Though not a natural constituent of air, zinc occurs in the air around zinc smelters and this airborne zinc may be hazardous to human health. Airborne zinc occurs mostly in the form of white zinc oxide fumes. Scrap zinc refineries also emit zinc into the air, as do brass and bronze refineries.
Some zinc is also produced as a by-product in the refining of copper, lead and steel from scrap. Therefore, these refineries can act as a source of air pollution of zinc. Open hearth furnaces have been estimated to emit 20-25 gm of zinc per hour from the refining of galvanized iron scrap.
Unlike zinc which is an essential micronutrient at low concentrations and has a rather low order of toxicity, cadmium is toxic even at extremely low concentrations and is known to accumulate in human kidney and liver. It causes hypertension, lung irritation, produces emphysema and can cause serious renal damage.
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Like zinc and mercury, cadmium is not a natural air pollutant but occurs in air as a result of industrialization and human activities. Some major sources of emission of cadmium in air include the metal industries engaged in extraction, refining, electroplating and welding of cadmium- containing materials, and in the refining of zinc, lead and copper.
Some cadmium can also be released into the air during the manufacture of pesticides and phosphate fertilizers and plants are known to be able to absorb cadmium from soils.
Mercury is volatile, is found in rocks and soils, and is undoubtedly present in the air as a contaminant, largely as a result of human activities such as the use of mercury-containing fungicides, paints and cosmetics.
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Safety limits for mercury are not known, but less than 0.1 µg/m3 of organic mercury may be permissible provided that no methylmercury is allowed at all.