Page 390 - Environment: The Science Behind the Stories

fAQ Does exposure to a toxic substance
cause genetic resistance to the
substance?
When a population of organisms is exposed to a toxicant,
such as a pesticide, a few individuals often survive while the
vast majority of the population is killed. These individuals sur-
vive because they possess genes (which others in the popula-
tion do not) that code for enzymes that counteract the toxic
properties of the toxicant. Because the effects of these genes
are expressed only when the pesticide is applied, many peo-
ple think the toxicant “creates” detoxification genes by mutat-
(a) Exposure through toys ing the DNA of a small number of individuals. This is not the
case. The genes for detoxifying enzymes were present in the
DNA of resistant individuals from birth, but their effects were
seen only when pesticide exposure caused selective pressure
(pp. 68–70) for resistance to the toxic substance.

Individuals vary in their responses
to hazards

Some of the defenses described above have a genetic basis.
As a result, individuals may respond quite differently to
identical exposures to hazards because they happen to have
different combinations of genes. Poorer health also makes
an individual more sensitive to biological and chemi-
cal hazards. Sensitivity also can vary with sex, age, and
(b) Exposure through cosmetics
weight. Because of their smaller size and rapidly devel-
Figure 14.10 Many soft plastic children’s toys and many oping organ systems, younger organisms (for example,
cosmetics contain phthalates, a hormone-disrupting fetuses, infants, and young children) tend to be much more
chemical. Banned in Europe, phthalates remain widespread in the sensitive to toxicants than are adults. Regulatory agencies
United States.
such as the U.S. Environmental Protection Agency (EPA)
typically set human chemical exposure standards for adults
vital activities such as eating, drinking, and breathing. Once in and extrapolate downward for infants and children. How-
the organism, they are distributed widely by the circulatory and ever, many scientists contend that these linear extrapola-
lymph systems in animals, and by the vascular system in plants. tions often do not offer adequate protection to fetuses,
Organisms possess biochemical pathways that use infants, and children.
enzymes to detoxify harmful chemicals. Some pathways
break down, or metabolize, toxic substances to render them
inert. Other pathways make toxic substances water soluble The type of exposure can affect the
so they are easier to excrete through the urinary system. In response
humans, many of these pathways are found in the liver, so
this organ is disproportionately affected by intake of harmful The risk posed by a hazard often varies according to whether
substances such as excessive alcohol. a person experiences high exposure for short periods of CHAPTER 14 • Envi R onm E n TA l H EA lTH A nd T o xi C ology
Some toxic substances cannot be effectively detoxified or time, known as acute exposure, or low exposure over long
made water soluble by detoxification enzymes. These chemicals periods of time, known as chronic exposure. Incidences of
are sequestered in fatty tissues and cell membranes to keep them acute exposure are easier to recognize, because they often
away from vital organs. Heavy metals, dioxins, and some insec- stem from discrete events, such as accidental ingestion, an
ticides (including DDT) are stored in body tissue in this manner. oil spill, a chemical spill, or a nuclear accident. Toxicity
These defenses can protect organisms against low lev- tests in laboratories generally reflect acute toxicity effects.
els of some toxicants but can be overwhelmed if exposure However, chronic exposure is more common—and more
exceeds critical levels. For other toxicants, harm occurs with difficult to detect and diagnose. Chronic exposure often
any exposure if organisms have no defense against the sub- affects organs gradually, as when smoking causes lung can-
stance. Defense mechanisms for natural toxins have evolved cer, or when alcohol abuse leads to liver or kidney damage.
over millions of years. Organisms have not had long-term Pesticide residues on food or arsenic in drinking water also
exposure to the synthetic chemicals that are so prevalent in pose chronic risk. Because of the long time periods involved,
today’s environment, so the impacts of these toxic substances relationships between cause and effect may not be readily
can be severe and unpredictable. apparent. 389

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