Environmental Engineering Reference
In-Depth Information
residential buildings may be at their lowest values in the winter. This
includes, in many cases, airborne mold as well. However, because of induced
convective soil gas air flows, radon levels in residential buildings may reach
a peak during cold winter weather. Radon sampling may be desirable under
such circumstances if one's objective is to maximize the probability that
elevated radon levels will be measured, or undesirable if the objective of
sampling is to obtain low test results for a real-estate transaction (as some
would be wont to do).
The selection of a sampling time for contaminant measurements should
be made with some knowledge of the range of variation that may occur with
time. Assuming resources are limited, it is, in many cases, desirable to mea-
sure contaminant concentrations in the upper end of their range under
normal building operating/environmental conditions. This is of particular
importance when IAQ guidelines are being used to determine whether expo-
sures are within acceptable limits or not.
Sampling duration is the amount of time required or
selected to collect a sample. It reflects limitations of sampling and analytical
methods used and potential exposures. With real-time or quasi-real-time
sampling devices, sampling duration may be limited to the time necessary
to collect the sample or stabilize instrument readings. With other instruments
and analytical methods, the sampling duration may be determined by the
time required to collect sufficient contaminant to meet the LOD. When the
NIOSH chromotropic acid method is used to sample HCHO, a sampling
duration of 1.5 hr is necessary to measure 0.02 ppmv at the optimum sam-
pling rate of 1 L/min, and a sampling duration of 1 hr is needed to accurately
measure a concentration of 0.05 ppmv. Using the DNPH-HPLC method, a
sampling duration of 0.5 hr would be sufficient to meet a similar LOD value.
Relatively short sampling durations (0.25 to 2 hr) are often used to
determine contaminant concentrations which may be responsible for acute
symptoms. Sampling durations of a week to months may reflect the need
to evaluate the potential for chronic effects, particularly for diseases such
as cancer.
d.
Duration.
A sufficient number of samples should be collected
to ensure measured values are reasonably reflective of exposure conditions.
Though there is no magic formula for determining the number of samples
to be collected, it is advisable to collect multiple samples (which includes
different locations) to avoid difficulties associated with bad samples or sam-
ple loss. As the number of samples increases, so does the time and cost of
their collection and analysis. Investigators must use their professional judg-
ment to determine the number of samples required to obtain meaningful
results within resource constraints.
The number of samples may be determined by sampling objectives. In
the case of radon, most homeowners choose to determine the average con-
centration near the center of the building by using one passive sampler. In
e.
Sample number.
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