Microbes in Air and
Bioaerosols
ENVR 133
Mark D. Sobsey
Bioaerosols
• Microorganisms or particles, gases, vapors, or
fragments of biological origin (i.e., alive or released
from a living organism) that are in the air.
• Bioaerosols are everywhere in the environment.
• Some bioaerosols, when breathed in, can cause
diseases including pneumonia, asthma, rhinitis (e.g.
cold, hay fever), and respiratory infection.
• Some bioaerosols can also infect the eyes and vai
ingestion (swallowed)
Some Examples of Bioaerosols
Living Source
Examples
Microorganisms (microbes):
• Bacteria
Legionella, Actinomycetes, endotoxins
• Fungi
Histoplasma, Alternaria, Pencillium,
Aspergillus, Stachybotrys aflatoxins,
aldehydes, alcohols
• Protozoa
Naegleria, Acanthamoeba
• Viruses
Rhinoviruses (colds), Influenza (flu
• Algae
Chlorococus
Green plants Ambrosia (ragweed) pollen
Arthropods
Dermatophagoides (dust mites) feces
Mammals
horse or cat dander
Diseases Caused by Bioaerosols:
Hypersensitivity or Allergic Diseases
• Result from exposure to antigens (of indoor
bioaerosols) that stimulate an allergic response by the
body's immune system.
• Susceptiblity varies among people.
• Diseases usually are diagnosed by a physician.
• Once an individual has developed a hypersensitivity
disease, a very small amount of the antigen may cause
a severe reaction.
• Hypersensitivity diseases account for most of the
health problems due to indoor bioaerosols.
Hypersensitivity or Allergic Diseases
• Building-related asthma: result in complaints of chest
tightness, wheezing, coughing, and shortness of breath.
– symptoms may occur within an hour of exposure or 4-12
hours after exposure.
– Can be caused by airborne fungi such as Altemaria,
glycoproteins from fungi, proteases (digestive enzymes that
cause thebreakdown of proteins) from bacteria, the algae
Clorococus, ragweed pollen, dust mites, and dander from
cats.
• Allergic rhinitis: stuffiness of the nose, clear discharge from
the nose, itchy nose, and sneezing. Itching and puffy eyes.
– All the indoor bioaerosols listed under building-related
asthma except the bacteria proteases alsocause rhinitis.
Hypersensitivity or Allergic Diseases
• Hypersensitivity pneumonitis (extrinsic allergic alveolitis):
– Can be an acute, recurrent pneumonia with fever, cough, chest
tightness, and fluids entering the lungs.
– Or, can be a cough that progresses to shortness of breath,
fatigue, weight loss and thickening and scarring of the lungs.
– microorganisms associated with hypersensitivity pneumonitis:
fungi such as Penicillium and Sporobolomyces, bacteria such
as Thermoactinomyces, and protozoa such as Acanthamoeba.
• Humidifier fever: fever, chills, muscle aches, and malaise
(general feeling of being unwell), but no lung symptoms.
– The symptoms usually start within 4-8 hours of exposure and
end within 24 hours without long-term effects.
Airborne Microbes and Aerosols
• Airborne transmission is possible for essentially all
classes of microbes: viruses, bacteria, fungi and
protozoans.
• Any respiratory pathogen able to survive aerosolization
and air transport is considered a potential cause of
airborne disease.
• Aerosols: Airborne particles, either solid or liquid,
about 0.5 to 20 microns in diameter, that remain
airborne for extended periods of time.
• Droplets: >20 (usually 100+) microns in diameter; settle
rapidly or evaporate to form droplet nuclei in the
aerosol size range.
Regions of the Respiratory System
• The cellular composition as well as geometry of the
respiratory system influence particle deposition.
