Environmental Engineering Reference
In-Depth Information
from complaint and noncomplaint areas. Since outdoor air commonly has
elevated biological contaminant levels that reflect outdoor sources, such
samples should be collected to provide a reference for determining whether
certain genera and species are being amplified in the indoor environment.
Duplicate samples are recommended for each culture medium used at each
location.
ble sampling devices vary in flow rates used and sampling duration recom-
mended. Sampling rate is an important determinant of sampling duration,
which may be influenced by biological aerosol concentration and the poten-
tial drying of culture media. Excessive collection of mold spores and particles
may result in multiple mold particle deposition at the same collection site,
overgrowth of colonies, colonies too numerous to count, and, even with
multiple-hole correction procedures, a potential underestimation of airborne
levels. It is often necessary to adjust sampling duration to assure a minimum
level of detection in some cases and prevent media overload in others. Low
sample volumes increase data variability; higher volumes obtained with
longer sampling durations result in decreased sample counts due to the
apparent desiccating effects of high velocity air flows on media and viable
particles. Sampling durations of 0.5 to 2 minutes at a flow rate of 28.3 L/min
are considered optimal.
iv. Disinfection. Unique to biological aerosol sampling is the need to
disinfect the sampler prior to field use. The sampler is subject to mold
contamination on aerosolized media that deposits on sampler surfaces. Dis-
infection is commonly accomplished by wet wiping sampler surfaces (par-
ticularly interior surfaces) with a 70% ethanol solution.
v. Calibration. As with other instruments, culturable/viable sam-
pling devices need periodic calibration. Calibration must be conducted with
devices that can measure high flow rates. A variety of calibration approaches
have been recommended including pitot tubes, vane anemometers, and
rotometers calibrated against primary or secondary standards. Wet test
meters of sufficient volume work well under laboratory conditions. A wet
test meter and an orifice cover connected to plastic tubing used to calibrate
an N-6 Andersen sampler is illustrated in
Figure 9.10
.
vi. Handling and analysis. After collection, samples can be incu-
bated, enumerated, and individual organisms identified to specific taxa by
the investigator, or as is commonly the case, sent to a laboratory that provides
such services.
Mold sample plates are typically incubated at room temperature for 5
to 7 days before counts are made and taxa identified. Colony counts obtained
from sieve plate impactor samplers (e.g., Andersen sampler) must be
