Chemistry Reference
In-Depth Information
iron, uranium, and sometimes manganese, cadmium,
and lead. Whether local contamination exists, ingestion of
metals via food and drinking water is ordinarily the main
pathway of exposure for the general population.
A route of exposure that must not be forgotten is
the inhalation of tobacco smoke, which contains a
number of metals, including cadmium, nickel, arsenic,
and lead. It has been shown in a number of studies that
cadmium in cigarette smoke may have contributed a
third of the total body burden found at age 50 (Friberg
et al.
, 1985; WHO, 1992). The relative contribution of
cadmium made by tobacco smoke will, of course, be
considerably less when the intake of cadmium via food
or air is large. Some data clearly show that external
metal contamination of cigarettes in industries where
an exposure to metals through air already occurs may
increase the workers' inhalation dose of metals sever-
al fold. In areas where tobacco is grown and soils are
contaminated by cadmium, substantial exposure may
occur through smoking (Cai
et al.
, 1995).
found in fi sh from acid lakes were higher than in lakes
with waters with neutral pH (Wood, 1985; Chapter 13).
It is obviously of great importance to understand the
ecological cycles of metals to evaluate potential threats
to human health from consumption of food and drink-
ing water. There are still considerable gaps in our
knowledge in this scientifi c fi eld.
Essential metals may be toxic if exposure is exces-
sive (e.g., molybdenum). Other metals are not known
to have an essential function, and they may give rise to
toxic manifestations even when intakes are only mod-
erately in excess of the “natural” intake. Metal toxic-
ity is explainable on the basis of the interference with
cellular biochemical systems. Metals often interact at
important sites such as the SH groups of enzyme sys-
tems. They may also compete with other essential met-
als as enzyme cofactors (Chapters 3, 5, 7, and 9). Thus
the effects of a toxic metal may mimic the defi ciency
of an essential metal. For example, cadmium does not
penetrate into the fetus to any considerable degree but
causes an effect on the fetus most likely as a result of
a secondary zinc defi ciency. Cadmium has been found
to be high in the placenta and most likely blocks the
zinc uptake of the placenta. Several of the toxic metals
have a long biological half-time and tend to accumu-
late in the body. For cadmium, the biological half-time
in man is estimated to be 15-20 years or more. This
long half-time may be related to the fact that cad-
mium is the most potent inducer of the synthesis of
metallothionein, a high-affi nity metal-binding protein
(Kägi
et al.
, 1984; Nordberg, 1984; 1998; Nordberg and
Kojima, 1979). With constant exposure, the long biological
half-times imply that accumulation will sometimes
take place over an entire lifetime.
2.4 Essentiality of Metals
Metallic elements are found in all living organisms
and play a variety of roles. They may be structural ele-
ments, stabilizers of biological structures, components
of control mechanisms (e.g., in nerves and muscles),
and, in particular, are activators or components of
redox systems. Thus, some metals are essential ele-
ments, and their defi ciency results in impairment of
biological functions. Essential metals, when present in
excess, may even be toxic (Chapter 9). Although this
Handbook is concerned primarily with human health
effects resulting from excessive exposure to metals and
their compounds, it should be recognized that metals
might also have deleterious effects on other animal
species and plants. Such effects may lead to modifi -
cation of an entire population or species assembly in
an ecosystem. Such effects of metals may be of great
signifi cance to human life and should be considered
in the total evaluation of environmental pollution by
metals and their compounds (see Chapter 13).
It is diffi cult to rid the environment of a metal with
which it has been contaminated. Two striking examples
are mercury in the bottom sludge of lakes and cadmium
in soil. In addition, several metals and metalloids may
undergo methylation during their environmental and
biological cycles (e.g., tin, palladium, platinum, gold,
thallium, arsenic, selenium, and tellurium), but mer-
cury is the only metal presently known to undergo bi-
omagnifi cation in food chains. The potential infl uence
of acid precipitation on the methylation process for
mercury has been considered as one possible explana-
tion for the fact that concentrations of methyl mercury
2.5 Human Health Effects
Our information on metal toxicity, as far as humans
are concerned, is derived from industrial health, large-
scale, severe episodes of poisoning via contaminated
drinking water and food (MethylHg, Cd, Pb) and
recently from recognition of widespread occurrence
of less obvious but adverse effects on large population
groups. There are at least 20 metals or metal-like ele-
ments that can give rise to rather well-defi ned toxic
effects in man. These are dealt with in the specifi c chapters
of this Handbook. Arsenic, cadmium, lead, manganese,
and mercury have been studied most thoroughly, but
other metals are also of concern. Molybdenum brings
about goutlike signs, and aluminum has been shown
to have serious effects on the central nervous system
(CNS) under certain circumstances. Antimony and
cobalt may have effects on the cardiovascular system.
Some organometallic tin compounds give rise to effects
