Aerosol Science & Engineering 

http://www.aerosols.wustl.edu/education/

This interactive and animated computer program is designed for aerosol science and engineering education in the introductory level. Each module in the program contains a narrative of the background/principles and a web calculator/simulator. Through the various modules, you'll be able to learn what aerosol is about, how aerosol moves, how we can measure or collect aerosol, how the aerosol in the atmosphere influences life, and how we can assess the health effects of aerosol.

Learning About Cyclones	Optical Particle Counter	Respiratory Deposition
Atmospheric Aerosol	Aerosol Transport - Inertia	Aerosol Instrumentation
Aerosol in Health Care	Thermophoresis	Bioaerosol

Bioaerosol Basics

While all bioaerosols are biological in origin by definition, an important attribute is whether the bioaerosol is living. Based on this attribute, bioaerosols are categorized into two very important classifications: viable and non-viable. Non-viable bioaerosols are not currently alive and, therefore, cannot multiply; aerosolized pollen, animal dander and saliva, and insect excreta are all forms of non-viable bioaerosols. In contrast, viable bioaerosols are living organisms that demonstrate microbiological activity and have the potential to multiply. These include airborne bacteria, fungi, and viruses, of which bacteria and fungal spores are the two most prevalent bioaerosols present.

Individual bioaerosol particles can range in size from approximately 0.02 to 100 micrometers in diameter, depending on the type and source. However, they also frequently agglomerate in clusters, thereby forming larger particles.

Figure 3: Individual particle sizes for some common bioaerosols.

Important properties characterizing bioaerosols are size, viability, infectivity, allergenicity, toxicity, and pharmacological activity. For a bioaerosol to be infectious or pathogenic (cause disease), it must be viable. However, non-viable bioaersols can still cause allergies or toxic reactions. Note that an innate characteristic of bioaerosols is that these properties may change with time, which can play an important role in sampling, especially for the viable organisms.

The following sections will cover the different types of bioaerosols, including a quick review of microbiology, to aid in your understanding of viable bioaerosols and concepts related to them.

III. VIRUSES

Another type of microorganism of utmost important to the field of bioaerosols are viruses. A virus is simply a DNA strand inside of a protein capsule. Viruses require a host for survival and do not carry out metabolic processes. They do not belong to any of the three domains of life because viruses are not self-sustaining entities. Viruses are significantly smaller than bacteria and fungi, generally ranging in the 20-300 nanometer range. Their small size poses significant challenges in attempts to sample for their presence. Infamous virus pathogens include the influenza, SARS, smallpox, and chickenpox viruses. A schematic of the HIV virus is shown in the figure below.

Figure 6: Schematic of the HIV virus.³

I. HEALTH EFFECTS

Bioaerosols can produce a wide range of health effects. Recall from earlier that for a bioaerosol to be infectious (pathogenic), it must be viable. However, non-viable bioaersols can still cause allergies or toxic reactions. Allergy sufferers can be affected by airborne biological matter. People with compromised respiratory systems, such as those with asthma and emphysema, can suffer respiratory sensitization attacks caused by bioaerosols as well. As seen in Figure 7 below, the respiratory tract may contract (known as bronchiconstriction) in the presence of allergens or other irritants, making breathing difficult. Extreme health effects associated with bioaerosols involve epidemics or bioterrorism transmitted via the airborne route.

On a daily level, countless people are afflicted with allergies or respiratory sensitization reactions, such as asthma, caused by interactions with fungi, pollen, and dander. Allergies are estimated to cost our economy close to $7 billion annually.⁴ There are typically welfare effects associated with many of the health effects.

Figure 7: Nasal and respiratory reactions to allergies.

A good example of the effect that bioaersols can have on the respiratory system is the effects of the Red Tide Blooms in the Gulf Coast, which are large blooms of algae that are often dangerous to local aquatic life. These algae contain a toxin that is released when the cells are broken up by the energy associated with the waves. The toxins are released into the air as non-viable bioaerosols, causing extreme eye and respiratory irritation. This irritation is especially a concern for those with compromised respiratory systems.

The more extreme health effects include the resulting illnesses due to pathogenic bioaerosols. Sources of pathogenic bioaerosols include humans, animal houses, wastewater treatment plants, and biosolids storage units. Examples of pathogenic organisms that are transmitted through the airborne route and the resulting disease are listed in the following table. More details on pathogenic microorganisms and their transmission will be included in Section 6.