• Nasopharynx Region: the head region, including the
nose, mouth, pharynx, and larynx
• Tracheobronchial Region: includes the trachea,
bronchi, and bronchioles
• Pulmonary (Alveolar) Region: comprised of the
alveoli; the exchange of oxygen and carbon dioxide
through the process of respiration occurs in the
alveolar region
Lung Anatomy and Physiology and Particle Deposition
Trachea:
• largest airway; lined by a ciliated epithelium covered by
mucus and serous secretions produced by the cells and
glands of the epithelium.
• The secretions may trap and/or dissolve some inhaled
particles
• Beating of the cilia tends to drive secretions upward toward
the mouth where they are ingested or expectorated.
• Particles deposited in this portion of the lung will also be
expelled in this manner.
Lung Anatomy and Physiology and Particle Deposition
Tracheobronchial Region:
• As the airways divide and become smaller in diameter,
the composition of the epithelium also changes
• Becomes thinner with less ciliated cells and increasing
numbers of Clara or non-ciliated bronchiolar cells.
• Clearance via the mucociliary escalator is less likely
than in the trachea.
• The Clara cells do produce secretions and have
metabolic capabilities.
– Can detoxify locally absorbed chemicals
Lung Anatomy and Physiology and Particle Deposition
Pulmonary (Alveolar) Region:
• Airways become even smaller in diameter (respiratory and
terminal bronchioles) and open into the alveolar region.
• Lined primarily by very thin Type I epithelial cells.
• Other epithelial cell type found less frequency: pulmonary
macrophage.
– Specifically recruited (by migration or cell division) to the alveolar region
when there is a local particle burden.
– Engulf the particles and either migrate into the lung interstitium or move up
the airways to be caught and removed by the mucociliary escalator.
• Region of primary lung function: gas exchange.
• Only a layer of surfactant, the Type I cell layer, and a thin
interstitium containing fibroblasts and endothelial cells separate
air from blood.
Regional Deposition in Respiratory Tract and Particle Size
Aerosols and Respiratory
Deposition
• Aerosols 5 microns in diameter are removed in the
upper respiratory tract, especially the nose.
– Particles are brought to the pharynx by mucociliary
activity of the upper respiratory epithelial mucosa,
where they are expectorated or swallowed.
• Swallowed particles containing enteric microbes can
initiate enteric infections.
Aerosols and Respiratory Deposition
• Particles <5 microns in diameter, esp. 1-3 microns
diameter, penetrate to the lower respiratory tract
– Can be deposited in the bronchioles, alveolar ducts and
alveoli.
– Deposition efficiency in lower respiratory tract is ~50% for
particles 1-2 microns diameter.
– Particles <0.5 microns dia. can also be deposited in the lower
respiratory tract, especially particles <0.25 microns dia.
– Particles deposited in the lower respiratory tract can be
phagocytized by respiratory (alveolar) macrophages
• can be destroyed or
• carried to the ciliary escalator, where they are transported
upward to the pharynx.
Hydroscopicity and Aerosol
Deposition in the Respiratory Tract
When inhaled, aerosol particles derived from aqueous fluids
pick up moisture (water) while traveling in the respiratory
passageways, thereby increasing in size.
– Increased size changes deposition site
H2O
H2O
H2O
Respiratory Deposition - Impaction
• Each time airflow changes due to a bifurcation in the
airways, suspended particles tend to travel along their
original path due to inertia and may impact on an airway
surface.
• This mechanism is highly dependent on aerodynamic
diameter, since the stopping distance for very small
particles is quite low.
• Occurs mostly for larger particles that are very close to
airway walls, near the first airway bifurcations.
• Therefore, deposition by impaction is greatest in the
bronchial region.
• Impaction accounts for the majority of particle deposition on
a mass basis.
Respiratory Deposition - Sedimentation
• Settling out of particles in the smaller airways of the
bronchioles and alveoli, where air flow is low and
airway dimensions are small.
• Rate of sedimentation is dependent on the terminal
settling velocity of the particles
• Sedimentation plays a greater role in the deposition
of particles with larger aerodynamic diameters.
