PRINCIPLES OF TOXICOLOGY

534 CONTROLLING OCCUPATIONAL AND ENVIRONMENTAL HEALTH HAZARDS
The purpose of conducting exposure monitoring needs to be determined in each case because the
strategy and techniques may differ if one is attempting to demonstrate compliance rather than control
effectiveness. For example, personal exposure information, determined from air samples taken in the
breathing zone of workers, is often needed to demonstrate compliance. Stationary area samples are
required to determine compliance with some standards, such as cotton dust, but usually they are located
near potential exposure sources to determine sources of exposure with the intent to design the most
efficient control systems.
Monitoring is also performed for hazardous physical agents as well as toxic chemicals. Noise,
ionizing and nonionizing radiation, and heat and cold can all be monitored and compared with exposure
limits.
Today, the most widely-practiced form of exposure monitoring for toxic chemicals is air sampling.
This method assesses exposure by the inhalation route only. Personal air samples are collected by
attaching a sampling apparatus to the worker so that the concentration of the contaminant inside the
individual’s breathing zone is quantified. Typically, a small battery-powered air sampling pump is
attached to the worker’s belt. Tubing is routed from the pump to sampling media, which is attached to
the shirt lapel so that it is within a one-foot radius of the head (i.e., the “breathing zone”). Figure 22.1
shows such an arrangement.
The choice of sampling media, flow rate, and duration of sampling depend on the substance to be
collected and how it is used. The specific choice of sampling media needs to be made under the
supervision of an experienced industrial hygienist and in consultation with the analytical laboratory.
Certain conditions on the day of the survey, such as temperature and relative humidity, may affect the
choice of sampling media.
Usually, the sampling media is analyzed to determine the total mass of the contaminant present (in
milligrams). Since the pumps are set at a calibrated flowrate, the total volume of air sampled is known
(in cubic meters). The exposure results are then reported in either milligrams of analyte per cubic meter
(mg/m 3 ) of air or parts per million (ppm) by volume, both of which are time-weighted average (TWA)
concentrations for the period of time the sampling was conducted.
One purpose of the walk-through survey should be to determine how many personal and/or area
air samples need to be collected to adequately characterize exposure patterns. The exposed employees
should be assigned to homogeneous exposure groups, and then the exposure patterns of the groups
should be assessed. Statistics provides guidance on how many air samples need to be collected in order
to achieve stated degrees of confidence. For example, when exposure levels are near the exposure limit
and the variability in the exposure during a day is large, many samples may be needed to be assured
that the exposure is less than the exposure limit with 95 percent confidence. If smaller numbers of
samples are collected, the 95 percent confidence interval is larger, and may include the exposure limit
within the range, so there is less confidence that the actual exposures or concentrations are below the
exposure limit.
Unfortunately, statistical analysis of air sampling data is an often-neglected exercise in industrial
hygiene. One reason for this has to do with cost and available resources; it is often simply too
time-consuming and expensive to collect the large numbers of air samples necessary to minimize the
standard errors of the measurements, particularly when variability is high. Another reason is that
variations between days is typically greater than variations within a day. Therefore, multiple days of
sampling may be necessary to fully characterize exposure patterns. One potential way around this
quandry is to take extraordinary steps to ensure that the samples that are collected are representative.
A conservative (i.e., health-protective) definition of a representative sample is one, which is collected
under the “worst-case scenario,” but realistic conditions. Thus, “representative” should not be
confused with “average.” If exposures have been measured under a worst-case scenario, some degree
of confidence can be achieved that exposures during routine operations are not higher. Determining
what constitutes a representative sample is often a most difficult task. The results of the interviews are
especially helpful in arriving at a working definition of representative conditions. Of course, an
employee should never be intentionally overexposed for the purposes of collecting an air sample.