Potassium triboron pentaoxide

Endpoint summary

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - oral (%):

100

Absorption rate - dermal (%):

0.5

Absorption rate - inhalation (%):

100

Additional information

Orthoboric acid, potassium salt is manufactured in highly refined mineral oil and further diluted with other lubricant additives and base oil in the finished fluid. This mineral oil and the other additives in the package are expected to help stabilize the substance from hydrolysis to boric acid. Additionally, the registered substance is not vaporized or aerosolized during manufacture and use so inhalation is not a relevant route of exposure. Oral is not a route of exposure for workers either and so the only relevant route of exposure is dermal. Worst case dermal absorption assumptions would estimate equal absorption to boric acid. Boric acid is not absorbed dermally with <0.5% percutaneous absorption in human. Therefore, the toxicokinetic considerations are minimized based on exposure considerations. As a worst case example, boric acid is used as read-across for the anticipated ADME of orthoboric acid, potassium salt.

There is little difference between animals and humans in absorption, distribution, and metabolism to boric acid. A difference in renal clearance is the major determinant in the differences between animals and humans, with the renal clearance in rats approximately 3 times faster than in humans.

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.

 

Read Across

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