The Facts on Cadmium
What is Cadmium?
In its pure form, cadmium is a silvery white, malleable metal with a bluish hue. It is found naturally in the earth's crust and is a relatively rare metal, ranking 67th in abundance among the 90 naturally occurring elements on Earth. Cadmium is odorless and tasteless, and chemical analysis is most often required to detect its presence. Because it readily reacts with other elements, cadmium is rarely found in its pure, or elemental, form but rather coupled with other elements in a variety of compounds, including some that are extremely toxic. Two forms of cadmium compounds, cadmium sulfates and cadmium chlorides, dissolve so easily in water that people rarely find them in nature in their solid form.
Cadmium is primarily found in zinc-containing ores, but it may also be found in lead and copper ores. The primary mineral form of the metal is greenockite or cadmium sulfide. Other mineral forms such as otavite or cadmium carbonate exist but are fairly rare.
Pure cadmium metal was first identified by German chemist Friedrich Stromeyer (1776-1835) in 1817. Stromeyer had been heating samples of zinc carbonate in his laboratory and observed that they were becoming discolored at high temperatures. Knowing that pure zinc carbonate does not discolor, Stromeyer reasoned that some impurity must have caused the reaction. He created an experimental procedure to identify the impurity and was able to isolate the silver-blue metal.
Where is Cadmium Found?
Cadmium is extremely rare in the Earth's crust, with less than one-fifth of a gram of the metal about one fifth of the metal in a thumbtack in every ton of crustal material. Cadmium can be spread through the air by the wind through natural processes such as erosion, or through the combustion of cadmium-containing ores in volcanic emissions. Airborne particles of cadmium settle onto the ground and waterways as dust. Though surface waters can contain some dissolved cadmium, concentrations tend to be low since the metal is readily absorbed by sea life, especially shellfish. Because of this absorption of cadmium by aquatic life, the U.S. Environmental Protection Agency (EPA) has issued fishing advisories related to cadmium for coastal waters around New York, Connecticut, and New Jersey.
Humans play a significant role in creating concentrated sources of cadmium and releasing it into the environment through activities such as mining, smelting and refining metal ores particularly zinc, lead and copper. Cadmium is also emitted into the atmosphere from fossil fuel burning, waste incineration and steel production. Soil and water near industrial areas or waste sites may contain higher concentrations of cadmium. All told, an estimated 4,000 to 13,000 tons of cadmium are released into the environment every year as a result of human activities.
Most commercial cadmium is obtained as a byproduct of zinc ore refining, though cadmium can also be recovered from recycled materials such as nickel-cadmium batteries and metal scrap. The United States is the world's primary producer of cadmium, generating an estimated 1,100 tons of the metal per year.
What are the Uses of Cadmium?
Until the mid-1900s, cadmium was used primarily as a leather tanning agent or as a pigment in dyes, but it was not widely used even in these applications. Today, cadmium is used primarily in rechargeable batteries, usually in combination with nickel or silver oxides (The term "Ni-Cad" or "Ni-Cd" refers to batteries containing nickel and cadmium.). Solar rechargeable nickel-cadmium batteries have been used on space exploration missions, including the Magellan probe that explored the planet Venus.
The use of cadmium in batteries accounts for roughly three quarters of the cadmium consumption in the United States. Since it is a disposable consumer product, Ni-Cad batteries also account for over half of the cadmium waste produced. Most consumer products contain sealed Ni-Cad batteries as opposed to the vented Ni-Cad batteries used in aircraft, buses and diesel locomotives, which emit a significant amount of cadmium to the environment.
Because it does not react with alkali (basic) chemicals and it reacts only slowly with hot hydrochloric acid, cadmium plating provides excellent protection from highly corrosive chemicals like acids and bases. Cadmium is often used to electroplate steel, copper, iron and brass in applications where those metals will be exposed to weather or to corrosive materials.
Cadmium plays a critical role in several cutting-edge technologies such as solar cells, through applications that take advantage of its unique physical properties.
Cadmium is grouped among the elements known as transition metals. Certain transition metals can act as both electrical conductors and insulators depending on slight alterations in their chemical structure. Silicon and germanium are good examples of such transition metals. These elements can also absorb certain wavelengths of electromagnetic radiation, turning the sun's rays into usable electric energy. Since cadmium is a transition metal with a chemical structure similar to silicon and germanium, it possesses some of the same semi-conducting qualities. By evaporating a compound of cadmium known as cadmium telluride onto glass or plastic, engineers can create a semi-conducting layer only several atoms thick. The resulting product serves as an ultra-thin, highly energy efficient solar cell. Such thin-film solar cells may provide a cost-effective and efficient solution to the energy needs of the future.
