Environmental Engineering Reference
In-Depth Information
toxin. Some individuals are sensitive to a number of
toxins—a condition known as multiple chemical sen-
sitivity (MCS). This genetic variation in individual re-
sponses to exposure to various toxins raises a difficult
ethical, political, and economic question: When regu-
lating levels of a toxic substance in the environment,
should the allowed level be set to protect the most
sensitive individuals (at great cost) or the average
person?
Five major factors can affect the harm caused by a
substance. One is its solubility. Water-soluble toxins
(which are often inorganic compounds) can move
throughout the environment and get into water sup-
plies and the aqueous solutions that surround the cells
in our bodies. Oil- or fat-soluble toxins (which are usu-
ally organic compounds) can penetrate the mem-
branes surrounding cells because the membranes al-
low similar oil-soluble chemicals to pass through
them. Thus, oil- or fat-soluble toxins can accumulate in
body tissues and cells.
A second factor is a substance's persistence. Many
chemicals, such as DDT, are used precisely because of
their persistence or resistance to breakdown. Of course,
this persistence also means they can have long-lasting
harmful effects on the health of wildlife and people.
A third factor for some substances is bioaccumula-
tion, in which some molecules are absorbed and stored
in specific organs or tissues at higher than normal lev-
els. As a consequence, a chemical found at a fairly low
concentration in the environment can build up to a
harmful level in certain organs and tissues.
Arelated factor is biomagnification, in which levels
of some potential toxins in the environment become
magnified as they pass through food chains and webs.
Organisms at low trophic levels might ingest only
small amounts of a toxin, but each animal on the next
trophic level up that eats many of those organisms will
take in increasingly larger amounts of that toxin (Fig-
ure 9-16, p. 197). Examples of chemicals that can be
biomagnified include long-lived, fat-soluble organic
compounds such as DDT, PCBs (oily chemicals used in
electrical transformers), and some radioactive isotopes
(such as strontium-90).
A fifth factor is chemical interactions that can de-
crease or multiply the harmful effects of a toxin. An
antagonistic interaction can reduce harmful effects. For
example, there is preliminary evidence that vitamins E
and A can interact to reduce the body's response to
some cancer-causing chemicals.
A synergistic interaction multiplies harmful effects.
For instance, workers exposed to tiny fibers of asbestos
increase their chances of getting lung cancer 20-fold.
But asbestos workers who also smoke have a 400-fold
increase in lung cancer rates. In such cases, one plus
one can be a lot greater than two.
The type and amount of health damage that result
from exposure to a chemical or other agent are called
the response. An acute effect is an immediate or rapid
harmful reaction to an exposure—ranging from dizzi-
ness to death. A chronic effect is a permanent or long-
lasting consequence (kidney or liver damage, for ex-
ample) from exposure to a single dose or to repeated
lower doses of a harmful substance.
A basic concept of toxicology is that any synthetic
or natural chemical can be harmful if ingested in a large
enough quantity. For example, drinking 100 cups of
strong coffee one after another would expose most
people to a lethal dosage of caffeine. Similarly, down-
ing 100 tablets of aspirin or 1 liter (1.1 quarts) of pure
alcohol (ethanol) would kill most people.
The critical question is this: How much exposure to a
particular toxic chemical causes a harmful response? This
is the meaning of the chapter-opening quote by the
German scientist Paracelsus about the dose making
the poison.
People vary in terms of the dose of a toxin they
can tolerate without incurring significant harm, be-
cause of differences in their genetic makeup. For this
reason, a better way to state Paracelsus's principle of
toxicology is this: The dose makes the poison, but dif-
ferently for different individuals.
Your body has three major mechanisms for reduc-
ing the harmful effects of some chemicals. First, it can
break down (usually by enzymes found in the liver),
dilute, or excrete (for example, in your breath, sweat,
and urine) small amounts of most toxins to keep them
from reaching harmful levels. However, accumu-
lations of high levels of toxins can overload the ability
of your liver and kidneys to degrade and excrete such
substances.
Second, your cells have enzymes that can some-
times repair damage to DNA and protein molecules.
Third, cells in some parts of your body (such as your
skin and the linings of your gastrointestinal tract,
lungs, and blood vessels) can reproduce fast enough to
replace damaged cells.
Science: Effects of Trace Levels
of Toxic Chemicals
Trace amounts of chemicals in the environment or
your body may or may not be harmful.
Should we be concerned about trace amounts of vari-
ous chemicals in air, water, food, and our bodies? The
honest answer is that, in most cases, we do not know
because of a lack of data and the difficulty of determin-
ing the effects of exposures to low levels of chemicals.
Some scientists think that trace levels of most
chemicals are not harmful. They point to the dramatic
increase in average life expectancy in the United States
since 1950. Other scientists are not so sure and suggest
that much more research is needed to evaluate the pos-
sible long-term harm caused by exposure to low levels
of the thousands of new synthetic chemicals that we
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