Water Quality Standards

Boron compounds are used in the manufacture of glass, soaps, and detergents, and as flame retardants. Concentrations of boron in water vary widely and depend on the surrounding geology and wastewater discharges.

Boric acid and borax are absorbed from the gastrointestinal tract and the respiratory tract, as indicated by increased levels of boron in the blood, tissues, or urine or by systemic toxic effects of exposed individuals or laboratory animals. Clearance of boron compounds is similar in humans and animals. Elimination of borates from the blood is largely by excretion; 90% or more of the administered dose is eliminated via the urine, regardless of the route of administration. Excretion is relatively rapid, occurring over a period of a few, or possibly several, days.

Short- and long-term oral exposures to boric acid or borax in laboratory animals have demonstrated that the male reproductive tract is a consistent target of toxicity. Testicular lesions have been observed in rats, mice, and dogs given boric acid or borax in food or drinking-water. Developmental toxicity has been demonstrated experimentally in rats, mice, and rabbits. In a developmental toxicity study in rats, the NOAEL was 9.6 mg of boron per kg of body weight per day, based on a decrease in fetal body weight at the next higher dose (13 mg of boron per kg of body weight per day).

Negative results in a large number of mutagenicity assays indicate that boric acid and borax are not genotoxic. In long-term studies in mice and rats, boric acid and borax caused no increase in tumour incidence.

The TDI of boron is derived by dividing the NOAEL (9.6 mg of boron per kg of body weight per day) for the critical effect, which is developmental toxicity (decreased fetal body weight in rats), by an appropriate uncertainty factor, which is judged to be 10 × 6 = 60. The value of 10 for interspecies variation (animals to humans) was adopted because of lack of toxicokinetic and toxicodynamic data to allow deviation from this default value. The intraspecies (individual variations) factor of 10 (default value) is made up of two components, 3.2 each, for the inter-individual variability in toxicodynamics and toxicokinetics for a particular compound in humans. However, available toxicokinetic data support reduction of the default value for intraspecies variation from 3.2 to 1.8; there are no data to serve as a basis for replacement of the default value of 3.2 for the toxicodynamic component of the uncertainty factor for intraspecies variation. Hence, the total uncertainty factor for intraspecies variation is 1.8 × 3.2 = 5.7 (rounded to 6).

Using an uncertainty factor of 60, the TDI is therefore 0.16 mg of boron per kg of body weight. [The Working Group on Chemical Substances in Drinking-Water took note of the lower uncertainty factor proposed by an IPCS Task Group (see: Boron. Geneva, World Health Organization, 1998: 146-148 (Environmental Health Criteria, No. 204)), but decided to use the more conservative factor of 60 for the purposes of recommending a guideline value for boron in drinking-water.] With an allocation of 10% of the TDI to drinking-water and assuming a 60-kg adult consuming 2 litres of drinking-water per day, the guideline value is 0.5 mg/litre (rounded figure).

Conventional water treatment (coagulation, sedimentation, filtration) does not significantly remove boron, and special methods would have to be installed in order to remove boron from waters with high concentrations. It may be possible to achieve substantial reduction with ion exchange and reverse osmosis processes, but these are likely to be prohibitively expensive. Blending with low-boron water supplies might be the only economical method to reduce high boron concentrations. 

The guideline value of 0.5 mg/litre is designated as provisional because, with the treatment technology available, it will be difficult to achieve in areas with high natural boron levels.