Cadmium as a pollutant

Cadmium occurs in zinc-lead-copper ores. Zinc ores constitute the main industrial source of cadmium and the metal is fractionated during the smelting or electrolytic processes employed for the refining of zinc. There is, therefore, the possibility of a considerable release of cadmium into the environment during zinc-refining operations and, to a lesser extent, during lead and copper smelting. Man has, therefore, been releasing cadmium into the environment from the time he was first able to smelt and refine these metals thousands of years ago. Cadmium is now used industrially as an antifriction agent, as a rust proofer, in plastics manufacture, in alloys, as an orange colouring agent in enamels and paints, in alkaline storage batteries, and for many other purposes.

World production of cadmium has increased heavily during this century. Only marginal amounts of cadmium were refined before 1920; in the late 1970s, world production was of the order of 15,000 tons per annum. The marked increase in cadmium use during the last three decades has caused a corresponding increase in environmental contamination and in problems caused by exposure at different states. The main pathways of cadmium to man are via inhalation or food intake. Cadmium is released into the air as a result of incineration or disposal of cadmium-containing products (for example, rubber tyres and plastic containers) and as a byproduct in the refining of other metals, primarily zinc.

Near smelters, atmospheric cadmium concentration can be as high as 0.5 micro g/m3 (microgram per cubic metre). High concentrations have also been encountered in some working environments, although more typical levels are now around 0.05-0.02 micro g/m3. One study showed that of 58 cities in the USA, cadmium was found in the air of 36, in concentrations ranging between 0.002 and 0.370 micro g/m3 (38). Of 29 nonurban areas, 17 showed cadmium levels of 0.004 to 0.026 micro g/m3. In nearly all cases cadmium was associated with zinc. Cadmium concentrations are also higher in soil and fresh water around smelters and industries processing materials that contain the metal. The amount of waterborne cadmium is affected by acidity, but concentrations up to 10 micro g/l are found in some mining areas, while in neutral or alkaline waters suspended particulate matter can contain as much as 700 micro g/l of cadmium. Concentrations in sediments can exceed 100 micro g/g. Plants vary in their ability to take up cadmium from soil, but some grasses, wheat, and lettuce do so fairly readily, and plant/soil ratios for most crops range between 0.5 : 1 and 2 : 1. When rice is grown in an environment highly contaminated by cadmium, concentrations can reach 0.5-1 micro g/g, which is 10 to 15 times higher than in noncontaminated areas. The sensitivity of plants to cadmium also varies, spinach, lettuce, and soya bean being affected when levels reach 3 to 4 micro g/g, while other species may tolerate concentrations ten or a hundred times greater. Aquatic organisms vary more widely in their sensitivity The main impact of prolonged human exposure to cadmium is on the kidney, although obstructive lung disorders can also result from respiratory exposure. The effect on the kidney is due to accumulation of cadmium in the renal cortex, leading to tubular protein urea. Cadmium absorbed by the body is only slowly excreted; as a consequence, cadmium toxicity is markedly cumulative so that there is the possibility of chronic cadmium poisoning among industrial workers regularly exposed to this metal or its compounds. An epidemiological survey of workers exposed to cadmium dust found excessive protein urea due to kidney damage in 68% of a group of male workers with over 20 years' exposure. Occupational exposure to cadmium oxide dust has been suggested to increase the risk of prostate cancer in man.