Table 1: Examples of Pathogenic Microorganisms and their Resulting Diseases

Legionella pneumophila

Legionnaires' Disease

Mycobacterium tuberculosis

Tuberculosis

Bordetella pertussis

Whooping Cough

Yersinia pestis

Pneumonic Plaugue

Bacillus anthracis spore

Anthrax

Variola vera

Smallpox

Herpesvirida, HHV-3

Chickenpox and Shingles

Measles virus

Measles, Mumps, and Rubella

SARS coronavirus

Severe Acute Respiratory Syndrome (SARS)

	Species	Approximate Size	Resulting Disease
Bacteria		0.6 micron^5
			2-4 micron in length^6
			0.45 micron^7
			2 micron in length^8
			1-3 micron^9
	Virus		200-300 nm^10
			100-200 nm^10
			125-250 nm^10
			80-160 nm^10

II. PROTECTION

Protection varies depending on the location and type of bioaersol present. Personal protection refers to protective measures used for individuals. Collective protection refers to efforts made on a larger scale, such as a building.

Respirators are often used for personal protection. Simple respirators are often sufficient in many scenarios to prevent transmission, but a self-contained breathing apparatus (SCBA) might be necessary in the instance of extreme cases such as biological warfare.

Other personal protection includes gloves, hand-washing, and changing clothes following exposure. In the event of an infection, antibiotics are useful against pathogenic bacteria. Antibiotics are not effective against viruses, but vaccinations exist for some viruses though it must be administered prior to exposure. People with allergies and asthma can use medications but may have to be more careful during certain seasonal times of the year.

Collective protection targets a much wider audience. For instance, in buildings such as hospitals where nosocomial infections are common, UV units can be set up in the central air system or in the room to inactivate bioaerosols and prevent their growth. Other methods to prevent distribution through the central air system include filtration and electrostatic precipitators (ESP) .

Figure 8: Varying degrees of respirator protection.¹¹

The top left represents a simple particulate respirator, designed only for low-hazard situations. The top right portrays a gas mask respirator. The bottom right portrays a SCBA and the tank which provides its clean air supply. The bottom left portrays a Powered Air-Purifying Respirator

Welfare Effects of Bioaerosols

The welfare effects of bioaerosols are extensive, ranging from crop and livestock damage to the economic effects due to health effects.

Airborne fungi, bacteria, and viruses can be a serious detriment to the agricultural sector. One of the most infamous welfare effects of bioaerosols was due to the fungus, Phytophthora infestans, which devastated Ireland in the middle of the 19th century. Over a period of three years, this airborne fungus decimated the potato crop of Ireland, leading to a famine that claimed the lives of an estimated 500,000-1,000,000 people. Another 2,000,000 people left the country because of the dire circumstances in what is known as the Irish Potato Famine. This fungus single-handedly altered the course of history for Ireland. Figure 9: The start of the Irish Potato Famine caused by a foreign fungus carried by ships and wind to Ireland. The fungus multiplied on the potato plants, causing its leaves to rot and wither, and was then transported by the wind to surrounding plants.

Livestock can also be infected with pathogenic microorganisms, similar to humans. In the close quarters of typical animal houses, diseases are easily transmitted from animal to animal. In 1991, Bovine Respiratory Disease cost the cattle industry an estimated $624 million in cattle deaths in the US.

Another example of economic loss is the airborne toxins created from Red Tide algal blooms discussed in the previous section. The effects of the bioaerosols are so uncomfortable, the tourism industry in the many vacation towns on the Gulf of Mexico shore can take a big economic hit.

Other facets of welfare effects caused by bioaerosols are those related to health. Fully grasping all of the costs associated with health care, lost productivity, and general discomfort due to allergies, respiratory sensitization, and pathogenic bioaerosols would be difficult. Allergies are believed to cost our economy close to $7 billion annually, with close to $6 billion resulting from medications and medical visits and another $1 billion in lost productivity⁴. Beyond that, many sufferers would find it difficult to place a cost on the misery of the experience.

Pathogenic Transmission

Because pathogenic bioaerosols can cause such extreme health effects in humans, many members of the bioaerosol field study their behavior and their effects. Pathogenic transmission can occur through several routes: person-to-person, waterborne, foodborne, vector-borne (rodent- and insect-based), airborne, or a combination of several of these. Although there are many potential forms of transmission, scientists in the air quality field of bioaerosols focus largely on pathogenic aerosols that propagate via airborne transmission. Pathogens that are transmitted primarily through the airborne route include SARS and Bird Flu, two diseases that have the potential to create epidemics.

