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Spacecraft Water Exposure Guidelines: For Selected Contaminants, Volume 2
The average zinc concentration in tap water across the United States is 0.245 milligrams per liter (mg/L) (NRC 1980). The highest mean value reported for tap water from galvanized pipes is about 2 mg/L (Sharrett et al. 1982).
Occupational exposure to zinc by means of inhalation occurs extensively in zinc mining, smelting, welding, and the manufacturing of galvanized metals, paints, tires, and certain personal consumer products. ZnCl2 is primarily used in making batteries, zinc silicate in phosphors of cathode ray tubes, and ZnO in the rubber vulcanizing process. Exposure to zinc through drinking water can take place in areas near where these activities occur.
Zinc is an essential food element. Dairy products, grains, meats, fish, and poultry are the richest sources of zinc (Tanner and Friedman 1977). The Recommended Dietary Allowance (RDA) of zinc for nonpregnant women is 12 mg per day (d), and for men it is 15 mg/d. A typical mixed diet provides at least 65-80% of the daily RDA (Bowerman and Harrill 1983). According to the Food and Drug Administration (FDA) Total Diet Study (1991-1997), the mean intake of zinc from food by males between 31 and 50 years (y) of age is 13.38 ± 0.16 mg/d (n = 1,805). Daily intake by women of that age range is 8.51 ± 0.11 mg/d (n = 1,733) (see Appendix E in IOM 2001). Zinc occurs in all living cells as a constituent of metalloenzymes involved in major metabolic pathways (NRC 1989). Zinc controls several enzymes of intermediary metabolism, DNA and RNA synthesis, gene expression, and immunocompetence. Zinc can interact with almost all hormones and plays a significant role in homeostasis of hormones such as thyroid and steroid hormones, insulin, and pituitary hormones like prolactin (Brandao-Neto et al. 1995).
Summary reports on humidity condensates collected from the Mir space station and water recycled from Mir during the years 1995-1998 indicate that zinc was present at concentrations ranging from 1.26 mg/L to 5.3 mg/L in the humidity condensates and at concentrations ranging from 10.4 to 475.0 micrograms (µg) per liter in the processed (recycled) water (Pierre et al. 1999). Although the average concentrations did not exceed the U.S. Environmental Protection Agency’s (EPA’s) secondary maximum contaminant level (SMCL) of 5 mg/L, zinc was found very frequently in the recycled water. Concerns exist about potential system breakthroughs. This document will be limited to addressing the adverse effects of extraneous zinc that may leach into the drinking water through the water processing system (from distribution lines), the humidity condensate heat exchangers (through corrosion), or as a result of the failure of the ion-exchange resins to remove metal ions completely.