Elemental boron was isolated in 1808. It typically occurs in nature as borates hydrated with varying amounts of water. Important compounds are boric acid and borax. Boron compounds are also used in the production of metals, enamels, and glasses. In trace amounts, boron is essential for the growth of many plants, and is found in animal and human tissues at low concentrations. Poisoning in humans has been reported as the result of accidental ingestion or use of large amounts in the treatment of burns. Boron as boric acid is fairly rapidly absorbed and excreted from the body via urine. The half-life of boric acid in humans is on the order of 1 day. Boron does not appear to accumulate in soft tissues of animals, but does accumulate in bone. Normal levels of boron in soft tissues, urine, and blood generally range from less than 0.05 ppm to no more than 10 ppm. In poisoning incidents, the amount of boric acid in brain and liver tissue has been reported to be as high as 2000 ppm. Recent studies at the National Institute of Environmental Health Sciences have indicated that boron may contribute to reduced fertility in male rodents fed 9000 ppm of boric acid in feed. Within a few days, boron levels in blood and most soft tissues quickly reached a plateau of about 15 ppm. Boron in bone did not appear to plateau, reaching 47 ppm after 7 days on the diet. Cessation of exposure to dietary boron resulted in a rapid drop in bone boron. The analytical methodology developed and validated for these tissues consisted of microwave digestion of samples in concentrated nitric acid, followed by inductively coupled argon plasma emission spectroscopy analyses. The recovery of added boron from tissue samples was quantitative. The precision and accuracy were typically better than 6% and 12%, respectively. The method detection limit is typically 0.02 to 0.04 ppm of boron, depending on the sample matrix. -Environ Health Perspect 102 (Suppl 7): 113-117 (1994)

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An animal model using rats was developed to initiate investigations on the bioavailability of different sources of environmental lead. Lead must be absorbed and transported to target organs like brain, liver, kidney, and bone, before susceptible cells can be harmed. The bioavailability and therefore the toxicity of lead are dependent upon the route of exposure, dose, chemical structure, solubility, particle size, matrix incorporation, and other physiological and physicochemical factors. In the present study male F344 rats were fed < or = 38 microns size particles of lead sulfide, lead oxide, lead acetate, and a lead ore concentrate from Skagway, Alaska, mixed into the diet at doses of 0, 10, 30, and 100 ppm as lead for 30 d. No mortality or overt symptoms of lead toxicity were observed during the course of the study. Maximum blood lead concentrations attained in the 100 ppm groups were approximately 80 micrograms/dl in rats fed lead acetate and lead oxide, and were approximately 10 micrograms/dl in those fed lead sulfide and lead ore concentrate. Maximum bone lead levels in rats fed soluble lead oxide and lead acetate were much higher (approximately 200 micrograms/g) than those seen in rats fed the less soluble lead sulfide and lead ore (approximately 10 micrograms); kidney lead concentrations were also about 10-fold greater in rats fed the more soluble compared to the less soluble lead compounds. However, strong correlations between dose and tissue lead concentrations were observed in rats fed each of the four different lead compounds. Kidney lesions graded as minimal occurred in 7/10 rats fed 30 ppm and in 10/10 rats fed 100 ppm lead acetate, but not at lower doses or from other lead compounds. Similarly, urinary aminolevulinic acid excretion, a biomarker for lead toxicity, was increased in rats fed 100 ppm lead acetate or lead oxide, but was unaffected at lower doses or by the less soluble lead compounds. Although the histological and biochemical responses to lead toxicity were restricted to the more soluble lead compounds in this study, lead from Skagway lead ore concentrate and lead sulfide was also bioavailable, and accumulated in proportion to dose in vulnerable target organs such as bone and kidney. Longer-term studies with different mining materials are being conducted to determine if tissue lead continues to increase, and whether the levels attained are toxic. Data from such studies can be used to compare the toxicity and bioavailability of lead from different sources in the environment.

Elemental boron was isolated in 1808. It typically occurs in nature as borates hydrated with varying amounts of water. Important compounds are boric acid and borax. Boron compounds are also used in the production of metals, enamels, and glasses. In trace amounts, boron is essential for the growth of many plants, and is found in animal and human tissues at low concentrations. Poisoning in humans has been reported as the result of accidental ingestion or use of large amounts in the treatment of burns. Boron as boric acid is fairly rapidly absorbed and excreted from the body via urine. The half-life of boric acid in humans is on the order of 1 day. Boron does not appear to accumulate in soft tissues of animals, but does accumulate in bone. Normal levels of boron in soft tissues, urine, and blood generally range from less than 0.05 ppm to no more than 10 ppm. In poisoning incidents, the amount of boric acid in brain and liver tissue has been reported to be as high as 2000 ppm. Recent studies at the National Institute of Environmental Health Sciences have indicated that boron may contribute to reduced fertility in male rodents fed 9000 ppm of boric acid in feed. Within a few days, boron levels in blood and most soft tissues quickly reached a plateau of about 15 ppm. Boron in bone did not appear to plateau, reaching 47 ppm after 7 days on the diet. Cessation of exposure to dietary boron resulted in a rapid drop in bone boron.(ABSTRACT TRUNCATED AT 250 WORDS)

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