Many pathogens can be transmitted through multiple methods. For instance, the Black Plauge of the 14th century was caused by a bacterial pathogen (Yersinia pestis), which was transmitted by both vector-borne (fleas) and airborne (sneeze or cough) transmission. Fleas containing blood from infected rats served as reservoirs for the disease. If a carrier flea then bit a person, the disease could be transmitted via the vector route. If an infected person sneezes or coughs, the bioaerosols formed can be inhaled by another person, thus propagating the disease via the airborne transmission route.

Figure 10: Example of the many paths of transmission during the Black Plague.

Enteric diseases (fecal-oral transmission) can also be acquired from bioaerosols. Common enteric pathogens include Escherichia coli and Salmonella. If a bioaerosol is inhaled, a human's natural defense mechanisms can dislodge it from the respiratory system and redirect it toward the mouth. At that point, the pathogen is preferably exhaled but potentially swallowed. If swallowed, the pathogen can then enter the enteric route and propagate the disease.

Please refer to Respiratory Deposition module for more information about the body's natural defense mechanisms against bioaerosols.

Indoor Bioaerosols

Indoor air quality is a significant concern to professionals in the bioaerosol field. Office buildings, hospitals, dormitories, airplanes, and similar climate-controlled structures that rely largely on re-circulated air sustain abnormally large numbers of viable bioaerosols, often including pathogenic bioaerosols.

There are certain natural inactivation mechanisms for microorganisms that are very effective. Sunlight and natural oxidants (e.g. ozone and free radicals) in the ambient air can inactivate them, and many microorganisms do not survive well in low relative humidity. However, ductwork provides airborne microorganisms with a moist environment, protected from sunlight and free radicals, thereby eliminating the most effective means of inactivation. Once a pathogenic bioaerosol is released indoors, the ductwork can preserve it. The central air system then distributes them to the entire structure, spreading the pathogens incredibly effectively and exposing numerous people as a result (see animation below). Therefore, the threat of pathogenic bioaerosols are especially potent in indoor scenarios.

Indoor air quality is especially a concern in hospitals, where airborne pathogens are common. Infections that are acquired while in the hospital are known as nosocomial infections. Approximately 2.5 million hospital patients each year acquire nosocomial infections¹², prompting hospitals to take a serious look at how to minimize the problems. One response of collective protection has been to provide duct work with UV light to inactivate the airborne pathogens.

Figure 11: Central air systems can protect and distribute bioaerosols.

Bioaerosols as Agents of Bioterrorism

Several pathogenic bioaerosols are currently considered to be potential agents of bioterrorism. As has already been discussed, there are many pathogens that can be transmitted via the airborne route. The airborne transmission of pathogens can be quick and effective, especially in indoor scenarios, causing heightened concerns about their potential use.

Perhaps the most infamous bioaerosol in contemporary times is the spore of the bacteria Bacillus anthracis. The spore formation makes the pathogen resistant to inactivation, and the release into a building with central air quickly contaminates the entire structure.

Another pathogenic bioaerosol of concern is smallpox. Although it was once presumed to be eradicated following a successful global immunization campaign, many countries maintained small stocks for scientific purposes. In the wrong hands, the pathogen could be dangerously potent, especially considering that smallpox immunizations have not been required in the US since the mid-1970s.

People are not the only direct targets of bioterrorism. Airborne crop pathogens also have the potential to devastate countries. Recall the horrendous effect of the fungi in Ireland during the Irish Potato Famine. Although crops and economies are more diverse in present time, the destruction of a portion of a large crop in America could still prove devastating.

I. SAMPLING CONSIDERATIONS

Due to the health and welfare effects of bioaerosols, obtaining exposure limits is crucial to public safety. Therefore, any sampling method for bioaerosols must do well in three categories. Firstly, the sampler must have high inlet efficiency, meaning that the sampler takes in a representative number of aerosols. Secondly, the sampler must have a good physical collection efficiency. The physical collection efficiency describes the sampler's ability to remove the bioaerosols from the air stream; it depends on size, shape, and aerodynamic behavior of the particles¹³. Lastly, the sampler must have a high biological collection efficiency. The biological collection efficiency describes the sampler's ability to maintain the true viability and then quantify the viable count. While inlet efficiency and physical collection efficiency are standards for any aerosol sampling method, the biological collection efficiency is unique to bioaerosol samplers.

