Page 388 - Environment: The Science Behind the Stories

pesticides that were virtually untested for health effects were assume that all artificial substances are unhealthy and that all
indiscriminately sprayed over residential neighborhoods and natural substances are healthy. In fact, the plants and animals
public areas, on an assumption that the chemicals would do we eat contain many chemicals that can cause us harm. Recall
no harm to people. Carson brought together a diverse collec- that plants produce toxins to ward off animals that eat them. In
tion of scientific studies, medical case histories, and other domesticating crop plants, we have selected (p. 70) for strains
data to demonstrate that the insecticide DDT in particular, with reduced toxin content, but we have not eliminated these
and artificial pesticides in general, were hazardous to people, dangers. Furthermore, when we consume animal meat, we
wildlife, and ecosystems. Most consumers had no idea that the ingest toxins the animals obtained from plants or animals they
store-bought chemicals they used in their houses, gardens, and ate. Scientists are actively debating just how much risk natural
crops might be toxic. toxicants pose, and it is clear that more research is required on
The chemical industry challenged Carson’s book vigor- these questions.
ously, attempting to discredit the author’s science and personal
reputation. Carson suffered from cancer as she finished Silent
Spring, (p. 192) and she lived only briefly after its publication. Toxic substances come in different types
However, the book was a best-seller and helped gener-
ate significant social change in views and actions toward the Toxicants can be classified based on their particular effects
environment. The use of DDT was banned in the United States on health. The best-known are carcinogens, which are sub-
in 1973 and is now illegal in a number of nations. stances or types of radiation that cause cancer. In cancer,
U.S. chemical companies still manufacture and export malignant cells grow uncontrollably, creating tumors, damag-
DDT, because developing countries with tropical climates ing the body, and often leading to death. Cancer frequently
use DDT to control disease vectors, such as mosquitoes has a genetic component, but a wide variety of environmen-
that transmit malaria. In these countries, malaria represents tal factors are thought to raise the risk of cancer. Indeed, in
a greater health threat than do the toxic effects of the pes- 2010 the President’s Cancer Panel concluded that the preva-
ticide. New technologies are promising to reduce the need lence of environmentally induced cancer has been “grossly
for pesticides to control malaria, however. In 2012, research- underestimated.” In our society today, the greatest number of
ers reported that they genetically modified a type of bacteria cancer cases is thought to result from carcinogens contained
found in the digestive tract of mosquitoes so that it produced in cigarette smoke. Polycyclic aromatic hydrocarbons (PAHs;
a protein that impairs the hatching of the malarial parasite p. 47) comprise some of the carcinogens found in cigarette
from its egg sacs in the mosquito’s gut. The bacteria were smoke. PAHs also occur in charred meats and are released
introduced into mosquitoes by mixing them in a sugar solu- from the combustion of coal, oil, and natural gas.
tion for mosquitoes to drink. While 90% of mosquitoes that Carcinogens can be difficult to identify because there
didn’t drink the solution were later found to contain malarial may be a long lag time between exposure to the agent and the
parasites, only 20% of the mosquitoes that drank the solu- detectable onset of cancer—up to 15–30 years in the case of
tion later contained parasites. The use of genetically modi- cigarette smoke. Moreover, as with all risks, only a portion
fied organisms is not itself without risk (pp. 279–286), but of people exposed to a carcinogen will eventually get can-
such research may provide non-chemical options for reduc- cer. Cancer is a leading cause of death that kills millions and
ing human mortality from malaria and speed the phase-out of leaves few families untouched. Two of every five Americans
DDT around the world. are diagnosed with cancer at some time in their lives, and one
of every five dies from it. Thus, the study of carcinogens has
played a large role in shaping the way that toxicologists pur-
WEIGHING THE ISSUES sue their work.
Mutagens are substances that cause genetic mutations in
A CIRClE Of POISON? Although the United States has the DNA of organisms (p. 47). Although most mutations have
banned the use of DDT, U.S. companies still manufacture and little or no effect, some can lead to severe problems, including
export the compound to developing nations. Thus, it is pos- cancer and other disorders. If mutations occur in an individu-
sible that pesticide-laden food can be imported back into the al’s sperm or egg cells, then the individual’s offspring suffer
United States in what has been called a “circle of poison.” the effects. CHAPTER 14 • Envi R onm E n TA l H EA lTH A nd T o xi C ology
How do you feel about this? Is it unethical for one country to Chemicals that cause harm to the unborn are called
sell to others a substance that it has deemed toxic? Or would teratogens. Teratogens that affect development of human
it be unethical for the United States not to sell DDT to African embryos in the womb can cause birth defects. One example
nations if they desire it for controlling malaria?
involves the drug thalidomide, developed in the 1950s to aid
sleeping and to prevent nausea during pregnancy. Tragically,
the drug turned out to be a powerful teratogen, and its use
caused birth defects in thousands of babies. Even a single dose
Not all toxic substances are synthetic, during pregnancy could result in limb deformities and organ
and not all synthetic chemicals are toxic defects. Thalidomide was banned in the 1960s once scientists
recognized its connection with birth defects. Ironically, today
Although many toxicologists focus on synthetic chemicals, the drug shows promise in treating a wide range of diseases,
toxic substances also exist naturally in the environment around including Alzheimer’s disease, AIDS, and various types of
us and in the foods we eat. Thus, it would be a mistake to cancer. 387

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