As(III), and As(V). The predominant form of inorganic As in aqueous and aerobic environments is arsenate (As[V] as H2AsO4– and HAsO42-), whereas in anoxic environments, arsenite (As[III] as H3AsO30 and H2AsO3–) predominates. Adsorption of arsenate on the surface of minerals such as ferrihydrite and alumina constrains its hydrologic mobility. The most common form of arsenite (AsO2–) is less strongly adsorbed to minerals, so its oxyanion is more mobile in environmental water (8). As has not been shown to be required for any physiologic functions in animals or humans, but deficiencies have been reported to result in myocardial damage. Its principal importance in the diet is as a toxin that can induce damage to the nervous and cardiovascular systems and increase the risk of cancer of the skin, lung, and bladder.
TABLE 16.1 RECOMMENDED DIETARY ALLOWANCES AND UPPER LIMITS
rapidly excreted in urine with small amounts of inorganic As eliminated in feces, sweat, skin desquamation, hair, and nails (22). The relative proportions of urinary As metabolites are 40% to 60% dimethylarsinic acid, 20% to 25% inorganic As, and 15% to 25% methylarsinic acid (23). One study, in which a single intravenous injection of radiolabeled trivalent inorganic As(III) was administered to human volunteers, showed that most of the As(III) was removed by urinary excretion within 2 days, and a small amount of excretion continued during the subsequent 2 weeks. The biologic half-life of As from fish is estimated to be less than 20 hours, with total clearance occurring over 48 hours. Blood concentrations may appear normal while levels in the urine remain elevated.
of crop failure throughout the world. Fungi synthesize antibiotics that contain a single B atom in their structure, and bacteria synthesize and release autoinducer AI-2, a quorum-sensing molecule with a single B atom (37).
in blood in low-B regions, but the ratio is lower in men living in higher-B regions (50). This finding suggests that borate/boric acid export transporters are present in the prostate or seminal glands. More than 90% of ingested B is eliminated as boric acid in the urine of humans and rats following first-order kinetics. The half-life of renal clearance is approximately 21 hours in humans, renal reabsorption occurs when the ratio of B to creatinine is less than 1 (51). Locksley and Sweet (52) conducted a dose-response mouse toxicity study using intraperitoneal injections of borax. Tissue B concentrations increased proportionally over a range of 1.8 to 71 mg B/kg. Ku et al (53) evaluated the tissue concentrations of male rats fed a diet containing 1575 mg B/kg for 7 days. After bone, the seminal vesicles accumulated the next highest concentration and are a known target of toxic exposure in rats.