Cadmium has also found its way into a variety of other products. It is sometimes used to chemically stabilize certain plastics, and it can also be used to produce special solder alloys that melt at relatively low temperatures. In paints, cadmium sulfide lends a yellow hue, while cadmium selenide produces colors ranging from yellow to red. Control rods and shields for nuclear reactors may contain cadmium because of its ability to absorb emitted particles. Cadmium is also used to make phosphor compounds that glow when bombarded with electrons, the technology that produces the image in a television tube.
Do we Need Cadmium for Health?
Though trace amounts of many metals are essential for the health of living things, there is no scientific evidence showing a nutritional role for cadmium.
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.
Kamioka Mine, Japan
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 are People Exposed to Cadmium?
According to the Agency for Toxic Substances and Disease Registry, foods account for more than 90 percent of human exposure to cadmium. On average, people consume about 30 micrograms of cadmium daily through a normal diet, absorbing 1 to 3 micrograms. There is currently no evidence that these trace levels pose a hazard to healthy adults.
Cigarette smokers, however, typically sustain significantly higher exposure. Cadmium inhaled through cigarette smoke is more easily taken up by the body than cadmium in food or water. From 40 to 60 percent of the cadmium inhaled in smoke is absorbed into the bloodstream as opposed to the 5 to 10 percent absorbed through foods. Each cigarette contains roughly 1 to 2 micrograms of cadmium, and smokers absorb an additional 1 to 3 micrograms of cadmium into their systems daily for every pack they smoke. Studies have shown that smoking more than 20 cigarettes daily can increase cadmium levels in the body tenfold.
Those who work in or live near industries that produce or use cadmium such as battery manufacturing, ore refining, metal soldering, or welding can also be exposed by drinking it in contaminated water or inhaling it from the air, though there are workplace and environmental regulations to protect people from these exposures.
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.
Exessive 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.
What are the Symptoms of Cadmium Poisoning?
The symptoms associated with cadmium exposure depend largely on how cadmium is encountered through eating or drinking, breathing or skin contact and over what period of time the exposure occurs. A short-term exposure to cadmium in the air can cause bronchial and lung irritation, and workers who have unknowingly done welding on cadmium alloys have even died from these complications. Cadmium in air presents a serious hazard because it is difficult to detect until significant damage to the lungs has occurred.
Exposure to highly contaminated food or water can cause can severe irritation of the digestive tract, including vomiting, diarrhea and even death. Other symptoms of short-term exposures include muscle cramps, sensory disturbances, liver injury, convulsions, shock and kidney failure.
Are there Ways to Reduce the Risk of Cadmium Exposure?
People who work in industries such as metal processing, electroplating, battery manufacture, and ore refineries should observe all workplace safety practices, and should also take care to avoid bringing cadmium dust into their homes on shoes, other clothing or tools, for example.
Families can reduce their exposure by storing products that may contain cadmium such as fertilizers and nickel-cadmium batteries away from children, and by using these products according to manufacturer's instructions.
Is There a Test for Cadmium Exposure?
There are several tests that indicate if someone has been exposed to or been harmed by cadmium exposure. Urine or blood samples can be tested to indicate current and past exposure and may even be useful in determining if kidney damage has occurred. Hair and fingernails or toenails are also excellent biomarkers biological indicators for cadmium exposure and can reveal past exposure to the metal.
What are the Government Standards and Guidelines on Cadmium?
To protect public health the federal government develops recom- mendations, or guidelines, as well as regulations that can be enforced by law. Generally these are expressed as "not-to-exceed levels in air, water, soil, or food. Federal agencies that develop regulations for toxic substances include the United States Environmental Protection Agency (EPA), the Occupational Safety and Health Administration (OSHA), and the Food and Drug Administration (FDA). Federal organizations that develop guidelines or recommendations for toxic substances include the Agency for Toxic Substances and Disease Registry (ATSDR) and the National Institute for Occupational Safety and Health (NIOSH).
Under EPA regulations, public drinking water supplies cannot exceed 5 parts of cadmium per billion parts of water (5 ppb). The EPA also limits how much cadmium can be put into lakes, rivers, dumps, and cropland, and does not allow cadmium to be used in pesticides. The FDA limits the amount of cadmium in food colors to 15 parts per million (ppm). The OSHA limit for the amount of cadmium in workplace air is 5 micrograms per cubic meter.
These levels are based on concentrations that affect animals and are adjusted to protect people. Regulations and recommendations are updated as new information becomes available. Not-to-exceed levels may differ among federal organizations because they assume different durations of exposure (an 8-hour workday or a 24-hour day), or are based on different animal studies, or other factors.