There are inadequate dermal toxicity data on zinc borate, zinc oxide, or boric acid to derive a risk value for dermal exposure.

There are inadequate inhalation toxicity data on zinc borate, zinc oxide, or boric acid to derive an inhalation RfC.

There are inadequate oral toxicity data on zinc borate to derive an oral RfD. However, zinc borate readily breaks down in the stomach to zinc oxide and boric acid. The subcommittee used the available dose-response data for both compounds to derive their RfDs and selected the more conservative RfD of the two values to characterize the health risk of zinc borate.

The EPA has established an oral RfD of 0.3 mg/kg-d for zinc (EPA 1999). That RfD is based on a LOAEL of 1.0 mg zinc/kg-d which was identified by Yadrick et al. (1989) who reported a decrease in erythrocyte superoxide dismutase activity in adult women following 10 wk of exposure to zinc supplements. The change in the enzyme activity reflects an alteration in copper levels. That RfD is supported by data from several other clinical studies demonstrating the effect of zinc on copper balance (Fischer et al. 1984; Prasad et al. 1978). The LOAEL was divided by an uncertainty factor of 3 (less than the default factor of 10 because the effects are not severe and zinc is an essential micronutrient) to yield an RfD of 0.3 mg zinc/kg-d.

Developmental toxicity appears to be the most sensitive endpoint for boric acid. Price et al. (1996a) identified the lowest LOAEL and the highest NOAEL