Introduction

Cadmium, a rare but widely dispersed element, is found naturally in the environment. Most cadmium ore (greenockite):

• exists as cadmium sulfide,
• is refined during zinc production, and
• occurs in association with zinc.

It is released into the environment through mining and smelting, its use in various industrial processes, and enters the food chain from uptake by plants from contaminated soil or water.

Sources of Cadmium

Cadmium has been widely dispersed into the environment through the air by its mining and smelting as well as by other man-made routes:

• usage of phosphate fertilizers,
• presence in sewage sludge, and
• various industrial uses such as NiCd batteries, plating, pigments and plastics (ATSDR 1999).

Release by Industrial Processes

The most important sources of airborne cadmium are smelters. Other sources of airborne cadmium include burning fossil fuels such as coal or oil and incineration of municipal waste such as plastics and nickel-cadmium batteries (which can be deposited as solid waste) (Sahmoun et al. 2005). Cadmium may also escape into the air from iron and steel production facilities.

Cadmium is used mainly:

• in metal plating,
• in producing pigments,
• in NiCd batteries,
• as stabilizers in plastics, and
• as a neutron absorbent in nuclear reactors.

Contamination of the Natural Environment

When released into the atmosphere by smelting or mining or some other processes, cadmium compounds can be associated with respirable-sized airborne particles and can be carried long distances. It is deposited onto the earth below by rain or falling out of the air. Once on the ground, cadmium moves easily through soil layers and is taken up into the food chain by uptake by plants such as leafy vegetables, root crops, cereals and grains (ATSDR 1999).

Cadmium concentrations in drinking water supplies are typically less than 1 microgram per liter (μg/L) or 1 part per billion (ppb) (ATSDR 1999). Groundwater seldom contains high levels of cadmium unless it is contaminated by mining or industrial wastewater, or seepage from hazardous waste sites. Soft or acidic water tends to dissolve cadmium and lead from water lines; cadmium levels are increased in water stagnating in household pipes. These sources have not been reported to cause clinical cadmium poisoning, but even low levels of contamination add to the body's accumulation of cadmium.

Cadmium oxide also exists as small particles in air (fume) which are the result of smelting, soldering, or other high-temperature industrial processes. A certain percentage of these particles are respirable.

Cadmium in the Food Chain

From the soil, certain plants (tobacco, rice, other cereal grains, potatoes, and other vegetables) take up cadmium more avidly than they do other heavy metals such as lead and mercury (Satarag et al. 2003).

Cadmium is also found in meat, especially sweetmeats such as liver and kidney. In certain areas, cadmium concentrations are elevated in shellfish and mushrooms (Jarup 2002).

Cadmium can also enter the food chain from water. In Japan, zinc mining operations contaminated the local water supplies with cadmium. Local farmers used that water for irrigation of their fields. The soil became contaminated with cadmium which led to the uptake of cadmium into their rice (Jarup 2002).

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