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There is little difference between animals and humans in absorption, distribution, and metabolism. 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

A number of these studies were conducted on an analogue substance. Read-across is justified on the following basis:

In aqueous solutions at physiological and acidic pH, low concentrations of simple inorganic borates such as boric acid B(OH)3, potassium pentaborate (K2B10O16.8H2O), potassium tetraborate (K2B4O7.4H2O), disodium tetraborate decahydrate (Na2B4O7.10H2O; borax), disodium tetraborate pentahydrate (Na2B4O7.5H2O; borax pentahydrate), boric oxide (B2O3) and disodium octaborate tetrahydrate (Na2B8O13.4H2O) will predominantly exist as undissociated boric acid. Above pH 9 the metaborate anion (B(OH)4-) becomes the main species in solution (WHO, 1998). This leads to the conclusion that the main species in the plasma of mammals and in the environment is undissociated boric acid. Since other borates dissociate to form boric acid in aqueous solutions, they too can be considered to exist as undissociated boric acid under the same conditions.

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. Some studies express dose in terms of B, whereas other studies express the dose in units of boric acid. Since the systemic effects and some of the local effects can be traced back to boric acid, results from one substance can be transferred to also evaluate the another substance on the basis of boron equivalents. Therefore data obtained from studies with these borates can be read across in the human health assessment for each individual substance. 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

 Dipotassium tetraborate (anhydrous)    K2B4O7    0.185  
 Dipotassium tetraborate (tetrahydrate)    K2B4O7.4H2O    0.1415  
 Potassium pentaborate (anhydrous)    B5KO8    0.244  
 Potassium pentaborate (tetrahydrate)    B5KO8.4H2O    0.1843  

 

References:

WHO. Guidelines for drinking-water quality, Addendum to Volume 1, 1998