Zinc is nutritionally an essential element and is required for the activity of a number of enzymes. Common sources of this metal are Zinc blende (ZnS), Smithsonite (ZnC03), Zincite (ZnO) and Ellemite (Zn,Si04).
Mining, processing and smelting of ores for the extraction of zinc constitutes the chief source of zinc pollution in the environment. Municipal wastes, sewage effluents, waste oils and trash from automobiles, batteries, construction material, paints and pigments, coating on iron, steel and brass alloys, pesticides containing zinc etc. have significant amount of zinc which is ultimately added to the environmental burden.
Combustion of coal and organic matter also introduces some zinc into the environment. Zinc content in aquatic invertebrates in fresh unpolluted water ranges between 25-200 mg per litre. Above 40 ppm level this metal imparts a faint but definite metallic taste and milky appearance to fresh water.
About 20-30% of the ingested zinc is absorbed. Within mucosal cells zinc forms metal- protein complexes. More than 70 different enzymes require zinc as a co-factor and its deficiency results in a wide spectrum of clinical effects. Toxicity consequent upon excessive intake of this metal is rather uncommon. Gastrointestinal distress and diarrhoea have been reported following intake of liquids stored in galvanized cans or from the use of galvanized utensils.
After ingestion of about 12 gms of elemental zinc, evidence of hemolytic, hepatic, and renal damages have been noticed in humans barely within two hours after the intake. Metal fume fever, resulting from excessive inhalation of freshly formed zinc oxide fumes in industrial workers, is characterized by chills, fever, profuse sweating and weakness after four or five hours. Other aspects of zinc toxicology are not yet well understood.
Experimental animals given about 100 times more zinc than their dietary requirement failed to show any discernible effects (Goyer et al, 1979). Though testicular tumours have been produced by direct intra-testicular injections in chickens and rats there is no convincing evidence of teratogenic, mutagenic or carcinogenic action of zinc in humans.
Zinc is an important plant nutrient. Paddy cultivated in soils with less than 20 ppm of zinc develops symptoms of zinc deficiency. Zinc content of an average paddy field in India ranges between 35-90 ppm. However, zinc content of about 460 ppm may result in an appreciable reduction in yield. A higher calcium and magnesium content in the soil is helpful in reducing zinc toxicity.