Environmental Science & Technology Air Pollution
Bioaerosols: Nature, Sources and Impact
Bioaerosols, meaning airborne particles derived from microbial, viral, fungal and actinomycete, come in a wide variety of sizes, shapes, compositions and classifications and can all be considered as pollutants.
Bioaerosols can cause three basic conditions:
INFECTIONS,
ALLERGIES,
POISONINGS.
Infections are generally the result of multiplication and growth of microbes inside humans while allergies are the result of exposures to antigens. Poisonings result from contact with microbial toxins, endotoxin and mycotoxin.
Not all infectious organisms cause pathogenic diseases in humans, but those that can are of concern. Well-known diseases associated with occupational exposures include anthrax, Q fever.
Air borne pathogens in this context are defined as “an infection acquired by inhalation of infectious particles, whose source is normally found in soil, organic debris and where the agents exists as a free living form”.
Person to person spread of the infection is almost unknown.
Antigens are capable of stimulating the production of antibodies that produce allergic diseases. Allergic reactions are the result of an antigen producing a response from the immune system.
Hypersensitivity disease is another term for the allergic reactions produced by these agents. These include hypersensitivity pneumonitis, allergic asthma, and allergic rhinitis. Sources of airborne antigens include bacteria, fungi, pollen, insect body parts, and skin scales (dander) and saliva of mammals. In these situations antibody assays on blood from affected individuals may be performed in conjunction with monitoring for bioaerosols.
Sometimes it is not the microbe itself that produces the harmful effect but the fact that it produces a toxin. Botulism is an example wherein the botulinum toxin is the responsible agent. When release of a toxin is involved, the organism can produce a disease without extensive multiplication or dissemination throughout the body.
Just as there are factors that can predispose individuals to the health effects caused by chemicals, certain persons are also at increased risk when exposed to bioaerosols if they are over 50 years old, drink alcohol excessively, smoke, or have preexisting respiratory disease or other illnesses such as diabetes or kidney disease.
Bioaerosols can exist in both viable (living) and nonviable states. Viable microorganisms such as bacteria, fungi, yeasts, and moulds originate from sprays or splashes of media, from the agitations of dusts, and from sneezes and coughs of which only the small particles (<10m) remain in the air long enough to travel any distance. Examples of nonviable agents that are occasionally sampled include pollens and insect parts. Grains, clusters of cells, and skin scales are much larger-sized particles (10—50 µm) than bacteria and viruses. Spores, which can be formed by fungi and certain bacteria, can be both viable and nonviable and are capable of causing disease in both forms. Most techniques attempt to sample for only viable particles as these can be cultured so that they multiply, making identification easier.
The specialized characteristics of viable agents require specialized sampling instruments in order to preserve the organisms for laboratory culture, which is the primary means of identification. Their fragility and temperature, moisture, and nutrient needs are the primary considerations when selecting a sampling device. While passive air sampling is simple and can be done by setting out plates containing culture media, it is not as effective as the use of active techniques involving the use of pumps.
There are two basic methods for collecting these air samples:
(1) Specialized instruments and (2) air sampling trains incorporating a personal air sampling pump, rotameter and media, such as is used for integrated chemical sampling.
The specialized instruments can be used to house culture media and therefore in most cases are preferred to integrated sampling techniques. Area air samples are more commonly collected for bioaerosols than personal samples, regardless of the type of situation being monitored, due to the need to house culture media inside of instruments specialized for sampling viable bioaerosols.
Most air sampling for bioaerosols is related to occupational exposures in hospitals,
laboratories, and research facilities;
certain industrial operations, such as
brewery fermentation,
cotton preparation and ginning,
wool sorting, hemp handling, and
sawmills; and
agricultural operations,
including hay preparation and
the use of biological insecticides and
wastewater and sewer treatment facilities.
pharmaceutical manufacturing plants,
animal laboratories, and food processing plants
air surveys in buildings incorporate both indoor and outdoor air samples for various agents.
Sampling usually attempts to determine whether the agents are being generated from a source within a building rather than from an outside source where they naturally occur.
It has been noted that the majority of fungal spores found indoors are derived from outdoor sources, such as decaying plant and animal materials, while the primary source indoors is human bacteria shedding.
Sources of microbes include organic materials; humidifiers; vaporizers; heating, ventilating, and air conditioning systems (HVAC); as well as their associated equipment, such as cooling towers.
Another situation of growing interest that incorporates both occupational and environmental exposures is bioremediation of contaminated soils and waters using specially engineered "super bugs." These techniques have proved very successful for treating petroleum compounds. In these situations, the release of volatile organic compounds (VOCs) is also a concern, so a boundary line monitoring strategy would need to incorporate both sets of agents.
Industrial wastewater treatment has incorporated the use of various microbes for many years.
