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Environmental Science & Technology
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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, gramnegative 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. Hotwater 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.
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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).
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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.
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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.
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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. Candida albicans 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.
Fungal-related diseases can be divided into two types: mycosis and
mycotoxicosis. Mycosis represents a variety of toxic effects, including
dermatitis, hypersensitivity pneumonitis, and some systemic diseases that
result from an infection by the organisms themselves. Mycotoxicosis is
produced by metabolites of various fungi and causes diseases such as
toxic aleukia and yellow rice disease.
Occupations associated with exposure to fungi include sawmill,
sugarcane, and cork workers as well as jobs where seeds and textile fibers
are handled. Other work environments conducive for the growth and
sporulation of fungi are farming, grain handling, mushroom cultivation,
insect rearing, and pharmaceutical manufacturing.
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Like other microbes, fungi have specific nutritional requirements that vary
among the species and produce metabolic products, a classic example
being penicillin produced from the mold penicillium. Fungi are also
dependent on having water present. The presence of a moldy odor is
suggestive that fungi are growing.
A unique stage of some fungus' life cycle is the spore stage consisting
of a wide variety of shapes in a very broad size range (<2 mm to >100
mm).l9 In this stage they form a durable coating over the exterior and
become dormant. Spores can be classified by size, morphology, and color,
allowing them to be categorized into different taxonomic groups. Since
spores are relatively hardy structures, they can survive in dry
environments and become airborne when disturbed. Fungal spores are
released into the air either by mechanical means, such as wind or other
agitation, or biologically by specialized (active) spore discharge
mechanisms usually occurring during periods of high relative humidity.
When airborne, fungal spores tend to travel as single units.
Certain foods such as peanuts and animal feed contain fungal spores that
begin growing and producing aflatoxins when environmental conditions
(time, temperature, moisture, nutrients, and pH) are favorable. Aflatoxins
are a group of chemically similar compounds known to be acutely toxic
and carcinogenic at low doses, and are metabolites of two common fungi:
Aspergillus favus and Aspergillus parasiticus. If fungal spores are
suspected, water reservoirs should be identified and bulk samples should
be collected at all suspected sources. Suitable niches for growth and
sporulation include stored food, house plants, air conditioners,
humidifiers, cold air vaporizers, books and papers, carpets, and damp
areas.
The primary air sampling tools used for fungi spores are slit impactors
and filters. Screening samples can be collected with a centrifugal
impactor.
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VIRUSES
Viruses represent a unique class of agent and are different from cellular
organisms. A virus alternates in its life cycle between two phases: one
extracellular and the other intracellular. In its extracellular phase, a virus
exists as an inert, infectious particle, or virion. A virion consists of one or
more molecules of nucleic acid, either DNA or RNA, contained within a
protein coat, or capsid. In its intracellular phase, a virus exists in the form
of replicating nucleic acid, either DNA or RNA. Viruses utilize the host
cell for replication (reproduction) and thus are intracellular parasites. In
the extracellular phase, some viruses are quite stable and resistant to heat
and light.
Viral infections may be acquired from vectors such as needles or from
handling of animals or animal products and from humans. Laboratory
acquired infections may result from needle sticks; animals; clinical or
autopsy specimens; or contaminated glassware. Diseases include rabies,
cat- scratch disease, and viral hepatitis (both serum and infectious).6
Viruses survive best in situations where high humidity and moderate
temperatures are present. Situations where water containing a virus is
being aerosolized are especially conducive to viral multiplication.
Collection of viruses often requires very specific techniques, although
some have been collected on filters. Viruses have also been collected on
the slit sampler and the multistage cascade impactor.
Viruses are usually measured as either infectious units or total particle
numbers. While it is important to be aware that viruses may be a cause of
an outbreak of illness, it is unlikely that air sampling would be useful in
most situations, due to complicated analytical techniques usually
requiring that a live species be injected and the degree of specialization
necessary to perform these analyses. Instead, a more common technique is
to identify the symptoms associated with a suspect virus and determine if
the disease is present through a physician's clinical evaluation.
Collection of viruses often requires very specific media, although some
have been collected on filters.
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OTHER MICROORGANISMS
Spirochetes have a unique cell structure relative to other bacteria in that
they have very long, wormlike bodies. Thus, they can swim in liquid
media and are found in mud and water.
Mycoplasmas, the smallest known cellular organisms, are a large and
widespread group. The first member of this group to be identified was the
agent of bovine pleuropneumonia.
Factors That Affect the Survival and
Dispersion of Bacteria and Viruses in Wastewater Aerosols
Relative humidity
Bacteria and most enteric viruses survive longer at
high relative humidities, such as those occurring
during the night. High RH delays droplet evaporation and retards organism die-off.
Sunlight
Sunlight, through UV radiation, is deleterious to
microorganisms. The greatest concentration of
organisms in aerosols from wastewater occurs at
night.
Open air
It has been observed that bacteria and viruses are
inactivated more rapidly when aerosolized and
when the captive aerosols are exposed to the open
air than when held in the laboratory.
Wind speed
Low wind speeds reduce biological aerosol
transmission .
Temperature
Increased temperature can also reduce the viability
of organisms in aerosols, mainly by accentuating
the effects of RH. Pronounced temperature effects
do not appear until a temperature of 80°F (~6°C)
is reached.
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