Boron trichloride

Endpoint summary

Additional information

Boron trichloride

Boron trichloride (BCl3) is rapidly and completely hydrolysed in water to form hydrogen chloride and boric acid. Endpoint data for BCl3 could therefore be replaced by data on the 2 degradation products.

Boric acid

Boric acid is not metabolised in either animals or humans, owing to the high energy level required (523 kJ/mol) to break the B - O bond (Emsley, 1989). Other inorganic borates convert to boric acid at physiological pH in the aqueous layer overlying the mucosal surfaces prior to absorption.Most of the simple inorganic borates exist predominantly as undissociated boric acid in dilute aqueous solution at physiological and environmental pH, leading to the conclusion that the main species in the plasma of mammals is un-dissociated boric acid. Since other borates dissociate to form boric acid in aqueous solutions, they too can be considered to exist as un-dissociated boric acid under the same conditions. Additional support for this derives from studies in which more than 90 % of administered doses of inorganic borates are excreted in the urine as boric acid. Absorption of borates via the oral route is nearly 100 %. For the inhalation route also 100 % absorption is assumed as worst case scenario. Dermal absorption through intact skin is very low with a percent dose absorbed of 0.226 ± 0.125 in humans. Using the % dose absorbed plus standard deviation (SD) for boric acid, a dermal absorption for borates of 0.5 % (rounded from 0.45 %) can be assumed as a worse case estimate.

In the blood boric acid is the main species present and is not further metabolised. Boric acid is distributed rapidly and evenly through the body, with concentrations in bone 2 - 3 higher than in other tissues. Boric acid is excreted rapidly, with elimination half-lives of 1 h in the mouse, 3 h in the rat and < 27.8 h in humans, and has low potential for accumulation. Boric acid is mainly excreted in the urine.

Interspecies differences in toxicokinetics based on data for boron clearance rates in rats versus humans and intraspecies differences in human toxicokinetics based on data on human variability in glomerular filtration rates (GFR) are critical determinates in evaluating human toxicity of boric acid. GFR was identified as the primary determinant of boron clearance rates. A toxicokinetic adjustment factor for boron for human variability is based on the variability in GFR during pregnancy (Dunlop, 1981; Krutzén et al., 1992; Sturgiss et al., 1996) ensuring adequate coverage of the sensitive subpopulation of preeclamptic women (US. EPA 2004; Dourson et al. 1998; Maier et al. 2014).

 

For comparative purposes, exposures to borates are often expressed in terms of boron (B) equivalents based on the fraction of boron in the source substance on a molecular weight basis. As noted previously, only boric acid and the borate anion are present at environmentally and physiologically relevant concentrations. Read-across between the different boron compounds can be done on the basis of boron (B) equivalents. Conversion factors are given in the table below.

Substance	Formula	Conversion factor for equivalent dose of B (multiply by)
Boric acid	H3BO3	0.1748
Boric Oxide	B2O3	0.311
Disodium tetraborate anhydrous	Na2B4O7	0.2149
Disodium tetraborate pentahydrate	Na2B4O7•5H2O	0.1484
Disodium tetraborate decahydrate	Na2B4O7•10H2O	0.1134
Disodium octaborate tetrahydrate	Na2B8O13·4H2O	0.2096
Sodium metaborate (anhydrous)	NaBO2	0.1643
Sodium metaborate (dihydrate)	NaBO2·2H2O	0.1062
Sodium metaborate (tetrahydrate)	NaBO2·4H2O	0.0784
Sodium pentaborate (anhydrous)	NaB5O8	0.2636
Sodium pentaborate (pentahydrate)	NaB5O8∙5H2O	0.1832

Hydrogen chloride

Hydrogen chloride and its aqueous solution hydrochloric acid are corrosive and irritating and cause direct local effects on the skin, eye and gastro-intestinal or respiratory tract after direct exposure to sufficiently high concentrations. The chemistry of this substance is well understood; as an inorganic salt it dissolves in water to form hydrogen and chloride ions, both of which are physiological electrolytes.  

The absorption, distribution, metabolism and excretion of acids and chloride are well known from human and mammalian physiology. Both ions are regulated within a narrow range to maintain homeostasis.  

Complete absorption by oral route can be expected. Dermal absorption of dissolved salts on the other hand is generally limited. Related to extreme water solubility and logP value below 0, the ions are too hydrophilic to cross the lipid rich environment of the stratum corneum. Dermal uptake for such solutions of salts in water will be low.  

Concentrated hydrogen chloride is corrosive to skin. At concentrations lower than those that cause corrosion, hydrogen chloride will have no systemic toxicity. Dermal exposures should be controlled on the basis of the potential risk to local effects (irritation, corrosion) to the skin. Concentrations that are lower than those that are irritant will only add to the body’s pool of electrolytes.