• Hygroscopic particles may grow in size as they
pass through the warm, humid air passages, thus
increasing the probability of deposition by
sedimentation.
Respiratory Deposition - Interception
• Occurs when a particle contacts an airway surface due
to its physical size or shape.
• Unlike impaction, particles that are deposited by
interception do not deviate from their air streamlines.
• Most likely to occur in small airways or when the air
streamline is close to an airway wall.
• Interception is most significant for fibers, which easily
contact airway surfaces do to their length.
– Furthermore, fibers have small aerodynamic
diameters relative to their size, so they can often
reach the smallest airways.
Respiratory Deposition - Diffusion
• Primary mechanism for particles <0.5 microns in diameter
• Governed by geometric rather than aerodynamic size
• Net transport of particles from a region of higher to lower
concentration due to Brownian motion.
– Brownian motion: random wiggling motion of a particle
due to the constant bombardment of air molecules.
• Occurs mostly when particles have just entered the
nasopharynx
• Also most likely to occur in the smaller airways of the
pulmonary (alveolar) region, where air flow is low.
Agents of Respiratory Infectious Diseases
Viruses: influenza, measles (rubeola), chickenpox (herpes
varicella-zoster) and rhinoviruses (colds); Hantavirus
(from a rodent; mouse)
Bacteria: Legionella spp., tuberculosis and other
mycobacteria (Mycobacterium spp.), anthrax (Bacillus
anthracis), and brucellosis (Brucella spp.).
Fungi: diseases: histoplasmosis, cryptococcosis,
blastomycosis, coccidiodomycosis, and aspergillosis
Protozoans: Pneumocystis carinii pneumonia; prevalent in
immunodeficient hosts such as AIDS patients.
Acanthamoeba encephalitis; primary amebic
meningoencephalitis (PAM)
Reservoirs and Amplifiers of Airborne Microbes
Reservoirs:
Wide range, overall
Depends on the microbe
–
–
–
–
–
–
humans,
animal,
soil
dust
water
air
Amplifiers:
• Places where microorganisms multiply or proliferate.
• Most reservoirs are potential amplifiers.
Disseminators
• Devices causing microbes to enter airborne state or be
aerosolized; often the reservoir or amplifier.
• Any device able to produce droplets and aerosols:
–
–
–
–
–
–
Humans and other animals: coughs and sneezes, esp.
Mechanical ventilation systems
Nebulizers and vaporizers
Toilets (by flushing)
Showers, whirlpools baths, Jacuzzi, etc.
Wet or moist, colonized surfaces (wet walls and other
structures in buildings)
– Environments that are dry and from which small particles can
become airborne by scouring or other mechanisms:
• Vacuuming or walking on carpets and rugs
• Excavation of contaminated soil
• Demolition of buildings
Airborne Microbes and their Reservoirs
Viruses:
• Mostly humans but some animals
• Some rodent viruses are significant: ex: Lassa Fever Virus and
Hantavirus.
Bacteria:
•
•
•
•
Humans (TB & staphylococci),
other animals (brucella and anthrax),
water (Legionella)
soil (clostridia).
Fungi:
•
•
•
•
•
soil and birds (Cryptococcus and Histoplasma)
dead plant material
wet surfaces (wood and other building materials)
indoor air (mycotic air pollution)
stagnant water for the opportunistic fungi (e.g., Aspergillus sp.).
Legionella: Legionellosis and Pontiac Fever
Reservoirs and amplifiers:
•
•
•
•
•
Hot water systems
circulating water ventilation systems (cooling towers)
Plumbing (e.g., shower heads).
Hot tubs, whirlpools, etc.
Produce fresheners
Cleveland Auto plant outbreak, March, 2001:
• Plant cooling tower is considered a possible source of the
outbreak.
• But, more than 100 other internal water sources -- favorite
breeding grounds for the Legionella bacteria -- were also
under investigation….
