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June 14, 2005

How does EPA determine drinking water exposure guidelines?

Both the scientific community and the public need to develop a thorough understanding of the mechanism used to calculate the guidelines (as Oral Reference Dose, or RfD) used to limit drinking water exposure levels. The EPA can use a variety of uncertainty factors in the derivation of the RfD. These uncertainty factors are based subjectively on study methods, toxicological data bases, and other information, and typically are given values of 1, 3 or 10. Whether these factors are fairly applied on a systematic basis can be a subject of considerable debate.

One important issue is determining, for a given exposure situation, what is the critical endpoint in the critical study. It is in the EPA's authority to choose that critical data point. In addition, if there is a dose at which no adverse effects are observed, this becomes the NOAEL (no-observed-adverse-effect-level) upon which the reference dose (RfD) is based. Otherwise, the RfD is based upon the LOAEL (lowest-observed-adverse-effect-level).

The following relationships can be used to derive the RfD, which in turn is translated into a Drinking Water Equivalent Level (DWEL) related to individual body weight.  The DWEL is found by multiplying the RfD times the body weight in kg , divided by the volume of water consumed per day.  This in turn is equal to the term below, in which the U factors are typically 1, 3 or 10 (greatest uncertainty).

NOAEL or LOAEL (mg/kg-day)                                                                     UFl x UFa x UFh x UFd x UFs                        

where:

UFl  =  the LOAEL uncertainty factor, intended to compensate for the use of a LOAEL rather than a NOAEL as the basis for the RfD

UFa = interspecies uncertainty factor, intended to account for uncertainty involved in extrapolating from animal (e.g. rat) to humans.

UFh = intraspecies uncertainty factor, intended to account for variation in response among the human population for the end-point in question, with the particular intention of protecting sensitive subpopulations.

UFd = database uncertainty factor, essentially the completeness of the overall database especially as relates to the critical study.

UFs = subchronic to chronic uncertainty factor, intended to account for the uncertainty involved inextrapolating from a less-than-chronic NOAEL to a chronic NOAEL when a chronic NOAEL of sufficient quality is not available. [Note - this factor was not used by the EPA in the derivation of an RfD for perchlorate because the mechanism of perchlorate action is such that the effects of subchronic exposure fully account for the effects of chronic exposure.]

June 14, 2005 in Environmental Guidelines, Toxicological Relevance | Permalink

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