in the operation or process, which constitutes the greatest hazard, and the duration of
the hazard.
d. Determination of whom to sample is closely related to where to sample.
The most effective way to determine actual worker exposure is to collect breathing zone
samples from all workers. Some work areas have a large number of workers and
collection of breathing zone samples from all the workers is not feasible. Samples
would then be collected from the maximum risk workers. The maximum risk worker is
normally the worker closest to the source of the contaminant. Other considerations are
work patterns, work habits, worker mobility, and air movement patterns.
e. Depending upon the physical demands of the work, workers inhale from 5 to
20 cubic meters of air during an 8-hour day. However, the volume of the air sample
(and hence, the length of the sampling period) need not approach the volume of air
actually inhaled by the worker. All that is needed is a representative sample of the air,
large enough to contain sufficient contaminant to be analyzed and identified. Therefore,
the volume of the air sample (and the duration of the sampling period) will be dependent
upon the TLV of the contaminant, sensitivity of the analytical procedure to be used, and
the estimated concentration of the contaminant in the work place atmosphere. The
sample volume and flow rate is determined by using the USAEHA Sampling Guide TG
141 (see Appendix D). The duration of the sampling run depends on the flow rate of the
sampling pump and the volume required and can be calculated by using the formula:
Volume
Duration = ────────
Flow rate
f. There is no set rule for determining the number of samples needed to
evaluate a worker's exposure, provided that a minimum number characterizing the
exposure in time and space is taken. A single grab sample is never sufficient even if it
is believed that the concentration at the time of sampling will be maintained throughout
the work period. If the indicated concentration is near or above the TLV, repeated
sampling should be done. Ordinarily, contaminants are not generated at uniform rates,
so the concentration can vary from time to time. In addition, the type of contaminant
will, to some extent, dictate the number of samples to be taken as well as the type of
instrumentation to be used for sampling. The number of samples to be collected will
also depend upon the type of evaluation being conducted. If the objective is merely to
determine the presence of a contaminant, a few samples will suffice. On the other
hand, if a comprehensive survey is being conducted, more samples will be required.
g. Before we can decide when to sample, we must consider the purpose of the
evaluation (for example, what information is desired) as well as the overall nature of the
particular operation or process being evaluated. Since the concentration of contaminant
may vary considerably throughout a 24-hour period, it is necessary to take samples
during each shift of operation. In addition, when Army installations are located in areas
which experience wide seasonal variations in temperature, it may be necessary to
MD0165
2-19