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Description of key information

A 2 year carcinogenicity study of boric acid (NTP, 1987) equivalent to OECD Guideline 451 was carried out in mice. The study showed that boric acid was non-oncogenic by the oral route. The NOAEL is equivalent to 201 mg B/kg bw/day.

Key value for chemical safety assessment

Carcinogenicity: via oral route

Endpoint conclusion
Dose descriptor:
823.8 mg/kg bw/day

Justification for classification or non-classification

No classification is required for potassium pentaborate regarding carcinogenicity as all results for boric acid were negative.

Additional information

In long term feeding studies on boric acid and disodium tetraborate decahydrate in both rats and mice, no carcinogenic effects were observed (Weir, 1966a, b; Weir and Fisher, 1972).

An OECD 451 equivalent study in B6C3F1 mice consisting of 50 per sex per group treated in diet for 103 weeks with 0, 2500 or 5000 ppm boric acid showed no evidence of carcinogenicity (NTP classification meaning no chemically related increase in benign or malignant neoplasms).

Based on the mouse NTP-study (1987) boric acid is not regarded carcinogenic. Although not carried out according to modern standards, nor to GLP, the 2-year studies in rats and dogs support this finding. While in the 2-year rat studies, only 10 animals/sex of the control and high-dose group were macroscopically and histologically examined, only 1-2 animals/sex/dose/time were examined in the 2-year studies in dogs, which limits the conclusions that can be derived from these studies. However, they were well performed and reported and are adequate to evaluate the carcinogenicity of boric acid and sodium borates. It can be concluded that boric acid and sodium borates are not carcinogenic and there is no concern for carcinogenic effects in humans.

Since all the borates will exist as undissociated boric acid under physiological and environmental conditions, the toxicology of all these simple borates is similar on an equivalent boric acid basis or boron basis. Therefore the data for boric acid and disodium tetraborate decahydrate can be read across to the other borates for toxicological effects. 

The following data were obtained (NOAEL): 

Potassium pentaborate (anhydrous) 823.8 mg/kg bw/day

Potassium pentaborate (tetrahydrate) 1090.6 mg/kg bw/day

Read Across

This study was 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 under CSR section 5.1.3, which corresponds to IUCLID section 7.1 (toxicokinetics, metabolism and distribution endpoint summary).


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

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