Boron in drinking-water
Background document for development of WHO Guidelines for Drinking-water Quality
Kinetics and metabolism in laboratory animals and humans
Numerous studies have shown that boric acid and borax are absorbed from the gastrointestinal tract and from 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. The ratio of mean clearance values as a function of dose in non-pregnant rats versus humans is approximately 3- to 4-fold — i.e. similar to the default value for the toxicokinetic component of the uncertainty factor for interspecies variation [Report of informal discussion 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] (WHO, 1994). Elimination of borates from the blood is largely by excretion of >90% of the administered dose via the urine, regardless of the route of administration. Excretion is relatively rapid, occurring over a period of a few to several days, with a half-life of elimination of 24 hours or less. The kinetics of elimination of boron have been evaluated in human volunteers given boric acid via the intravenous and oral routes (Jansen et al., 1984; Schou et al., 1984). Absorption is poor through intact skin but is much greater through damaged skin.