Potassium triboron pentaoxide

Endpoint summary

Description of key information

A number of sub-chronic and chronic studies on boric acid and disodium tetraborate decahydrate were carried out in rats, mice and dogs. In some cases these studies are research studies (Weir and Fisher, 1972; Dixon et al, 1976; Seal and Weeth, 1980; Lee et al., 1978; Treinen and Chapin, 1991; Ku et al., 1993), but most support that boron can cause adverse haematological effects and that the main target organ of boron toxicity is the testis.  The NOAEL is equivalent to 17.5 mg B/kg bw/day.

Key value for chemical safety assessment

Repeated dose toxicity: via oral route - systemic effects

Endpoint conclusion

Endpoint conclusion:

adverse effect observed

Dose descriptor:

NOAEL

100 mg/kg bw/day

Study duration:

chronic

Species:

rat

Additional information

A number of studies on boric acid or disodium tetraborate decahydrate in diet or via drinking water for periods of 30 days to two years in rats, mice and dogs are available, however, the majority of these studies do not comply with current test guidelines, and they lack essential information regarding e.g. histological descriptions and statistical evaluations of the results. Most studies support that boron can cause adverse haematological effects and that the main target organ of boron toxicity is the testis. Other effects observed at high doses include rapid respiration, hunched position, bloody nasal discharge; urine stains on the abdomen, inflamed bleeding eyes, desquamation and swollen paws and tail, reduced food consumption and body weight gain. Treatment with boric acid and disodium tetraborate decahydrate disrupted spermiation, induced degeneration of testicular tubules and caused testicular atrophy. For effects on the blood system extramedullary haematopoiesis, reduced red cell volume and haemoglobin values and deposition of haemosiderin in spleen, liver and proximal tubules of the kidney were described. Several cases of anaemia have been observed in human poisoning cases. However, although doses in these poisoning cases are difficult to define, the effects occurred generally at relatively high concentrations.

Boric acid, the main species present under physiological conditions, acts as a Lewis acid and as such owns the ability to complex with hydroxyl, amino and thiol groups from diverse biomolecules, like e.g. carbohydrates and proteins (BfR, 2006). Such a mechanism could be involved in effects of boron on different enzyme activities (Huel et al., 2004).

A NOAEL for effects on testes and the blood system of 17.5 mg B/kg bw/day can be derived (with a LOAEL of 58.5 mg B/kg bwday) from two 2-year studies in rats on boric acid and disodium tetraborate decahydrate (Weir, 1966a,b).

Repeated dose toxicity: via oral route - systemic effects (target organ) urogenital: testes

Justification for classification or non-classification

There is no existing repeated dose toxicity study for orthoboric acid, potassium salt. As such, read across from boric acid in combination with intended use conditions is used to evaluate the potential of the substance to cause target organ toxicity. The substance to be registered is expected to have predictable adsorption, distribution, metabolism and excretion based on boric acid, meeting the criteria for inclusion.

The registered substance is manufactured in highly refined mineral oil and further diluted with lubricant additives and base oil in finished fluid. The oil and additives are expected to stabilize the substance to hydrolysis and hydrolysis to boric acid is not expected to occur under normal conditions of handling and use. However, under undesired hydrolytic conditions, including aqueous environments, the borated salt will quickly convert to boric acid. Exposure to boric acid is not expected under normal use conditions to the worker or consumer. The registered substance does not contain detectable levels of residual boric acid. The hazard communications requirement for classification is based on the constituents known to be present under normal conditions of use. Assuming worst case scenarios, if orthoboric acid, potassium salt were to hydrolyze completely, the only relevant route of exposure to workers is dermal and the dermal absorption of boric acid in humans is relatively low at <0.3% (Draize and Kelley, 1959). Animal ingestion studies in several species, at high doses, indicate that boric acid causes reproductive and developmental effects. The doses are many times in excess of those which humans would be exposed to. A human study of occupational exposure to borates showed no adverse effect on reproduction (Whorton et al., 1994). Therefore, orthoboric acid, potassium salt is not classified for target organ toxicity based on exposure considerations.