Silver is a disinfectant for non-spore forming bacteria at concentrations three or four orders of magnitude below the levels at which it is toxic to mammalian life (about 1 gm/day for humans or 500 mg/L in their drinking water). This extreme mammalian-to-bacterial toxicity differential is the definition of an oligodynamic material. Sterilizing contaminated water occurs at 40 to 200 µg/L and 250 µg/L is strongly germicidal to gram-positive and gram-negative bacteria. The low concentration necessary for oligodynamic activity allows silver or one of its insoluble salts to be used indefinitely in contact with sterile liquids without silver levels building up to concentrations harmful to people (Thompson 1973). Russian spacecraft used silver as a germicide to convert polluted wastewater to potable water (MRI 1975).