Environmental Engineering Reference
In-Depth Information
6.
Airborne particles
A variety of sampling and instrumental methods are available for measuring
particulate matter in the inhalable size range (
m). These include gravi-
metric methods, wherein airborne particles are collected on filters, with con-
centrations reported in micrograms or milligrams per cubic meter (
≤
10
µ
µ
g/m
,
3
mg/m
). Respirable particles can be measured with real-time or quasi-real-
time instruments using optical techniques. Some instruments measure par-
ticles in the 0.1 to 10
3
µ
m diameter range by forward light scattering; respirable
particles (
m diameter) are often measured by piezoelectric resonance.
In the latter devices, nonrespirable particles are removed by an impactor or
cyclone. Respirable particles are then electrostatically deposited on a quartz
crystal sensor. The difference in oscillating frequency between sensing and
reference crystals is determined and displayed as concentration in
≤
3.0
µ
µ
g/m
.
3
F.
Sampling biological aerosols
Biological aerosols comprise the fraction of airborne particles that are of
biological origin and remain suspended in air for some time. Bioaerosols
vary in their composition depending on sources present. They may differ
significantly between indoor and outdoor environments and from one
indoor environment to another. Of interest in indoor environments are bio-
aerosols that consist of a variety of viable and nonviable microorganisms,
reproductive propagules, and microbial fragments. Mold, bacteria, and act-
inomycetes are of special concern. There are two major bioaerosol sampling
methodologies used in conducting air sampling: culturable/viable and total
mold spore/particle sampling.
1.
Culturable and viable sampling
Culturable/viable sampling is used to determine airborne concentrations of
mold and bacteria (including actinomycetes) that are viable (alive) and can
grow on the culture media used. In such sampling, air is drawn at high
velocity through a multiholed orifice plate where inertial forces cause the
impaction of airborne particles (including viable organisms) onto a nutrient
agar surface. Various media are used depending on sampling objectives.
Collection media are incubated at room or other selected temperatures, and
colonies are allowed to grow. As colonies mature they may be identified to
genus or even to species. Counts may be made based on genus/species types
or on a total colony count basis. Concentrations are expressed as colony-
forming units per cubic meter (CFU/m
).
A variety of culturable/viable sampling devices and techniques are
sampling rate, and collection time used. They are similar in that a solid
culture medium is used to “grow out” culturable/viable particles for iden-
tification and enumeration. Samplers differ in their apparent collection effi-
ciencies. Overall collection efficiency determines the suitability of a device
for a given sampling application. It is determined by both the efficiency of
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