To derive the recommended reference dose – the maximum amount of a substance in drinking water that the U.S. Environmental Protection Agency (U.S. EPA) considers “safe” if consumed every day for a lifetime of exposure – the National Academies of Science (NAS) Committee selected as its starting point the No Observed Effect Level (NOEL) dose of 0.007 milligrams per kilogram per day. This level is the lowest dose used in an adult clinical study by Greer et al. (2002). At this dose, there was no difference in the amount of iodide uptake inhibition by the thyroid between people exposed to perchlorate and those not exposed to perchlorate. NAS indicated that this effect, inhibition of iodide uptake by the thyroid, is the first effect to occur before any other effects of exposure can occur. However, NAS is clear that iodide uptake inhibition is not an adverse effect (See the NAS report, “Health Implications of Perchlorate Ingestion.”) Using standard default EPA conversion practices, the RfD of 0.007 milligrams per kilogram per day is equivalent to 245 ppb in drinking water. NAS divided this dose by a safety factor of 10 to account for sensitive populations – fetuses, particularly those of pregnant women who have hypothyroidism or iodide deficiency. (See Page 172 of the NAS report, “Health Implications of Perchlorate Ingestion.”) NAS resulting recommended RfD (0.0007 milligrams per kilogram of body weight per day) would be equivalent to 24.5 ppb in drinking water.

The NAS Committee acknowledges using an unconventional approach to develop a Rfd from a dose that causes no effects at all. This dose is 57 times lower than the dose that the NAS Committee identified as the minimum dose before any theoretical adverse effects of perchlorate exposure could occur (14,000 ppb).