Human Intake of Cadmium
Cd2+ is rather soluble in water, unless sulfide ions are also present to precipitate the metal as CdS. Thus humans usually ingest only a small proportion of their cadmium directly from drinking water or from air, except for individuals who live near mines and smelters, particularly those that process zinc. The maximum containment level (MCL) for cadmium in drinking water is 5 ppb in the United States and Canada (Table 15-2).
Smokers are also exposed to cadmium rhat is absorbed from soil and irrigation water by tobacco leaves and then released into the smoke stream when a cigarette is burned. Heavy smokers have approximately double the net cadmium intake of nonsmokers.
Owing to its similarity to zinc, plants absorb cadmium from irrigation water. The use on agricultural fields of phosphate fertilizers, which contain ionic cadmium as a natural contaminant, and of sewage sludge contaminated with cadmium from industrial releases increases the cadmium level in soil and subsequently in plants grown in it. In the future, cadmium may be removed from phosphate fertilizer before it is sold to the consumer (see also Chapter 16). Soil also receives cadmium from atmospheric deposition. Since cadmium, uptake in plants increases with decreasing soil pH, one effect of acid rain is to increase cadmium levels in food.
For most of us, the greatest proportion of our exposure to cadmium comes from our food supply. Seafood and organ meats, particularly kidneys, have higher cadmium levels than do most other foods. However, the majority of cadmium in the diet usually comes from potatoes, wheat, rice, and other grains, since most people consume so much more of them than of seafood and kidneys. An exception is the Inuit people of Canada's Northwest Territories; a prized component of their diet is caribou kidneys, organs which are highly contaminated by cadmium that has reached the Arctic regions on the wind from industrial regions in Europe and North America.
Historically, all episodes of serious cadmium contamination resulted from pollution from nonferrous mining and smelting. The most acute environmental problem involving cadmium occurred in the Jintsu River Valley region of Japan, where rice for local consumption was grown with the aid of irrigation water drawn from a river that was chronically contaminated with dissolved cadmium from a zinc mining and smelting operation upstream.
Hundreds of people in this area, particularly older women who had home many children and who had poor diets, contracted a degenerative bone disease called itai'itai or "ouch-ouch," so named because it causes severe pain in the joints. In this disease, some of the Ca2+ ions in the bones are replaced by Cd2+ ions since they have the same charge and are virtually the same size. The bones slowly become porous and can subsequently fracture and collapse. The intake of cadmium by itai-itai sufferers was estimated at about 600 fig per day, about 10 times the average ingestion of North Americans.
Continue reading here: Protection Against Low Levels of Cadmium
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