Studies in humans, particularly infants, show that boron (as boric acid) can be lethal following ingestion. Infants who ingested formula accidentally prepared with 2.5% aqueous solution of boric acid died within 3 days after exposure (Wong et al. 1964). It was estimated that the amount of boric acid consumed ranged from 4.51 to 14 g. Although 5 of 11 infants died, the authors provided histopathological data and weights for only 2 infants who had ingested 9.25 g (505 mg boron/kg/day) and 14 g (765 mg boron/kg/day) (Wong et al. 1964). Infants became lethargic and developed vomiting and diarrhea. Degenerative changes were seen in the liver, kidney, and brain. Acute exposure to dose levels of 895 mg boron/kg as boric acid was not lethal in one adult (Linden et al. 1986).
In animals, boron (as boric acid and borax) is lethal following acute, intermediate, and chronic oral exposures. Estimates of oral LD50 in rats were 898 and 642 mg boron/kg (as boric acid and borax, respectively) (Smyth et al. 1969) and 510 and 550 mg boron/kg as borax and boric acid (Weir and Fisher 1972). No deaths were reported in dogs exposed to 696 mg boron/kg as boric acid and 738 mg boron/kg as borax (Weir and Fisher 1972). In a 14-day repeated-dose feeding study in male mice, doses of 2,251 and 3,671 mg boron/kg/day (as boric acid) were lethal in 20% and 60% of males, respectively (NTP 1987). The mice were lethargic and the spleen, liver, and renal medullae were discolored. Hyperplasia and dysplasia of the forestomach were also observed (NTP 1987).
Survival was also reduced in mice following intermediate-duration exposure. Males (10%) died after exposure to a dose of 288 mg boron/kg/day (as boric acid) in the diet, while 80% of males and 60% of females died at 577 mg boron/kg/day (NTP 1987). Hyperkeratosis and/or acanthosis in the stomach and extramedullary hematopoiesis of the spleen in both sexes were observed at the highest dose tested (577 mg boron/kg/day). There was 100% mortality in rats fed 263 mg boron/kg/day for 90 days (Weir and Fisher 1972). Congestion of liver and kidneys, small gonads, and brain swelling were reported. When male mice consumed 48 and 96 mg boron/kg/day (as boric acid) for 103 weeks, mortality was 40% and 56%, respectively, compared to 18% in untreated controls (NTP 1987). No clinical signs were reported; however, boron caused increased incidence of testicular atrophy and interstitial hyperplasia. Mortality in female mice was 30% and 24% (48 and 96 mg boron/kg/day) compared to 34% in the untreated controls (NTP 1987).
The LD50 values and the highest NOAEL values in animals and the lowest level at which death was reported in humans and the duration categories are recorded in Table 2-2 and plotted in Figure 2-2.
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