Some populations are especially vulnerable to the potential health effects of toxic chemical exposures, including the devloping fetus.
The timing of a toxic chemical exposure can change the presence, type, or severity of the health effects from that exposure. Exposures during gestation (development in the womb), in particular, can have serious health consequences.
Prenatal development, particularly brain development, proceeds at a blistering pace. “Neuroendocrine chemicals are very busy within the first months of pregnancy,” said Larry Silver, MD.
The developing fetus is extraordinarily vulnerable to the potential effects of exposure to neurotoxic chemicals for several reasons. As Drs. Grandjean and Landrigan noted in an article published in the Lancet, “during the nine months of fetal life, the human brain must develop from a strip of cells along the dorsal ectoderm of the fetus into a complex organ consisting of billions of precisely located, highly interconnected, and specialized cells” and thus “windows of unique susceptibility to toxic interference arise that have no counterpart in the mature brain, or in any other organ.”12
Also, the placenta (a temporary organ that provides the fetus with oxygen and nutrients) does not shield the fetus from all toxic chemical exposures. The fetus has limited ability to metabolize and excrete toxins compared to adults, and the blood-brain barrier that protects mature brains from many chemical exposures is still forming until about six months after birth.74
Chemical Case Study:
Thyroid and brain development
The thyroid gland controls how quickly the body uses energy, makes proteins, and controls how sensitive the body should be to other hormones. Good thyroid functioning is critical for good health. Healthy functioning of the thyroid can be damaged by exposures to chemicals such as PCBs,75,76 dioxin, perchlorate,74 bisphenol A,77 and polybrominated diphenyl ethers (PBDEs).78
Low thyroid hormone levels in the fetus have been associated with developmental delays, visual impairment, memory and cognition problems, and lowered IQ. The fetus must rely on maternal thyroid hormones until its own thyroid is functioning, usually in the second trimester.74 If the mother herself is low in the thyroid hormone (T4), she may be unable to provide the amount of thyroid hormone necessary to the fetus, whose neurological system may consequently be impaired.74
Perchlorate is a chemical used in the manufacture of rocket fuel, road flares and airbags. We are exposed to perchlorate through drinking water, produce, and dairy products. Perchlorate can inhibit the production of T4 by preventing the thyroid from taking up a building block for T4, iodine, from the blood stream. Iodine is used in the body’s production of T4.79 We obtain iodine from eating iodine-containing foods such as fish, dairy products, and plants grown in soil rich in iodine.
The CDC has found that 36% of women in the United States have less than optimal levels of iodine in their bodies. The CDC also reports that we all carry detectable levels of perchlorate in our bodies, which means that those women with already low iodine levels may be more likely to have a dysfunctional thyroid, i.e., a thyroid not producing adequate amounts of thyroid hormone (TH).80 Despite this, there are no federal standards regulating the amount of perchlorate in our drinking water.
Larry Silver, MD states, “We know that alcohol use and smoking might impact the fetus so we tell the parents please don’t smoke, don’t drink, and don’t use drugs… We know that the thyroid can be harmed by a variety of toxic chemicals but we don’t protect women of child bearing age from these chemicals. Low thyroid functioning in the mother may impact the fetus, and if we can prevent exposures to these toxic chemicals, then we would potentially have an enormous impact on the rates of learning and developmental delays.”
Chemical Case Study:
Prenatal BPA exposure associated with aggression, hyperactivity in two-year-old girls
Researchers from the University of North Carolina and Simon Fraser University in British Columbia measured BPA concentrations in the urine of about 250 pregnant women in Cincinnati, Ohio at 16 and 26 weeks of gestation, and again at birth. Two years later, the behavior of the women’s children was assessed using the Behavioral Assessment System for Children.
After adjusting the data for confounding factors, the researchers found that BPA concentrations at 16 weeks of gestation were more strongly associated with externalizing scores than concentrations at 26 weeks or birth. “Externalizing” behavior includes hyperactivity and aggression. The association between higher BPA concentrations at 16 weeks and above-average externalizing scores was much stronger in female offspring.37
Researcher Bruce Lanphear, MD, MPH, Faculty of Health Sciences at Simon Fraser University, was quoted as saying, “The girls showed a definite difference in temperaments. Their behavior was actually much more like [the behavior of] boys at the same age.”81