Routes of Exposure, Dose, and Metabolism of Metals - Toxicology of Metals - page 60

et al. , 1976; Marsh et al. , 1981; Shahristani et al. , 1976).
Methylmercury is incorporated into the newly form-
ing hair in such a way that the concentration in hair
is directly proportional to the simultaneous concentra-
tion in blood. Once incorporated into the hair strand,
the concentration of mercury does not change. Thus,
the concentration profi le along the strands of hair
represents a recapitulation of previous blood concen-
trations. The hair growth rate is approximately 1 cm/
month so that the distance from the scalp in centim-
eters corresponds to the number of months of expo-
sure before the time of collection of the sample. In the
example given in Figure 18, it was possible to recapit-
ulate, from the analysis of one hair sample, the past
exposure over a period of almost 2 years, including the
entire period of pregnancy. The close parallel between
maternal hair and blood concentrations is obvious. It
has been possible to use the approach of analyzing seg-
ments of hair to obtain an estimate of interindividual
variability in biological half-times of a great number of
individuals (Figure 13).
Caution must be observed when applying this
approach. Hair samples are liable to external contami-
nation, and satisfactory washing procedures have not
been established. Furthermore, each metal should be
tested to see whether hair concentration is propor-
tional to blood concentration. Thus, it is diffi cult to
fi nd an indicator medium that is useful for estima-
tion of exposure or concentration in critical organs for
a wide range of metals. Blood is probably the single
biological medium that has greatest general applica-
bility. In some instances, blood values are not easily
interpretable without detailed information on previ-
ous exposure history, etc. An example of this is cad-
mium, for which metal there is no direct relationship
between blood concentration and the concentration in
critical organs. Blood cadmium may thus be used to
estimate exposure but does not necessarily reveal any-
thing about risk of adverse effects if exposure has been
highly variable over a period of years.
Because some biological samples (e.g., urine sam-
ples) are readily available, measurements performed
on such samples may also be useful as a tool in the
control of exposure (i.e., if analytical results obtained
refl ect recent exposure) but not concentration in criti-
cal organs. Considerations of biological monitoring for
a number of metals and their compounds have been
presented by Clarkson et al. (1988) and in Chapter 4.
When embarking on a program of biological moni-
toring, it is advisable to acquaint oneself with the
toxicokinetic model and patterns of toxicity for the
particular metal compound to be analyzed (see the spe-
cifi c chapters on individual metals). This is necessary
to make the best possible choice.

Diffi culties in the chemical analysis should also be
given due consideration, because faulty chemical anal-
ysis can make an entire monitoring program entirely
useless (WHO, 1979; see also Chapters 2 and 4).

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