IN 1985, perchlorate contamination was discovered in wells at California Superfund sites; however, perchlorate contamination of water sources nationwide was not recognized until 1997. Today, more than 11 million people have perchlorate in their public drinking-water supplies at concentrations of at least 4 ppb (4 µg/L).1 No national drinking-water standard for perchlorate exists, and the concentration at which a standard should be set is being debated. The U.S. Environmental Protection Agency (EPA), which has the responsibility for establishing national drinking-water standards, has issued draft risk assessments of perchlorate. However, the assessments have come under criticism on the grounds that the conclusions presented in them are based on flawed scientific studies and that not all available data have been incorporated appropriately into them.

In view of the controversy surrounding the concentration at which perchlorate should be regulated, EPA, the Department of Defense (DOD), the Department of Energy (DOE), and the National Aeronautics and Space Administration (NASA) asked the National Research Council (NRC) to assess independently the adverse health effects of perchlorate ingestion from clinical, toxicologic, and public-health perspectives. They also asked the NRC to evaluate the relevant scientific literature and key findings underlying EPA’s 2002 draft risk assessment, Perchlorate Environmental Contamination: Toxicological Review and Risk Characterization.


The estimate of 11 million people is based on sampling data collected as of May 2004 by the U.S. Environmental Protection Agency (EPA) as required by the Unregulated Contaminant Monitoring Rule. The minimum reporting level for that data collection is 4 parts per billion (ppb) (4 micrograms per liter [µg/L]).

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