In addition to the possible neurological effects, some researchers theorize
that the hormone disrupters could be reducing men's sperm counts or
increasing diseases of the reproductive system.
Pesticide company representatives--and some toxicologists and other
scientists -- remain skeptical that commonly found levels of pesticides can
alter human thyroid and sex hormones.
"I'm kind of dubious that low-level exposures to chemicals are raising all
kinds of havoc with the endocrine system," said John McCarthy, vice
president of a group representing pesticide manufacturers, the American
Crop Protection Assn. "The human system has so many protective mechanisms,
and our bodies are bombarded with all kinds of things."
Still, he said, the industry is highly concerned about the findings
suggesting neurological damage, and would like to see a comprehensive
review to evaluate all existing studies and figure out what they
"We ought to be taking a very hard look at it," McCarthy said. "There's
almost a study a week of one type or another, and it's hard to see how it
all fits together. We have to take some time to say, 'OK, what does this
all mean? Is this something that should require some abrupt change [in
pesticides] or fine-tuning or more research?'"
No one knows how many pesticides out of 77,000 used in the United States
might alter sex or thyroid hormones.
The U.S. Environmental Protection Agency requires tests that screen
pesticides for cancer and birth defects--but not for hormone effects. A
committee last year devised new testing requirements--supported by the
pesticide industry--that are expected to take effect in 2001.
It has long been suspected that various environmental pollutants can damage
brain development. Industrial compounds called PCBs have been linked to
learning disabilities in children of women who ate contaminated Great Lakes
The link to pesticides is far from definitive, however, and big gaps in
knowledge remain. Questions abound: How do the contaminants disrupt thyroid
levels? What does that physically do to the brain?
What dose of exposure does it take? Does the human body have some defense
mechanism to fend off low levels of hormones? What do mixes of various
man-made and natural hormones do to people?
Some scientists suspect that the damage is passed from a mother to her
unborn child early in the first trimester, before most women even know they
Thyroid hormones guide the nerve cells that dictate how the brain of a
fetus develops and the number of brain cells created. One theory is that if
a mother receives a dose of pesticides during this critical phase, it can
interfere with her thyroid levels -- sometimes raising them, sometimes
lowering them--irreversibly altering the child's nervous system.
How the child's brain circuitry develops determines his or her hand-eye
coordination, motor skills and learning ability.
Thyroid hormones also can change behavior--an excess can make people quick
to anger, while a low count could have the opposite effect. The hormones
also can alter steroids that control aggression and immune systems.
"Thyroid hormones are important to brain development, and that's been known
for a long time," said Dr. Harley Kornblum, a pediatric neurologist at the
UCLA Medical Center. But, he and other neurologists say, it's debatable how
important the mother's thyroid level is to the fetus, and it's even more
uncertain what role environmental contaminants may play.
Porter said children up to age 8 who have developing brains and immune
systems are "especially vulnerable" to changes in thyroid hormones.
Some symptoms of children exposed to pesticides are similar to attention
deficit hyperactivity disorders, which have been increasingly diagnosed in
American children. Some medical research supports a link between thyroid
hormones and those disorders, but the connection, especially with
pesticides, remains unclear.
The implication for adults, and whether pesticides might cause thyroid
disorders, also is unknown.
In the study of 50 Mexican children, the scientists, led by anthropologist
Elizabeth Guillette of the University of Arizona, said genetic and social
factors--including income, education and health services--are so similar
between the farm valley and the foothills that they cannot explain the
differences in the youngsters' cognitive ability.
"These children share similar genetic background, diets, water mineral
contents, cultural patterns and social behaviors. The major difference was
their exposure to pesticides," Guillette and Mexican researchers said in a
report published in June in the journal Environmental Health Perspectives.
Most of the stick-figure drawings by the 4- and 5-year-olds in the farm
valley were unrecognizable as human beings--they look like the scribbles of
a 2-year-old. In contrast, drawings by the foothill children had heads,
eyes, torsos, arms and legs.
Experts say that the inability to draw a person indicates a breakdown
between the brain's ability to process visual information and its ability
to control fine muscles.
"Some valley mothers stressed their own frustration in trying to teach
their child how to draw. In addition, two valley children drew pictures
composed of boxes, arches and lines, claiming these pictures were people,"
the researchers reported.
Other tests pointed to recollection and stamina problems. One foothill
child could jump for 336 seconds--over three times longer than the
best-performing valley child.
Some scientists remain dubious of the results because the tests on the
children were unusual, and are intrinsically subjective and difficult to
Dr. Richard Jackson, director of the National Center for Environmental
Health at the federal Centers for Disease Control and Prevention, said his
staff was "unimpressed by the scientific rigor" of the work in the report.
McCarthy said that although the differences between the two populations of
children seem striking, other hidden factors, rather than pesticide
exposure, cannot be ruled out.
Other studies, meanwhile, show that pesticide exposure during the first
trimester of pregnancy increases birth defects.
The University of Minnesota and the federal EPA in a 1996 study found a
high rate of birth defects in the children of Minnesotans who work as
pesticide appliers as well as the general population of western Minnesota,
a major farm region with heavy pesticide use.
The defect rate was the highest among babies born nine months after the
spring season, indicating that the risk rises for children conceived during
the time when pesticide use increases.
In Porter's 5-year study of mice, the animals drank water containing a
mixture of two pesticides--aldicarb and atrazine--and nitrates from
The concentrations ingested by the mice were similar to those found in
ground water in many agricultural areas, Porter said. Aldicarb, atrazine
and nitrates are the three most abundant agricultural contaminants in the
United States, although they do not rank high in use in California.
While the mix of the three chemicals altered the mice hormones, each one
alone did not. That points out a gaping hole in the federal effort to
protect consumers--the EPA only tests for effects of pesticides
individually, not cumulatively.
The EPA tests, Porter said, "generate a great deal of false confidence in
the safety" of pesticides.
Source: Environmental Health Perspectives, June 1998.
Copyright 1999 Los Angeles Times. All Rights Reserved
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