Air sampling for bioaerosols may be combined with sampling for chemical agents in some situations such as indoor air surveys and exposures to wood dust or bark. In some situations, monitoring is performed for chemicals where there are metabolic by-products of the organism, such as endotoxin, released by certain bacteria. Biological monitoring may also need to be performed. For example, blood and urine samples have been collected in cases of suspected Legionnaires'- disease, since Legionella pneumophila infections cause the release of antigens to the urine.
Surface contamination may also be a concern.
BACTERIA
Bacterial infections are the most commonly seen in humans, such as those that occur in minor wounds and scratches. Diseases related to bacteria that are found in occupational exposures include those caused by anthrax, also called wool sorter's disease, transmitted by handling imported goat hair, wool, and hides; Brucella canis infections (brucellosis) from the contaminated blood of slaughtered animals; and Leptospira-induced disease (leptospirosis). associated with farm animals, dogs, and rodents. which is spread through contact with infected urine, animal tissue, or water.
Other bacteria of concern in sampling include Staphylococcus and Streptococcus, which are carried by humans and cause infections under the right conditions; Pseudomonas, which cause pneumonia; and bacillus, which is associated with hypersensitivity pneumonitis. Thermophilic bacteria of concern include Thermoactinomyces, known to cause hypersensitivity; and Micropolyspora, Thermomonospora, and Saccharomonospora. Another exposure of concern is the Salmonella bacteria responsible for food poisoning. In this case, ingestion rather than inhalation is the route of exposure. While not strictly occupational in nature, this may be a concern in indoor air investigations.
Rickettsiae are intracellular parasites in fleas, ticks, and lice that are considered to be bacteria. The tick is the most common reservoir and tick bites are the primary route of transmission. Rickettsiae do not appear to produce symptoms of disease in their hosts, but if they are transmitted to humans, a severe and often fatal infection may result. The major rickettsial disease of humans is epidemic typhus. (~ther human diseases are Rocky - Mountain spotted fever and Q fever, both transmitted by ticks, and scrub typhus, normally transmitted by mites to field mice, but also transmissible to humans. The chlamydias, other specialized bacteria, are carried in birds and the primary disease they are associated with, ornithosis, is transmitted by inhalation of dried discharges and droppings of birds.
Most bacteria are 1 µm to 5 µm in size
In order to select the proper sampling medium, it will be necessary to know what the specific characteristics are of the bacteria suspected of being present. It has been suggested that Gram-positive bacteria are more likely to survive during air sampling than gram negative.
Gram-negative bacteria produce endotoxins that are pyrogenic and induce local inflammatory responses. It is currently thought that endotoxins may be responsible for a number of diseases in workers.
Bacteria are widely present in the environment and the presence of certain odours slime, and foam on a surface are often an indication of bacterial growth. Certain types live in the human body while others live outdoors on vegetation; therefore, when sampling indoors, those bacteria associated with humans will predominate while outdoor samples will contain mostly the other type. Bacteria are spread primarily through inhalation, although bacteria that are very small are often dispersed on skin scales. It has been estimated that 7 million skin scales are shed per minute by humans, each containing an average of 4 viable bacteria.
Bioaerosols are usually associated with water, and an undisturbed source of water or a humid environment is highly conducive to growth. When water is aerosolized, droplets range in size, but larger droplets can evaporate and become smaller, thus increasing the potential for inhalation. Taps, showers, whirlpool spas, and cooling towers are all sources. When bacteria attach to particles, they are often protected against environmental stresses.
The primary tools for collecting bacteria for culture are impingers and cascade impactors. Screening samples can be collected with a slit or centrifugal impactor.
Legionella Pneumophila
Legionnaires' disease became a concern when it became apparent that aerosol from contaminated cooling towers could enter fresh air vents and spread this agent to the HVAC system in buildings. Legionella pneumophila, the agent in this case, is a rod-shaped, slow-growing, gram-negative bacterium. Ideal growth conditions require a temperature of 35—45°C and a pH of 6.9—7.0. The bacterium is ubiquitous in the environment. It can coexist with amoebae and can survive and grow on blue-green algae. The vast majority of outbreaks have been associated with Legionella sero group 1, but other sero groups, if detected, are also of concern, and control measures should be initiated if they are detected.
Legionella sero group 1 has been isolated from a variety of surface and potable aquatic habitats. It is viable in tap water for more than a year. Hot-water tanks, and in particular their bottom sediments, are excellent media for its survival and proliferation.
Cooling towers are especially susceptible to contamination, since their primary function involves inducing large amounts of air into large amounts of flowing water; thus, they act as air scrubbers, washing out dust, debris, pollen, insects, and plant materials. These bacteria have also been known to build up in water softeners.
