Bioaerosols - Aerosols: Measurement, Dosimetry, and Health Effects - page 322

replacement of the collection medium, which may be an agar plate, coated microscope slide, il-
ter, or tape. Several models of disposable impactors have recently become commercially available
(Table 12.2).
High impact velocity can injure microorganisms, and bacterial and fungal survival has been
found to decrease with increased impaction velocity or sampling time. 344,345 Bacterial culturability
also has been shown to decrease with increased jet-to-plate distance, possibly because the increased
jet dissipation desiccates a larger fraction of the agar surface. 346
Liquid impingers collect particles by impaction, diffusion, and interception followed by suspen-
sion in a collection luid. One impinger uses three curved jets that induce a swirling motion in the
capture liquid, which increases collection eficiency and decreases particle re-entrainment (three-
jet, tangential sampler, Table 12.2). 347 Liquid volumes from 5 to 50 mL may be used with luids as
diverse as sterile distilled water and nonevaporating mineral oil.
Centrifugal samplers and cyclones collect particles by impaction onto a solid medium or into
a liquid. In cyclones, air tangentially enters a cylindrical or inverted conical chamber, spins down
along the chamber walls, lows up through the center, and exits at the top (wetted and dry cyclone
samplers, Table 12.2). Large particles deposit on the cyclone walls and very large particles fall to
the bottom. Liquid is often pumped into a cyclone's inlet to wash the particles into a collection
container.
12.6.3.2 Filtration
Filter samplers collect particles of all sizes, the upper limit depending on the ilter holder's inlet
characteristics and addition of precollectors. Filter cassettes developed for sampling of inhalable
particles, for example, the Button sampler, 348 have commonly been used for both area and personal
bioaerosol sampling (ilter samplers, Table 12.2).
Most ilters have their minimum collection eficiency at the particle size of 0.01-0.03 μm
(see Chapter 24). 343,349,350 Many types of ilter materials can be used for bioaerosol collection, for
example, polycarbonate, polytetraluoroethylene (PTFE, Telon ® ), mixed cellulose ester (MCE),
and gelatin ilters. The choice of ilter material depends mainly on the type of analysis that fol-
lows. Porous membrane ilters are often chosen for culturing and immunostaining of bioaerosols.
Polycarbonate and PTFE ilters typically are used when the collected material must be washed from
a ilter. 350,351 Dehydration of vegetative bacterial cells during ilter sampling signiicantly reduces
their viability, but hardy bacterial and fungal spores are not as susceptible to damage. 352-354 Gelatin
ilters can reduce the desiccation effect caused by ilter sampling. 350,352 Capillary pore ilters have
smooth, lat surfaces suitable for the examination of particles with an optical or scanning electron
microscope (SEM, see Section 12.6.4.1). MCE ilters can be made transparent for analysis by light
microscope. 355 Investigators are increasingly using membrane ilters to collect bioaerosols in con-
junction with analyses not based on culture because of their ease of use and high collection eficiency
(Sections 12.6.4.3 through 12.6.4.6). However, some of these ilters have shown poor recovery for
speciic agents such as endotoxin for which glass iber ilters have been recommended. 356
12.6.3.3 Electrostatic Precipitation
The electrostatic precipitator is an example of an instrument long used for particle collection that
has been adapted for laboratory sampling of bioaerosols. Collection eficiency for bacteria onto
rectangular agar dishes was observed to vary from 50% to 90% depending on air low rate and
applied voltage. 272,273 Recently, a Wet Electrostatic Precipitator (WEP) was developed for bioaerosol
collection. 357 An electrostatic ield charges particles in an air stream and forces them to the wall of
the WEP collection tube from which they are washed and concentrated by a recirculating liquid. An
Electrostatic Precipitator with Superhydrophobic Surface (EPSS) achieved a concentration rate of
10 6 for latex particles. 358 The EPSS subsequently was found to be compatible with polymerase chain
reaction (PCR)-based sample analysis and to reach a collection eficiency of 72% for P. luorescens
and B. subtilis . 359