Legionnaire’s Disease and Pontiac Fever
Legionnaire's disease:
• Bacterial pneumonia caused by Legionella pneumophila.
• A type of pneumonia that affects the lungs and may also affect the
stomach and intestines, kidneys, and central nervous system.
• Incubation period: 2-10 days after exposure
• Frequently requires hospitalization
• Aerosol exposure from contaminated cooling towers, evaporative
condensers, whirlpools, shower heads, faucets, & hot water tanks.
Pontiac fever: also caused by Legionella.
• A "flu-like" illness with fever, chills, headache, myalgia (pain in the
muscles), cough, nausea, and breathlessness.
• Pneumonia does not occur.
• Usually lasts 2-5 days.
• Same sources as for Legionnaires' disease
Factors Influencing Airborne Infection
Aerosol Factors
• Particle size; <5 um dia.; "droplet nuclei" from coughing & sneezing
– Deposition site: depends on particle size and hygroscopicity
– Chemical composition of the aerosol particle
• Relative humidity (RH); dessication (loss of moisture)
• Temperature: generally greater inactivation at higher temperature
• Sunlight: UV inactivation of microbes
• Factors influencing air movement: winds, currents,
mechanical air handlers, etc.
• Seasonal factors: precipitation, air currents, pollen sources, etc.
• Air pollution:
– chemicals inactivating airborne microbes (OAF= Open Air Factor)
– enhancing their ability to cause infection in a host
Factors Influencing Airborne Infection
Microbe Factors:
• Size of microbe and of aerosol particle
– influences air transport
– influences deposition site: in environment and in host
• Composition:
– lipids, proteins (structural, enzymes), amino acids, etc.
– enveloped and non-enveloped viruses respond
differently to air pollution
• Protective forms:
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–
–
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spores
cysts
growth phase
moisture content
Host Factors
• Discussed previously in this class.
Air Samplers - Sedimentation and Slit Samplers
• Sedimentation methods: collection of aerosol particles on a
sticky surface; e.g., a petri dish containing agar or glycerol.
• Slit samplers: Sampled of air is directed through a slit
against a rotating collection surface. For bacteria, this could
be an agar medium petri plate. Rotation is intermittent so
that each impaction area represents a specific volume of
sampled air and a time series of samples can be collected.
Sedimentation - Agar Medium Plate
Slit Sampler
Air Samplers - Stacked Sieve (Anderson type) Sampler
• Six stages, each a
perforated plate located
above a petri dish.
• Diameter of air
passageway is smaller at
each successive level,
collecting progressively
smaller particles.
• Classifies and collects
particles according to size
• Used mostly for bacteria.
• Can be used for virus
sampling by collecting
onto a sticky surface in the
petri dish.
Air Sampling - Filtration Methods
• Pass air through a membrane filter of small enough pore
size to trap aerosol particles.
• After collection, the filter can be plated or particles can be
washed off.
• Dessication and inefficient washing/recovery of collected
particles can be problems.
Open Face Air Filter Cassette
with Cap
Air Samplers - Electrostatic Precipitation
• Air is drawn over
electrically charged
collection plates so that
charged particles are
attracted to and collected
on either a positively or
negatively charged,
wetted surface.
• Collected particles are
washed off into the
circulating collection
fluid on the charged plate
surface.
Air Samplers - Liquid Impingers
• Collects particles from sampled
air into a liquid medium.
• All glass impinger (AGI).
• Particles are drawn through a
small orifice that increases their
velocity, thereby causing them to
impinge on the bottom surface of
the container and be "scrubbed"
into the collection fluid.
• Excessive cooling and
evaporation and leakage (particles
not being retained) can be a
problem.
Air Samplers
• High volume liquid (cyclone) scrubber: Particles in air traveling at
high speeds through a progressively smaller, helical passageway
impinge against the container walls and are collected into a
recirculating collection fluid supplied by a pump