Most sampling for Legionella is done by collecting bulk samples of suspected sources. One source considers heavy contamination by Legionella to be counts greater than 10 colony-forming units per liter in a bulk sample. Sampling should be done in both suspected and background areas. As a precautionary measure, bulk samples of suspect sources of Legionella should be collected every 6 months. Chemical analyses of makeup water and system water should be performed monthly. Samples should not be collected following cleaning or immediately after startup. It is best to sample in the middle or end of each 6-month period during normal operation.
The best places are system water dead spots or slow flowing areas; however, these should be selected so they are not near incoming fresh water or biocide treatment points.
Testing should precede but not follow slug feed of biocide.
In the course of evaluating the potential for this microorganism to be the source of an outbreak, the operational procedures and facilities are considered as important as the sources of microbial contamination and dissemination. An investigation for Legionella in cooling towers would focus on the following areas:
Temperature : If the normal operating temperature of the water is greater than 30°C, there is a high risk of bacterial growth.
Contamination: Nonmetallic materials such as washers, coating, gaskets, linings, and sealants in the system can harbour bacterial growth.
Stagnation : If the water is left standing for more than 5 days at a time, there is a high risk of bacterial growth.
Particulate matter The sump can accumulate sludge, debris, scale and bacterial growth.
Aerosol generation : If there is a significant amount of spray the likelihood of spread is increased, especially if there is any possibility of aerosol escaping from the cooling tower.
Susceptible populations : If the cooling tower is associated with any buildings or sites occupied by susceptible people, such as hospitals or schools, the risk is increased.
Other factors of importance include whether a responsible person is in charge of the cooling system, the type of training provided to the staff responsible for its upkeep, and the availability of adequate record keeping.
If a routine test for Legionella is positive, the cooling tower should be cleaned immediately. Following startup, the system should be resampled within seven days. As Legionella require soluble iron for growth, control of corrosion is an important factor. Often it involves adjusting the pH of the water. Biocides are often used to kill Legionella and other waterborne microbes, but their effectiveness depends on controlling water chemistry including pH, alkalinity, and cycles of concentration (i.e., addition periods).
Endotoxin
Endotoxin is a distinct lipopolysaccharide (LPS) found in the outer membrane of gram-negative bacteria, and it varies among bacterial types.
Endotoxin can be present in several forms: the bacteria, fragments of bacteria membranes incorporated in dust, as well as the endotoxin molecule itself. Endotoxin is considered highly toxic and is suspected of causing pulmonary impairment in humans. Endotoxin has been implicated as having a significant role in the development of byssinosis from cotton dust exposures.
It has been found in agricultural, industrial, and office environments.
Endotoxin in air has been sampled using filters attached to personal sampling pumps. Bulk samples are useful when endotoxin is suspected. These must be collected in oven-baked glassware and must be analyzed promptly.
The most common analytical method for endotoxin is known as the Limulus assay.
Testing results are often reported as picograms per meter cubed (p/M3), which is an extremely small amount of material.
LPS is inactivated by filter media, including 5.0-mm PVC, 1.0-mm Teflon, 0.45-mm MCE and Polyflon, the result being reduced concentrations when testing is conducted.
It is of note that currently there are a number of variations both in the Limulus assay and in the extraction technique used to remove endotoxin from the filter media; therefore, comparison of results from different laboratories will depend on how similar their analytical techniques are.
Given the fact that an agreed upon method that eliminates the problem of loss of sample on filters has not yet evolved, the best approach to sampling is to collect background and source samples and compare the results rather than attempt to associate the hazard with some type of standard. The same method should be used for all samples and similar volumes should be collected.
FUNGUS AND MOLDS
The fungi class includes yeasts, mold, mildew, and mushrooms. Soil is the most common habitat of the fungi, although many of the primitive fungal groups are aquatic. Fungi occur on the surface of decaying plant or animal materials in ponds and streams or grow on top of aqueous industrial fluids such as metal-working coolants. They are common in grain-handling facilities, paper mills, fruit warehouses, and agricultural environments as well as indoor air environments. Fungal species commonly encountered include Aspergillus, which is ubiquitous in the soil and air, especially in agricultural products and in standing water, whose spores are known to cause a variety of pulmonary effects; and Histoplasma and Cryptococcus, found in bird droppings. Penicillium is a mold that grows on damp organic materials and standing water and is associated with hypersensitivity pneumonitis. Candidaalbicans is a yeast that is ubiquitous and known to cause Candidiasis, a disease of the skin and mouth that occurs in dishwashers, cooks, cannery workers, and others who frequently have their hands in food-contaminated water. In immunosuppressed individuals it can have systemic effects. Other fungi of concern are Alternaria, Aureobasidium, Chaetomium, Cladosporium, and Mucor.