The U.S. Environmental Protection Agency should examine whether combined exposures to chemicals known as phthalates could cause adverse health effects in humans, says a new report from the National Research Council. In addition, this analysis, called a cumulative risk assessment, should consider other chemicals that could potentially cause the same health effects as phthalates, instead of focusing on chemicals that are similar in structure, which is EPA's current practice. Furthermore, EPA should consider using the recommended approach for future cumulative risk assessments on other kinds of chemicals.
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Recent animal studies have increased understanding of the potential risks from phthalates, although few human studies on the health effects of phthalates are available, said the committee that wrote the report. To decide whether a cumulative risk assessment is warranted, two factors needed to be determined: whether humans are exposed to multiple phthalates at any given time, and whether sufficient evidence exists linking exposures to similar adverse health effects. The committee established that recent studies have shown widespread human exposure to multiple phthalates, including in utero exposure.
Then, the committee reviewed animal research and found that exposure to various phthalates in lab animals produced similar health outcomes, including a range of effects on the development of the male reproductive system. The most notable effects in male rats are infertility, undescended testes, malformation of the penis, and other reproductive tract malformations. However, the severity of effects differs among phthalates; some exhibit less severe or no effects. Furthermore, the age of the animals at the time of exposure is critical to the severity of the effects. For example, the fetus is most sensitive. Given that multiple human exposures to phthalates occur and that research shows exposure to different phthalates leads to similar outcomes in lab animals, a cumulative risk assessment is called for, the committee said.
The animal studies reviewed by the committee also indicated that some phthalates reduce testosterone concentrations. Depending on when this drop occurs, it can cause a variety of effects in animals that are critical for male reproductive development. Other chemicals known as antiandrogens, which prevent or inhibit male hormones from working, can produce similar effects in lab animals. The committee recommended that phthalates and other chemicals that affect male reproductive development in animals, including antiandrogens, be considered in the cumulative risk assessment. A focus solely on phthalates to the exclusion of other chemicals would be artificial and could seriously underestimate risk, the committee emphasized.
Currently when conducting cumulative risk assessments, EPA often considers only chemicals that are structurally related, on the assumption that they have the same chain of reactions that lead to a final health outcome. That practice ignores how exposures to different chemicals may result in the same health effects. The conceptual approach taken for phthalates -- to consider chemicals that cause similar health effects -- should also be applied when completing any cumulative risk assessment, the committee said. For instance, EPA could evaluate the risk of combined exposures to lead, methylmercury, and polychlorinated biphenyls because all contribute to cognitive deficits consistent with IQ reduction in children.
In addition, further research should be conducted to allow greater refinement of the cumulative risk assessment associated with phthalates and reduce uncertainty associated with such an assessment, the report says. Moving beyond the constraints of grouping chemicals based on structural similarity may appear challenging, but it is feasible to evaluate the multiplicity of human exposures. It also directly reflects EPA's mission to protect human health, the committee noted. Such a shift in approach would entail substantial efforts by EPA, such as defining and setting priorities among the most prominent adverse health effects. However, a focus on similar outcomes facilitates the process by identifying the groups of chemicals that should be included.
The National Academy of Sciences, National Academy of Engineering, Institute of Medicine, and National Research Council make up the National Academies. They are independent, nonprofit institutions that provide science, technology, and health policy advice under an 1863 congressional charter. Committee members, who serve pro bono as volunteers, are chosen by the Academies for each study based on their expertise and experience and must satisfy the Academies' conflict-of-interest standards. The resulting consensus reports undergo external peer review before completion. For more information, visit http://www.nationalacademies.org/xpedio/groups/nasite/documents/webpage/na_069618.pdf.
Copies of Phthalates and Cumulative Risk Assessment: The Tasks Ahead are available from the National Academies Press; tel. 202-334-3313 or 1-800-624-6242 or on the Internet at http://www.nap.edu.
Date: Dec. 18, 2008
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