Environmental Health Criteria 214

HUMAN EXPOSURE ASSESSMENT
hair include sample preparation, differing growth rates at different
locations on the body, unknown correlation with biological effects and
biological variability (Wilhelm et al., 1990; Kemper, 1993). Hair
grows approximately 1 cm/30 days (Que Hee, 1993) and can be evaluated
along the shaft to provide a profile of exposure over time. Since
growth rates of hair differ based on body location, standardization of
sampling location is crucial.
Because of concerns about contamination, exposure assessors
usually prefer toenails to hair; however, some authors prefer hair for
exposure assessment based on correlations with concentrations measured
in environmental media and ease of collection (Wilhelm et al., 1991).
When sampling nails, toenails are usually the medium of choice, since
they are thicker and less subject to contamination than fingernails.
Toenails can be collected by participants, shipped to distant research
sites and stored in paper envelopes prior to analysis. To reduce
problems associated with surficial contamination, the outer layer of
the toenail can be removed. Each toenail grows at a different rate and
therefore each nail may represent a different time period of exposure;
long-term average exposure can be evaluated using a composite of all
toenails (Garland et al., 1993).
10.6.6 Ossified tissue
10.6.6.1 Teeth
Teeth constitute a unique medium for assessment of past exposure.
Depending on the tooth type and part of the tooth, one can reconstruct
early childhood exposures to bone-seeking elements, such as lead
(Rabinowitz et al., 1989). However, sample collection opportunities
are limited to shed deciduous teeth in young children. Sample
collection and storage are easy; removing the outer layer of the tooth
minimizes external contamination.
10.6.6.2 Bone
Bone represents both past exposure to bone-seeking elements and
is a source for future internal exposure to these elements. The
concentrations of elements in bone represent long-term exposure and
storage of contaminants. For example, the half-life of lead in bone is
approximately 10-40 years (Rabinowitz, 1991). Although numerous
elements can be detected in bone tissue using destructive analyses
such as atomic absorption spectroscopy (AAS), in vivo measurement of
environmental contaminants in bone has been limited to lead (e.g.,
Somervaille et al., 1988; Hoppin et al., 1995). Lead concentration in
bone can be analysed non-invasively using a technique known as X-ray
fluorescence (XRF) (Hu et al., 1995). Bone lead concentrations in
adults correlate well with integrated measurements of blood lead
concentrations (Somervaille et al., 1988; Hu et al., 1995). Sources of
variation in bone lead measurement include the degree of bone
mineralization, the concentration of lead in bone, the overlying skin
thickness and subject movement during the measurement. Lead and other
elements can be mobilized from bone during physiological events such
as pregnancy, lactation and osteoporosis (Silbergeld, 1991).
10.6.7 Breast milk
Environmental studies have used breast milk to evaluate past
exposure to lipophilic chemicals (e.g., pesticides and PCBs) and
metals (WHO, 1996b) and to examine potential exposures for
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