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EC number: 234-522-7
CAS number: 12007-92-0
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.
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, disodium
tetraborate decahydrate, disodium tetraborate pentahydrate, boric oxide
and disodium octaborate tetrahydrate will predominantly exist as
undissociated boric acid. At about pH 10 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 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.
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.
Conversion factor for equivalent dose of B (multiply by)
Disodium tetraborate anhydrous
Disodium tetraborate pentahydrate
Disodium tetraborate decahydrate
Disodium octaborate tetrahydrate
Sodium metaborate (anhydrous)
Sodium metaborate (dihydrate)
Sodium metaborate (tetrahydrate)
Sodium pentaborate (anhydrous)
Sodium pentaborate (pentahydrate)
WHO. Guidelines for drinking-water quality, Addendum to Volume 1,
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