In bioaerosol sampling, there are multiple things that ought to be measured for health purposes depending on the type of bioaerosol. For instance, for non-viable bioaerosols, mass or number may be the most important. A person may experience an allergic reaction to pollen or fungi once exposed to a threshold mass of the allergen. For viable bioaerosols, the number of viable microorganism is critical because the infectivity of viruses is measured as a minimum threshold. To determine the viable count, the bioaerosols must be physically captured and then cultured in appropriate conditions. Viable bacteria and fungal spores will form colonies, while viable viruses will form plaques on their host cells.

To maintain high biological collection efficiency, special consideration must be given to preserving the viable count. Otherwise, the results will not be representative of actual levels of exposure to the pathogen. Therefore, the microorganisms should not encounter any harsh conditions, including disinfectants, such as chlorine or alcohol, or high-speed impaction onto sampler walls. The viability is also time-dependent, so immediate care must be taken when sampling. To physically collect bioaerosols, they must be collected into or onto some form of collection medium which will not harm them.

¹³Chapter 24: Biological Particle Sampling, Aerosol Measurement, Baron and Willeke, 2001

II. SAMPLING METHODS: IMPACTOR

Common methods of viable bioaerosol sampling include impactors, impingers, and filtration.

Impactors utilize the bioaerosol inertia to collect the bioaerosol onto a solid or semi-solid collection medium. The impactor forces the air stream to turn a tight corner. If the inertia of the bioaerosol is too great, the bioaerosol will not be able to follow the air flow lines and will instead impact onto the collection medium.

Figure 12: The principle of impaction.

Please visit the Aerosol Transport - Inertia online module to learn more about the principle of impaction. Once the bioaerosols are collected onto the collection medium, they can be cultivated to determine the viable count. Multi-stage impactors can be used to collect a wide range of bioaerosol sizes. Because the impactor utilizes inertia to physically collect particles, its physical collection efficiency is highly dependent on particle size.

III. SAMPLING METHODS: IMPINGERS

Liquid impingers also use inertia to physically collect the bioaerosols. However, they also use diffusion. Rather than having a solid or semi-solid collection medium like the impactor, the collection medium of the impinger is liquid. The air stream is similarly forced to take a tight corner, and the bioaerosols are collected into the liquid by inertial impaction. Bioaerosols can also diffuse out of the air stream into the liquid. Although liquid impingers rely partly on impaction, diffusion also contributes to the physical collection efficiency of very small bioaerosols.

IV. SAMPLING METHODS: FILTRATION

Filters can also be used to sample bioaerosols. Filters use inertia and diffusion to collect particles. Although they have high physical collection efficiency for a wide range of particle sizes, including viruses, extracting the bioaerosols for culturing can be challenging. In other words, filters have high physical collection efficiency but low biological collection efficiency.

V. SAMPLING TIME

Since many viable bioaerosols are cultivated with nutrient agar in petri dishes to determine the viable count, it is important for there to be a suitable number. This is a major limitation of bioaerosol sampling. When sampling bacteria, for instance,if there is too high of a surface density on the petri dish, the colonies might overlap, making it difficult to distinguish one colony from the next. If the surface density of collected bacteria is too low, the values become statistically insignificant. For bacteria, a surface density of approximately one colony per cm² is a good goal⁸. A viable bioaerosol that has the potential to multiply is known as a colony forming unit (CFU).

The following equation aids in determining a suitable sampling time to achieve this surface density⁸.

where t is time, s is the surface density (CFU per unit area), C_N is the average number concentration of bioaerosol particles, and Q is the sampler flow rate.

Recall that many different environments have different concentrations of bioaerosols. For instance, a livestock structure might have a very high concentration of culturable bioaerosols - perhaps 10⁵ viable units per cubic meter of air or more. Outdoor environments tend to have lower concentrations - perhaps in the range of 10²-10³ viable units per cubic meter of air.

Use the following webcalculator to determine appropriate sampling times for varying airborne bacteria concentrations using a single-stage impactor to achieve the goal surface density (1 CFU/cm²). The default impactor has a total deposition area of 75 cm² and operates at a flow rate of 20 Lpm (0.02 m³/min). Try different values to get an idea of how each variable affects the sampling time.

Summary

In summary, bioaerosols are ubiquitous parts of our everyday life. Pollen, mold, and dander are all types of everyday bioaerosols humans encounter. More extreme types of bioaerosols are pathogenic bacteria and viruses, including influenza, SARS, and anthrax. Health risks and welfare effects associated with pathogenic bioaerosols necessitate accurate sampling methods be available, including impaction, impingement, and filtration. Recent and historic events related to bioaerosols has raised the awareness of human interactions with them and lead to an increased quest for knowledge of their behavior.

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