WHAT IS A PERCHLORATE REFERENCE DOSE?

The reference dose is the current best scientific estimate of what constitutes a safe exposure. The initial 3.6 ppb reference dose was from a rudimentary literature review and had very low confidence -- it was a first step in the process and not intended for public consumption. Ideally you would revise your reference dose to a medium confidence level within a year or so after making the initial reference dose, but for political reasons perchlorate has dallied for 8 years between low confidence reference dose and medium confidence reference dose.

Medium confidence reference doses involve an external peer review and are considered good enough for the US EPA to post on the web (IRIS -- Integrated Risk Information System, I think). State and local authorities use IRIS to make up-to-date decisions about toxic contamination. For example, if the reference dose for perchlorate had been 1000 ppb and there were no obvious point sources for ClO4- near Yardley, then Pennsylvania authorities could safely ignore an isolated, transient hit of 2 ppb perchlorate; now that the reference dose is potentially 5 ppb, they are very accountable to you to insure that the 2 ppb is not part of a larger plume.

Theoretically, reference doses only take health considerations into account, while federally enforced Maximum Concentration Levels (MCLs) are political-legal entities that explicitly take economic impact into consideration. Thus you can have a high confidence reference dose for perchlorate that says there is some health risk at 5 ppb, but the MCL may end up to be 18 ppb because you can't shut down Phoenix, Las Vegas, Los Angeles, and San Diego just because they have a little ClO4- in their water. Perchlorate, as a new member of the EPA's contaminant candidate list, will be eligible for entering the MCL process in 2003. There's a discussion of all this somewhere on the federal EPA perchlorate page.

I think you can safely interpret the 32 to 5 ppb reference dose range as a firm statement by the feds to the states that they want this stuff monitored so that it's potential health effects can be evaluated epidemiologically. I think the EPA is taking a hard-line because perchlorate exists in the Colorado River where it is concentrating in a vital segment of the nation's food supply. But until I read the supporting studies, I just don't know what caused this low estimate: Last month's presentation showed no lowering of hormone levels in rats below the 1mg/kg/day or 32,000 ppb level. I'm sure the perchlorate industry will argue that the thyroid hyperplasia (cell proliferation) found in rat pups at 3200 ppb is irrelevant to human health risks.

Larry Ladd