cyclone samplers.

4. Impingers

• Impingers have been created to impact bioaerosols into liquids, such as deionized water or phosphate buffer solution, rather than onto a greased substrate or agar plate.
• Ehrlich et al. (1966) demonstrate that the collection efficiency of impingers are often higher than comparable single stage impactor designs.
• Impingers available for purchase include the AGI-30 (Ace Glass Inc.) and Biosampler (SKC, Inc).

5. Electrostatic precipitators

• Electrostatic precipitators, or ESPs, have lately attracted renewed interest for bioaerosol sampling due to their highly efficient particle removal efficiency and more gentle sampling procedure than impinging.
• ESPs charge and remove incoming aerosol particles from an air stream using a non-uniform electrostatic field and a strong field strength between two electrodes.
• This generates a zone of dense ions, known as a corona discharge, which charges entering aerosol droplets, and the electric field deposits the charged particles onto the collection surface.
• Considering that biological particles are often analysed by liquid-based assays (PCR, immunoassays, viability assay), it is preferred to collect samples directly into a liquid volume for subsequent analysis.
• Pardon et al. demonstrate sampling of aerosols down to a microfluidic air-liquid interface, while Ladhani et al. demonstrate sampling of airborne Influenza down to a tiny droplet of liquid.
• Low-volume liquids are useful for minimising sample dilution and have the potential to be coupled with lab-on-a-chip technologies for speedy point-of-care analysis.

6. Filters

• Filters are frequently used to capture bioaerosols due to their low cost and ease of usage. Filter collection is particularly advantageous for personal bioaerosol sample due to the fact that filters are light and unobtrusive.
• A size-selective input, such as a cyclone or impactor, can precede filters to remove larger particles and classify the bioaerosol particles by size.
• Aerosol filters are frequently referred to by their “pore size” or “equivalent pore diameter.”
• Note that the filter pore size does NOT reflect the smallest particle size that will be collected by the filter; in reality, aerosol filters typically collect particles that are significantly smaller than the nominal pore size.

Transport mechanisms of Bioaerosol

Ejection of bioaerosols into the atmosphere

• Typically, bioaerosols are delivered into the atmosphere by wind turbulence over a surface. In most situations, once airborne, they remain in the planetary boundary layer (PBL), but in some instances they reach the upper troposphere and stratosphere.
• Once in the atmosphere, bioaerosols can be transported locally or globally: similar wind patterns and intensities are responsible for local dispersal, whereas tropical storms and dust plumes can transport bioaerosols between continents.
• Over ocean surfaces, sea spray and bubbles form bioaerosols.

Small scale transport via clouds

• The PBL contains the largest concentration of bioaerosols close to the Earth’s surface. Here, turbulence in the wind induces vertical mixing, transporting particles from the ground into the atmosphere.
• Bioaerosols put into the atmosphere can cause the formation of clouds, which are then blown to other areas and precipitate as rain, hail, or snow.
• During and after rain events, increased amounts of bioaerosols have been recorded in rain forests. Marine bacteria and phytoplankton have been associated with cloud formation.
• However, for the same reason, bioaerosols cannot be transported over long distances in the PBL, as they will eventually be precipitated out by clouds. In addition, increased turbulence or convection at the upper limits of the PBL would be required to inject bioaerosols into the troposphere, where they might be transported over greater distances as part of tropospheric flow. This reduces the bioaerosol concentration at these elevations.
• Cloud droplets, ice crystals, and precipitation utilise bioaerosols as nuclei around which water or crystals can form or adhere to their surfaces. These interactions demonstrate that air particles can affect the global hydrological cycle, meteorological conditions, and weathering. Climate change can amplify the effects of these alterations, such as desertification. When pure air and smog meet, bioaerosols also mix, altering visibility and/or air quality.

Large scale transport via dust plumes

Possible worldwide highways for dust-borne bioaerosols include:

• Storms in Northern Africa can transport dust to the Americas or Europe across the Atlantic Ocean. There is a seasonal shift in the destination of transatlantic transport dust: North America in the summer and South America in the winter.
• Dust from the Gobi and Taklamakan deserts is transported to North America, mainly during the Northern Hemisphere spring.
• Australia’s dust is carried into the Pacific Ocean, where it has the potential to settle in New Zealand.

Community dispersal

• Transport and dispersion of bioaerosols are inconsistent across the world. Although bioaerosols may travel thousands of kilometres prior to deposition, their final distance and direction are determined by meteorological, physical, and chemical parameters.
• Observational measures were used to create an airborne bacteria/fungi map of the United States. The resulting community profiles of these bioaerosols were linked to soil pH, mean annual precipitation, net primary production, and mean annual temperature, among other things.

References

• Douwes, J., Eduard, W., & Thorne, P. S. (2008). Bioaerosols. International Encyclopedia of Public Health, 287–297. doi:10.1016/b978-012373960-5.00281-1