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- WASHINGTON, DC, August 6,
1999 (ENS) - Too little is known about the effects of exposure to low doses
of chemicals that change the balance of hormones in the human body, says
a new report from a National Research Council committee. These chemicals
include polychlorinated biphenyls (PCBs) and the pesticide DDT. In high
doses they are known to have harmful effects.
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- Called endocrine disrupters, these chemicals may affect
reproduction and development, the nervous system, the immune system, the
incidence of cancer, and other aspects of the biology of humans and wildlife.
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- New studies should be conducted that follow groups of
at-risk subjects from conception through adulthood, the Committee on Hormonally
Active Agents in the Environment recommends.
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- "Determining the risk to humans from contact with
these chemicals in the environment is difficult because ordinary exposure
to these agents has not been routinely monitored," said committee
chair Ernst Knobil, the Ashbel Smith Professor and H. Wayne Hightower Professor
in the Medical Sciences, Medical School, University of Texas, Houston.
"Determining what these exposures actually are is therefore of primary
importance."
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- Hormonally active agents include a wide variety of chemicals
that mimic the actions of sex hormones and have been associated with adverse
reproductive and developmental effects in wildlife.
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- The pesticide DDT, for instance, acts like the human
hormone estrogen, and so is called an estrogen mimic. In 1972, the Environmental
Protection Agency banned all uses of DDT, except for public health emergencies.
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- Exposure to hormonally active agents can occur from a
variety of sources. For example, eating food that is contaminated with
PCBs or DDT, and using commercial products such as cleaners, pesticides,
and food additives are ways in which people may come in contact with these
chemicals.
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- PCBs are a family of man-made chemicals that contain
209 individual compounds with varying toxicity. Because of their insulating
and nonflammable properties, PCBs have been used widely as coolants and
lubricants in transformers, capacitors, and other electrical equipment.
The manufacture of PCBs stopped in the United States in 1977 because of
evidence that PCBs accumulate in the environment and may cause health hazards
for humans.
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- Laboratory studies are needed to determine whether hormonally
active chemicals used in pesticides, cleaning solutions, and skin care
products have any affect on the immune system, the committee said.
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- The majority of the evidence available is based on studies
done on wildlife and laboratory animals. Very little is known about how
the compounds affect humans. In every area of investigation, the committee
said information about how humans are affected is sketchy and more well-designed
studies are needed.
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- Do They Cause Cancer?
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- Some researchers believe that exposure may increase the
incidence of breast cancer. There are also concerns that endocrine disrupters
may play a role in the reported declines in sperm counts, increased rates
of testicular and prostate cancer, and other male reproductive disorders.
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- The committee said that although some endocrine disrupters
have been associated with tumors of the thyroid, pituitary, and adrenal
glands in lab animals, an evaluation of the available data does not support
an association between breast cancer and adult exposure to the chemicals.
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- A recent study reported an association between the pesticide
dieldrin and breast cancer. Still, additional epidemiological and laboratory
studies are needed to help confirm or refute this possible relationship,
the committee said.
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- The current literature does not support associations
between hormonally active agents in the environment and other hormonally
sensitive cancers such as testicular, prostate, and endometrial cancer,
the report says. But few studies have measured the levels of these chemicals
in adult humans in relation to cancer risk, and no studies have been conducted
to examine associations between the risk of cancer and exposure to them
during fetal development.
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- Reproduction and Development
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- Adverse reproductive and developmental effects have appeared
in wildlife and laboratory animals due to exposures to some hormonally
active agents. In humans, fetal exposure to pesticides and other pollutants
from the mother's consumption of contaminated fish or other foods can cause
lower birth weights and premature births. Exposure has been linked to lower
IQs and memory deficits and delayed neuromuscular development.
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- No links between endocrine disrupters and increases in
the incidence of male reproductive disorders in humans, such as testicles
that have not descended to the scrotum and testicular cancer could be confirmed,
the committee said.
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- Evidence exists that links fetal exposure to high concentrations
of PCBs from industrial accidents, and exposure to PCBs and some pesticides
found in fish and other foods, with abnormalities in the developing nervous
system.
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- Monkeys exposed to hormonally active agents while in
the womb and during nursing have lapses in brain function when examined
14 months after exposure. Rats and mice exposed to the chemicals before
birth suffer impaired movement and learning ability.
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- Several endocrine disrupters affect a variety of elements
of the immune system in laboratory animals, the committee reported. There
is evidence that birds of the Great Lakes exhibited immune system dysfunction
after exposure to the chemicals, as did captive seals that were fed contaminated
fish from the Baltic Sea. Such immunosuppression is believed to be the
reason for the increased incidences of bacterial and viral infections in
seals in similarly contaminated waters.
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- Ecological Effects
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- Hormonally active agents (HAAs) in the environment probably
have contributed to declines in some wildlife populations, the committee
said. Fish and birds of the Great Lakes and alligators of Lake Apopka in
Florida have been affected. The chemicals are also likely to blame for
diseases and deformities in mink in the United States, river otters in
Europe, and marine mammals in European waters, the committee said.
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- These contaminants, along with inbreeding, may have contributed
to the low birth rate of the endangered Florida panther, and the increase
of embryonic death and deformities observed in snapping turtles of the
Great Lakes.
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- Only 30 to 50 panthers still remain in Florida, making
this one of the rarest and most endangered mammals in the world. (Photo
courtesy U.S. Fish & Wildlife Service) "It is difficult to determine
a clear causal relationship between these changes and exposure to HAAs
given all of the other environmental factors involved," the committee
said, and called for more wildlife studies on population size and life
expectancy.
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- Such research should examine the links between chemical
exposures and alterations in key developmental stages to understand how
they affect populations in the long term.
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- The study was funded by the U.S. Environmental Protection
Agency, the National Biological Service of the U.S. Department of the Interior,
and the Centers for Disease Control and Prevention.
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- The National Research Council is the principal operating
arm of the National Academy of Sciences and the National Academy of Engineering.
It is a private, nonprofit institution that provides independent advice
on science and technology issues under a congressional charter. A committee
roster follows.
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