Page 310 - Industrial Ventilation Design Guidebook

266 CHAPTER 5 PHYSIOLOGICAL AND TOXiCOLOGiCAL CONSIDERATIONS

Adipose tissue and bones function as storage sites for many substances.
Most chemicals have some tissue specificity with regard to their tissue binding.
In many cases, this property of a chemical is not important, but, especially for
lipid-soluble chemicals, adipose tissue often becomes an important storage de-
pot from which they are slowly released. Both accumulation and release of
compounds from the adipose tissue are slow processes, partly because adipose
tissue receives only 2% of the cardiac output. The accumulated compound may
be released if the size of the fat depot decreases. For example, lipid-soluble in-
secticides, such as chlordane, may even cause acute intoxication due to dieting,
and dieting also causes release of the supertoxic compound dioxin into the cir-
culation, Lipid-soluble compounds can also be released from their depots in the
adipose tissue during breast feeding of infants, and this may cause excessive ex-
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posure. ^ The features of absorption, distribution, and excretion have been
depicted in Fig. 5.36.
Another important storage depot for toxic compounds is the skeleton. In
particular, cadmium and lead bind and accumulate in the bone tissue from
which they are released very slowly. The half-life of elimination of cadmium is
several years, the half-life of lead is several months.
Theoretical volume of distribution (V^) of a chemical is the volume in
which the chemical would be distributed if its concentration were equal to a
theoretical steady-state plasma concentration (C 0 ) at time zero. The volume of
distribution is thus obtained quite similarly as the steady state concentration
of a compound in the workroom air:

where m is the mass of a chemical and C 0 is its theoretical plasma concentration
at time zero. Even though the compound does not ever reach the theoretical

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FIGURE 5.36 Schematic representation of absorption, distribution, and excretion of xenobtotics.