PRINCIPLES OF TOXICOLOGY

328 PROPERTIES AND EFFECTS OF METALS
Biological Factors (Biotransformation)
Speciation of metals and conversion from one form to another in the environment may also occur as
a result of biological action, termed biotransformation. This process may be exploited in engineering
applications for remediation purposes, for instance, when contaminated soils are seeded with plants
or fungi, which absorb metals and other inorganic substances from the soil and bind the metals in stable
forms within their tissues. Removal of the plants or fungi then effectively removes the metals from the
site. It is important to remember that since metals are elements, in most instances, they cannot be
removed by degradation processes, which often are effective for organic compounds.
The metabolic processes of many microorganisms may be manipulated so as to biotransform metal
compounds to less toxicologically important forms. Some yeasts, for instance, can be cultured in
conditions whereby they will reduce ionic mercury to elemental mercury, which is a liquid and which
typically is less bioavailable. However, vapor forms of elemental mercury may rapidly diffuse across
some cellular membranes. There are also many organisms that readily methylate ionic mercury in
selected environmental circumstances to the more easily absorbed and much more toxic organic
species, methylmercury and dimethyl mercury, thus creating a more hazardous form. Likewise,
inorganic lead may be biotransformed by some bacteria to tetraethyl lead, an organic compound that
is more easily transported across biological membranes.
14.3 PHARMACOKINETICS OF METALS
Absorption and Distribution
For a metal to exert a toxic effect on a human or other organism, there must be exposure and it must
gain entry into the body. There are three main routes of absorption: inhalation, oral, and dermal. The
significance of the route depends on the metal being considered. For instance, copper is very soluble
in water, so that ingestion (oral exposure) is a common pathway of exposure. Ingestion is also an
important exposure pathway of mercury, often through the consumption of fish or other animals in
which organic forms of this metal have bioaccumulated to a high level in their tissues. However, the
elemental form of mercury has a high vapor pressure so that inhalation exposure also may be significant
in certain situations. Dermal exposure to metals may cause local effects, but it is rarely a significant
consideration from an absorption perspective.
The route of absorption also may influence the subsequent distribution of the metal within the body
and, thus, may affect its metabolism, potential toxic effects, and excretion. While oral exposure to
copper may lead to the absorption of high levels through the gastrointestinal mucosa, several efficient
excretion mechanisms exist that may minimize the subsequent distribution of absorbed copper to the
bloodstream and other tissues. Thus, exposure through inhalation or dermal absorption may ultimately
pose the greatest danger of producing copper toxicity, since these routes may deliver the toxicant more
directly via the circulation to target tissues.
Inhalation
Inhalation is a primary route of exposure in occupational and, to a lesser extent, environmental settings.
A number of metals have the potential for producing toxic effects in the respiratory tract. Some metals
volatilize into the atmosphere where they exist for long periods as a vapor. Mercury is one such
example, being easily transported into the atmosphere when present in incinerated waste and potentially
traveling great distances before settling down back to earth. Mercury may also be present in the
breathing space of workers in an area where mercury is used or in the air around hazardous-waste sites
where mercury-containing materials are present.
The speciation of mercury has an important influence on its potential for inhalation exposure, and
subsequent distribution within the body. For instance, inhaled elemental mercury partitions mostly to