WASHINGTON -- A new report by the National Academies' National Research Council on the health effects of perchlorate, a chemical that in high doses can decrease thyroid function in humans and that is present in many public drinking-water supplies, says daily ingestion of up to 0.0007 milligrams per kilogram of body weight can occur without adversely affecting the health of even the most sensitive populations. That amount is more than 20 times the "reference dose" proposed by the U.S. Environmental Protection Agency in a recent draft risk assessment.

Environmental releases of perchlorate -- a component of rocket fuel and fireworks -- have been discovered in 35 states, and more than 11 million people have perchlorate in their drinking water at concentrations of 4 parts per billion or higher. As it considers a first-ever national standard for acceptable levels of perchlorate in drinking water, EPA has issued a series of draft risk assessments, each containing a reference dose upon which a standard could be based. Controversies over the scientific conclusions reached in the risk assessments, however, led the federal government to request that the National Research Council review the issue.

The most recent EPA risk assessment, published in 2002, proposes a daily reference dose of 0.00003 milligrams per kilogram (mg/kg) of body weight, which the agency said would correspond to a drinking-water concentration of 1 part per billion based on certain assumptions about body weight and daily water consumption. The committee that wrote the Research Council report did not include a corresponding drinking-water concentration with its reference dose because the assumptions that are used to derive drinking-water standards involve public-policy choices that were beyond the committee's charge.

Perchlorate inhibits the thyroid's uptake of iodide, which is essential for the production of thyroid hormones. One potential consequence of that effect is low thyro
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Contact: Bill Kearney
news@nas.edu
202-334-2138
The National Academies
11-Jan-2005

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