Human Health: Early Life Exposures & Lifetime Health
Endocrine disrupting chemicals can affect a range of hormonal systems in ways that interrupt multiple important systems in the body. Hormonal signaling is an essential component of prenatal and childhood development, so chemical disruption during these sensitive periods can have permanent adverse effects, including birth defects. In addition, children's systems for eliminating toxic chemicals may not be as developed as adults and children can have higher exposures to chemicals due to their behaviors and the ways that they interact with their environment, physiology and anatomy when compared to adults. For instance, infants and toddlers may chew on furniture and toys as a natural part of exploring their environment.
Early life exposure to endocrine disrupting chemicals can have a range of effects that show up in adulthood. For instance, studies in laboratory animals suggest that fetuses exposed to phthalates, chemicals used in plastics, can have multiple health issues as adults, including infertility, issues with genitalia development and sperm production problems. Also there are epidemiological studies showing developmental changes in children including early puberty in boys and girls compared to previous generations. Chemicals that are being researched for such effects include organotins (used in PVC and as biocides), BPA (Bisphenol-A, present in canned food and many plastics) and PFOA (perfluorooctanoic acid, present in consumer products such as stain resistant carpet and waterproof jackets).
Researchers have also been able to define vulnerable periods for early life exposures. For example, thyroid hormone levels of the mother are critical during the early stages of conception to maintain proper fetal thyroid function during the first trimester. Appropriate thyroid hormone levels in the mother ensure proper brain development until the fetal thyroid gland develops sufficiently to take over. EPA researchers are continuing to research this area to learn more about the stages of early development most susceptible to chemicals interfering with important processes in the body. The answer will vary depending on the type of chemical and the vulnerable developmental processes occurring at the time of exposure. Researchers are also learning more about the effects of exposure to mixtures of multiple chemicals and exposures occurring through multiple routes at different times as children become adults.
EPA researchers are studying the effects of early-life exposures to answer a variety of questions. What are the routes of exposure to these chemicals, what are the exposure levels, and do these chemicals accumulate in the human body over time or does the body metabolize them quickly? What are the effects of exposure to mixtures of these chemicals and how do these chemicals affect susceptible populations such as children? EPA researchers are studying phthalates, for instance, and are developing models that can evaluate the potential for these chemicals to accumulate in the body and are developing methods that can be used to test mixtures of these chemicals. This research gained momentum after a report in 2007 from the National Academies of Sciences recommended that phthalates be assessed because these chemicals are present in the human body and have the potential to accumulate. Such work is augmented by ongoing research designed to simplify testing of chemicals for their ability to disrupt early development. Rapid tests in cells are being designed to serve as the initial screen for potential early life effects. For instance, researchers are examining stem cells in culture and assessing their response (proliferation, etc).
Results and Impact
Humans are exposed to a range of chemicals at many points during fetal development and early childhood, and studies assessing the cumulative risk of chemicals will provide a more realistic assessment of the impact of endocrine disruptors on human health. Research assessing how such chemical exposures might interact with other environmental stressors, such as stress or a high-fat diet, will help researchers understand the real-life impact of endocrine disruptors. Ongoing toxicity tests using cells and animals will help identify chemicals that have effects on biological processes important to early life development and which life stages are most vulnerable. Together with exposure information gathered though predictive models and actual measurements (from blood, breast milk, urine, etc), this research will help identify chemicals with the potential to be toxic to early life development, estimate exposures of concern, assess chemical mixtures for potential risks, determine how children get these chemicals in their bodies and how their bodies metabolize them.