Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
Researchers have used both strategies for collecting dust samples
(Farfel & Rhode, 1995).
A common criticism of composite sampling is that toxicant
variation across a floor or throughout a residence cannot be
determined; toxicant "hot spots" may be missed. It must be
acknowledged, however, that any settled dust sampling strategy may
miss hot spots. The important issue is how much these hot spots
contribute to the total exposure of the average person. This question
has not been answered by scientific studies. In any case, the
statistical relationship between biological toxicant levels and
average toxicant levels in settled dust levels across large areas in
which a person may be exposed are likely to be better than the
relationship between biological levels and a potential high-dose
source of toxicant exposure for a short period of time. Davies et al.
(1990) used this assumption to design a sampling strategy that
collected settled dust "taken over all the exposed floor surface in
the rooms concerned" (thus, the average level was measured in a room)
rather than from small areas in the room, and found a relatively high
statistical relationship with children's blood lead levels
( r = 0.46).
Possibly the best measures of toxicants in settled dust for
exposure assessment purposes are averages of dust measurements taken
repeatedly over time. If one were to repeat sampling over time,
averages across space and time could be obtained. However, most
sampling strategies used in previous studies collected settled dust at
only one point in time. An obvious advantage to cross-sectional (one
time) studies is that they are less expensive than longitudinal
(repeated measures) studies, which require repeated visits to a
dwelling, greater occupant burden, and higher laboratory analysis
costs.
One possible, but untested, approach to strengthening estimates
of time-weighted average dust levels in cross-sectional studies may be
to measure exposure-weighted average levels based on the activity of
the person. This may be done by listing indoor locations where the
person spends time, then roughly estimating the percent of time spent
actively in each location, rounded to a convenient percentage. Samples
can then be composited from the specific areas by adjusting the
subsample areas to be proportional to the percent of time spent in
each area. An exposure-weighted average toxicant dust level could then
be estimated from the result.
Finally, laboratories performing the chemical analysis should be
consulted before settled dust samples are collected. This is
particularly true when collecting composite wipe samples. An excess of
towelette material may present problems during the laboratory
digestion phase of analysis, requiring more reagents and larger
beakers than normally used, and potentially reducing the toxicant
recoveries owing to matrix effects. Similarly, vacuum sampling may
collect more dust than is required for analysis. If this is the case,
techniques need to be employed by the laboratory to ensure that the
fraction of dust analysed represents the whole. Another potential
source of error in the results lies in how the dust is handled after
sampling and prior to analysis. If measurements of lead concentration
in dust are important for the objectives of the study, sampling
methods that present the dust to the laboratory in an easy-to-handle
form should be considered over alternate methods. These issues and
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