Boron in drinking-water
Background document for development of WHO Guidelines for Drinking-water Quality
Provisional guideline value
The TDI of boron is derived by dividing the NOAEL (9.6 mg/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 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. Available toxicokinetic data do support, however, reduction of the default uncertainty factor for intraspecies variation from 10 to 6 (WHO, 1994).
Interspecies (toxicokinetic) variations for boron relate primarily to clearance. The ratio of mean clearance values in non-pregnant rats versus non-pregnant humans for boron (based on all of the data considered suitable for inclusion) is 3–4. In view of the lack of adequate kinetic studies in rats and hence less than optimum confidence in much of the data that serve as the basis for the ratio, replacement of the default for the toxicokinetic component of the interspecies factor is considered premature at this time. The total uncertainty factor for interspecies variation is 10.
Intraspecies variation (toxicokinetics) for boron relates also primarily to variations in clearance. As the critical effect that serves as the basis for the TDI is developmental, pregnant women are the subgroup of interest in this regard. Based on pooled individual data from available studies, the mean glomerular filtration rate (GFR) in 36 healthy women was 145 ± 23 ml/minute in early pregnancy and 144 ± 32 ml/minute in late pregnancy. The standard deviation represented 22% of the mean value in late pregnancy. Based on division of the mean GFR (144 ml/minute) by the GFR at two standard deviations below the mean (80 ml/minute) to address variability for approximately 95% of the population, the ratio for the toxicokinetic component of interspecies variation is 1.8 (compared with the default value for this component of 3.2). As there are no data to serve as a basis for replacement of the default value for the toxicodynamic component of the uncertainty factor for intraspecies variation, the total uncertainty factor for intraspecies variation is 1.8 × 3.2 = 5.7 (rounded to 6) [Report of informal discussions to develop recommendations for the WHO Guidelines for drinking-water quality — Boron. Cincinnati, OH, 28–29 September 1997. Report available from WHO, Division of Operational Support in Environmental Health, Geneva].
Using an uncertainty factor of 60, the TDI is therefore 0.16 mg/kg of body weight. 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 boron concentrations. Ion exchange and reverse osmosis processes may enable substantial reduction but are likely to be prohibitively expensive. Blending with low-boron supplies might be the only economical method to reduce boron concentrations in waters where these concentrations are high (WRc, 1997).
The guideline value of 0.5 mg/litre is designated as provisional, because it will be difficult to achieve in areas with high natural boron levels with the treatment technology available.