Sometimes, cadmium escapes into the environment through air, probably in the form of small particles which have escaped the trapping process at the smelter. Although the present- day emission from a modern smelter may be immeasurably small at the source, it is, often, being released on a twenty-four hour basis. Large zinc smelters caused local pollution in Toyama, Japan, and Bristol, England. In both the cases, the air-borne contamination could be identified in the plants and soil in the surrounding district. In Japan, unfortunately, some of these plants happened to be rice, which is a principal constituent of the diet of the local people in the city of Toyama. In Britain, the surrounding area was not agricultural land and, hence, cadmium effects on humans have not been observed. It is not always possible to separate air-borne and water-borne pollution in the case of cadmium when the principal source is a smelter.
In Toyama, for example, much of the cadmium in the rice came from the irrigation water which, in turn, came from a river which passed close to the smelter. Turning again to Britain, another example concerns a small village called Shipham, near Bristol. It lies below an area which used to be intensively mined for lead. Lead ore also contain some cadmium. The water which drains from the mineral area contains, in addition to lead, significant amounts of cadmium which, over hundreds of years, have accumulated in the soil around the village. Vegetables, grown there do contain cadmium but no definite effects on the population have been observed.
This is due to the fact that most of the food for the village comes from outside the contaminated area. Cadmium can also enter the environment from many different industries. The most obvious of these is the electroplating industry. In these days, when brass is so expensive, it has been found that it is cheaper to plate steel with cadmium and, thus, provide corrosion resistance.
The liquid effluent from this process is potentially very dangerous. Harder to identify are the many other liquid sources associated with other industries. For these reasons, the effluent, from any industrial city, is almost certain to contain some cadmium whether or not zinc smelting is carried out. An example of this is seen in the results of work carried out by the Institute of Science, Bombay, on the aquatic environment of the Kalu River. Here, the cadmium concentration in the water was at an acceptable level (under 50 ppm).
There was evidence, however, that the cadmium concentration was much higher in the leaves and roots of plants growing near the river. A biological concentration of over one thousand fold was reported. Thus, after it enters the environment, this metal can slowly accumulate in sediments and sedentary organisms, like animals or plants. Mercury:Once again, for the most serious example of human poisoning, we must turn, to Japan. In this case, it was in the coastal town of Minimata, which is just south of Nagasaki that large numbers of people died or were permanently crippled, mentally and physically. This later became known as the “Minimata Disease”, caused by mercury which had accumulated in the bodies of fish which constituted an important part of the human diet in that area.
Mercury differs from cadmium in one respect, i.e., it can be converted by microorganisms into alkyl compounds.
Such a compound, methyl mercury, is actually more toxic than mercury itself. This conversion can take place in the sediment of river beds where the mercury compounds can enter the food chain. The worms which live in this mud are the main diet of many fish which are used for human consumption. An analysis of thirty species of fish from the Bombay and Thane areas showed that many species, in particular, Tilapin, Lates, Wallago, Mugil and Elops, carried mercury burden which approached or slightly exceeded the permissible level of 500 gm. Once again, we see here, an example of toxic- metal pollution, originating from a highly populated and developed area.