potentially alter estrogen, androgen and thyroid transmission
signals, according to Thaddeus Schug, PhD, a researcher at
the National Institute of Environmental Health Sciences,
and colleagues (Endocrinology, 2015). Studies in animals and
humans suggest it affects reproductive and social behaviors.
Neuroscientist Sarah Evans, PhD, of Mount Sinai Hospital, and
colleagues, conducted a study of 125 women with detectable
BPA in their urine at 27 weeks of pregnancy. The researchers
found a significant association between levels of BPA and rule-breaking, depression and conduct problems in boys age 6 to 10,
but not in girls (Neuro Toxicology, 2014).
Critical windows of exposure
Timing appears to matter as well as gender; a number of
studies show that prenatal exposure to toxins is more likely
to be associated with IQ drops and behavior issues in boys
than in girls. A team of researchers at Columbia University
Mailman School of Public Health measured pre- and post-natal
BPA exposure and behavioral measures in children at ages 3
to 5 and again at ages 7 to 9 (Environmental Research, 2015).
Boys exposed to higher levels of BPA showed more behavioral
problems at both time points, including aggression and rule-breaking, as well as sleep problems, though girls had no similar
detectable issues. With exposure in early childhood, though, the
chemical appeared to affect girls more than boys.
“Boys and girls develop very differently, and so when there
is disruption of the endocrine system, it’s going to affect them
differently,” Yolton says.
Those effects are even more striking when it comes to
reproductive health. Evidence in animal studies suggests that
very high levels of endocrine disruptors may lead to congenital
anomalies in males and early puberty in females, according to a
2012 report by the WHO. Phthalates specifically are associated
with uterine abnormalities and with reduced testicle weight and
sperm production, and flame retardants with cryptorchidism,
or undescended testes.
Humans can excrete BPA, but another class of chemicals
used as flame retardants in mattresses, furniture foam and
carpet backing tends to linger, particularly in fatty tissue.
Manufacturers began to phase out one of these chemical
classes, polybrominated diphenyl ethers (PBDEs), in 2004 after
scientists found it accumulated in our bodies, in breast milk and
in the larger environment. Particles of these chemicals migrate
from products and settle into dust, so crawling children who
often put fingers and items into their mouths are more likely to
come into contact with them, leading to body levels some three
to nine times that for adults.
PBDEs are chemically similar to thyroid hormones, which
Safe until proven toxic
explains why they may tamper with brain development. In a
study by epidemiologist Aimin Chen, MD, of the University of
Cincinnati College of Medicine, and colleagues, higher prenatal
exposure to PBDEs was correlated with lower IQs of up to
five points and higher hyperactivity in 5-year-old children,
The chemicals have also been implicated in autism. Led by
Janine LaSalle, PhD, a biologist at the University of California,
Davis, researchers studied female mice with a genetic mutation
associated with autism, and exposed some of them to PBDEs.
Their female offspring showed impaired learning, memory and
social skills (Human Molecular Genetics, 2012).
Though there are hundreds of animal studies involving
endocrine disruptors, studying the potential effects of
endocrine disruptors in humans is daunting. First, proving
causation is extremely difficult, involving an “incalculable
number of parameters,” says Susan Koger, PhD, professor of
psychology at Willamette University and co-author of the
forthcoming book “Psychology for Sustainability” (Scott, Amel,
Koger, & Manning, 2016).
“Endocrine disruptors are manipulating our biochemical
milieu and interacting with our own hormonal system, and it’s
very difficult to determine their specific impacts because there
are so many variables,” Koger says.
Because of the different ways they signal cell receptors,
some chemicals, including BPA, may have what’s called a
nonmonotonic dose response curve, with a small exposure
sometimes being more harmful than a larger one. Plus, there
are many ways humans take them in: via food, including breast
milk, through the skin or by breathing them. Exposures, even
to the same toxin, may affect one developmental stage but
not another, or their effects may vary according to genetic
makeup. And finally, effects may not show up until years — or
generations — later.
“The dream study involves measuring reliable biomarker
levels, including chemical mixtures, in a large cohort of
women starting prenatally and covering multiple time points
throughout pregnancy and beyond,” Miodovnik says. “In
addition, health outcomes in the offspring would be assessed
using the same, well-validated instruments. Finally, the analysis
would control for other confounding risk factors, including the
home environment, secondhand tobacco smoke, and family
history of psychopathology.”
The chemicals may also interact with each other in ways that
we don’t understand, as Yolton pointed out in a 2014 review in
Neurotoxicology and Teratology.
“When we study these combinations by putting multiple
exposures into our statistical models, we find different results
than when we examine one toxicant at a time,” she says.