A human health risk assessment of boron (boric acid and borax) in drinking water.

A human health risk assessment was conducted to derive an appropriate safe exposure level in drinking water of inorganic boron-containing compounds (boric acid and borax). Several regulatory agencies have set or plan to set drinking water guidelines or standards for boron (B). Recent publication of reproductive and developmental toxicity studies by the National Toxicology Program prompted this risk assessment, along with the understanding that boron may be nutritionally essential. A rat developmental toxicity study with a NOAEL of 9.6 mg B/kg/day was selected as the pivotal study on which to base this risk assessment, since it represents the most sensitive endpoint of toxicity. A detailed evaluation of these and other studies allowed modifications of the default values for uncertainty factors to account for the pharmacokinetic similarities among species, the lack of metabolism of inorganic boron-containing compounds, the similarity of the toxicity profile across species, the quality of the toxicological database, and other factors according to the approach described by Renwick previously. Benchmark dose calculations were performed, and the results were in close agreement with the NOAEL selected for this risk assessment. The Reference Dose was calculated to be 0.3 mg B/kg/day, resulting in an acceptable daily intake of 18 mg B/day. Considering that the U.S. average dietary intake of boron is 1.5 mg B/day, 16.5 mg B/day could be available for drinking water or other exposures, if any. A preliminary review of boron data in the National Inorganic Radionuclide Survey by the EPA indicates the median boron level in U.S. drinking water supplies to be 0.031 mg B/liter, and most exposures are less than 2.44 mg B/liter (99th percentile). It is concluded that boron in U.S. drinking water would not be expected to pose any health risk to the public.