Is cadmium harmful to humans or ecosystems?
Humans can be harmed by a single large exposure to cadmium, and by long-term exposure to higher-than-usual concentrations. Until the mid-1900s, cadmium had few industrial uses. People were rarely exposed to concentrated doses of cadmium and the metal was not recognized as a health concern. But as new uses for cadmium were found, and as the industrial processes that produce the metal increased worldwide, the toxic effects of cadmium began to surface.
Some of the earliest cases of cadmium poisoning were reported in Belgium in 1858 in workers who inhaled cadmium dust as a result of polishing silver with cadmium carbonate. This kind of exposure can cause severe respiratory distress, emphysema, and even death.
Public awareness of cadmium’s toxic effects rose with the post-World-War-II outbreak of the “Itai-Itai” Disease (“Ouch-Ouch” Disease) in Japan, which had been caused by a release of cadmium into the run-off water from the Kamioka mine. Farmers in the region used the run-off to irrigate rice patties and other crops. Cadmium quickly became concentrated in the crops, and before long local women began to experience pain in their bones and joints, which eventually became so excruciating that they were bed-ridden. The cadmium, it was later found, had interfered with calcium metabolism, leading to reduction in calcium content and the density and strength of their bones. Simple movements, in some cases, caused the weakened bones to break. Removing cadmium from industrial wastewater halted the incidence of this extremely painful type of chronic cadmium poisoning and no new cases have been recorded in Japan since. (Itai-itai occurred primarily in post-menopausal women who had several children and was probably related as well to vitamin D deficiencies, hormonal status and other factors.)
The toxicity of cadmium is attributed, in part, to its ability to accumulate in living things. Cadmium is rare in nature and consequently plants and animals have not evolved with efficient means of metabolizing large amounts of the metal. Small amounts of the metal are bound up by the protein metallothionein and are removed from the body, but since organisms are unable to isolate and remove large amounts efficiently, long-term exposure to high levels can result in accumulation in body tissues. Under these conditions, cadmium can remain in the body for years. Most of the metal accumulates in the bones, liver and kidneys, where it can damage the functioning of those organs.
Cadmium can also bioaccumulate in the ecosystem. Crops treated with cadmium-containing fertilizer or commercial sludge can accumulate above-normal cadmium concentrations and pass them on through the food web to higher organisms such as livestock and humans as in the case of the Kamioka mine in Japan.
Some organisms absorb cadmium better than others. Among plants, staple foods such as wheat, rice and potatoes have been shown to accumulate higher amounts of cadmium. The overall highest levels of cadmium in food can be expected in the livers and kidneys of animals and in shellfish such as oysters and clams.
How does cadmium harm living things?
Cadmium is known to accumulate in the kidneys, and some scientists believe that damage to kidney tissue may lead to kidney disease, high blood pressure and heart disease. Calcium related kidney damage leads to calcium deficiencies in the rest of the body, particularly in the skeleton. As the “Itai-Itai” syndrome made clear, in extreme cases cadmium can contribute to aching bones and joints, progressing to extreme deformities and brittleness of bones. Some humans with high blood pressure have been found to have abnormally high amounts of cadmium in their urine, and animals given cadmium in food or water developed kidney and liver disease, high blood pressure, iron-poor blood and nerve or brain damage. Fortunately there have been no reported cases of Itai-Itai since the 1960s.
Excessive cadmium exposure may weaken the body’s immune system, and it is also believed to be linked to lung cancer. Some studies suggest it causes prostate enlargement. Some scientists suspect that cadmium may be a reproductive toxin. Some studies have found that animals exposed to high levels of cadmium had a higher incidence of premature birth, low birth weight, stillbirth and spontaneous abortion. Animal studies also suggest that cadmium exposure is linked to behavioral problems and learning disabilities.
People whose diets are deficient in zinc, copper, iron, calcium and vitamin D may be at higher risk for health complications from cadmium. These elements, which look and behave in a way that is chemically similar to cadmium, can be replaced by cadmium when the essential elements are in short supply. Bodily proteins that capture and metabolize essential metals can also absorb cadmium particles due to its similar chemical behavior.