significant portion, of the absorbed zinc. In the Drinker and Drinker (1928, as cited in ATSDR 1994) study, the swallowing of zinc particles during grooming activities might also account for the increased tissue zinc levels.

No data are available on the oral absorption of zinc oxide. The estimated rate of oral absorption of zinc (all zinc compounds) in humans is between 8% and 81%, depending on an individual’s diet (ATSDR 1994). People who are not deficient in zinc absorb about 20–30%, while zinc-deficient individuals absorb more (ATSDR 1994). Two studies measured the peak blood concentrations of zinc in volunteers following oral ingestion of zinc sulfate; peak blood Zn²⁺ concentrations were reached within 3 hr (Neve et al. 1991; Sturniolo et al. 1991, as cited in ATSDR 1994). The presence of cadmium, mercury, copper, or other trace metals can diminish zinc absorption by inhibiting zinc transport across the intestinal wall (ATSDR 1994).

Zinc absorption in male Wistar rats was approximately 40–48% when diets contained 0.81 mg of radio-labeled zinc (as zinc chloride or zinc carbonate) per kg of body weight (Galvez-Morros et al. 1992). ATSDR (1994) noted that the fraction of ingested zinc absorbed in immature organisms usually exceeds the fraction of ingested zinc absorbed in adult organisms.

Wester et al. (1998) exposed the back of the hand of volunteers to a 5% aqueous solution of boric acid or borax and measured urinary boron concentrations to determine the extent of absorption, the flux, and the permeability constants (Kp) for intact skin. Following exposure to boric acid, 0.23% of the applied dose was excreted, flux was calculated as 0.01 µg/cm²/hr and Kp was 1.9×10⁻⁷ cm/hr. Following exposure to borax, 0.21% of the applied dose was excreted, flux was calculated as 0.01 µg/cm²/hr, and Kp was 1.8×10⁻⁷ cm/hr. Draize and Kelly (1959) has also reported low dermal absorption of boric acid, with no increase in urinary boron concentrations following a 4-hr exposure in a volunteer. Blood boron concentrations did not increase in infants after treatment with ointment (3% boric acid), indicating a lack of dermal absorption of boric acid (Friis-Hansen et al. 1982).

No absorption of boric acid (measured as boron in the blood) occurred 1–9 d after a single topical application of boric acid in an anhydrous, water-emulsifying ointment (Stuttgen et al. 1982). However, blood boron concentrations were increased within 2–6 hr after application of the same amount of boric acid in a water-based jelly, indicating that the vehicle in which boric acid is applied to the skin affects absorption.

Boron was detected in the urine of infants who had moderate to marked diaper rash, but not in the urine of infants who had minor or no diaper rash,