Zinc imparts an undesirable astringent taste to water at a taste threshold concentration
of about 4 mg/litre (as zinc sulfate). Water containing zinc at concentrations in
excess of 3–5 mg/litre may appear opalescent and develop a greasy film on boiling.
Although drinking-water seldom contains zinc at concentrations above 0.1 mg/litre,
levels in tap water can be considerably higher because of the zinc used in older galvanized
plumbing materials. No health-based guideline value has been proposed for
zinc in drinking-water