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Spring 2005 Vol. 5 No. 1

Table of Contents

©Photodisc Perchlorate

Setting a Safe Dose

When the National Research Council early this year issued its assessment of the health risks of perchlorate, a chemical contaminating water supplies in 35 states, both advocates and opponents of a tough federal drinking-water standard seized upon the study to support their position. Some environmental groups claimed the new study would lead to a standard near one part per billion, a level proposed in 2002 by the U.S. Environmental Protection Agency. But the defense community argued that the standard should be much higher. Perchlorate has mainly been used as a component in rocket fuel and munitions, and if a cleanup is required, the defense industry would largely be responsible for its cost.

The Research Council report, however, did not directly support either stance, according to the committee that wrote it. The report emphasizes that drinking-water standards are based on assumptions the committee was not asked to make, such as average water consumption and average body weight. Regulators also need to consider other sources of human exposure to perchlorate, such as contaminated food and milk.

What the report does propose is a safe daily dose of the chemical, regardless of the source. It says that up to 0.0007 milligrams per kilogram (mg/kg) of body weight can be consumed daily without adversely affecting even the most sensitive populations -- fetuses of pregnant women with thyroid problems.

Perchlorate inhibits the thyroid's uptake of iodide, which is crucial for the production of thyroid hormones. There have been some studies of the effects of perchlorate exposure on human populations, but such studies may not be applicable at the individual level. The committee therefore chose to base its safe reference dose primarily on five human clinical studies, particularly one issued in 2000 that found no inhibition of iodide uptake in adult participants who took 0.007 mg/kg of body weight per day. This led to the 0.0007 reference dose after the committee applied an uncertainty factor of 10 to protect the fetuses of pregnant women who might have a pre-existing thyroid condition or insufficient iodide consumption.

The fact that no inhibition of iodide uptake -- which needs to occur before an adverse health effect is triggered -- was observed at the level selected from the clinical study, coupled with the uncertainty factor, prompted the committee to call its determination of a reference dose a "conservative, health-protective approach to perchlorate risk assessment," although one committee member who thought the data were inadequate contended that an additional uncertainty factor of three should be applied.

EPA said in its draft assessment that a standard of one part per billion in drinking water corresponded with a reference dose of 0.00003 mg/kg -- 23 times less than the one recommended by the Research Council. EPA's reference dose was lower because it is based on animal studies -- which generally require higher uncertainty factors -- including one study in rats that the agency said showed perchlorate exposure caused thyroid tumors in two offspring. However, the Research Council committee concluded that it was highly unlikely that disruption of thyroid function from perchlorate exposure would lead to thyroid cancer in humans.

Besides the standards that interest groups declared would be supported by the committee's assessment, some press reports also offered standards that journalists said corresponded with the committee's reference dose. Reporters contacted for comment said they calculated the standards -- which varied from one news story to the next -- by applying perchlorate-exposure assumptions used by EPA or by states that had set a standard.

Six weeks after the report's release, EPA announced that it was officially adopting the committee's reference dose. The agency determined that the dose corresponds with a drinking-water equivalent concentration of 24.5 parts per billion, but that perchlorate concentrations in food and milk still need to be examined.
  -- Bill Kearney

Health Implications of Perchlorate Ingestion. Committee to Assess the Health Implications of Perchlorate Ingestion, Board on Environmental Studies and Toxicology, Division on Earth and Life Studies (2005, approx. 191 pp.; ISBN 0-309-09568-9; available from the National Academies Press, tel. 1-800-624-6242; $42.00 plus $4.50 shipping for single copies).

The committee was chaired by Richard B. Johnston Jr., associate dean, research development, and professor, department of pediatrics, University of Colorado School of Medicine; and executive vice president for academic affairs, National Jewish Medical and Research Center, Denver. The study was funded by the U.S. Environmental Protection Agency, U.S. departments of Defense and Energy, and NASA.

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Copyright 2005 by the National Academy of Sciences