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Cadmium is in its elemental form a soft, silver-white metal. It is not usually present in the environment as a pure metal, but most often as complex oxides, sulphides, and carbonates in zinc, lead, and copper ores.
Cadmium is not an element that is used by the body, and it is toxic. It mainly affects kidneys and bones. It is also a carcinogen by inhalation. Cadmium can accumulate in, liver, kidneys and bones, which may serve as sources of exposure later in life.
In the environment, cadmium is toxic to plants, animals and micro-organisms. Being a simple chemical element, cadmium is persistent – it cannot be broken down into less toxic substances in the environment.
Cadmium is produced mainly as a by-product of mining, smelting and refining of zinc and, to a lesser degree, as a by-product of lead and copper production. It is therefore primarily produced in relation to zinc production rather than direct cadmium demand. Global cadmium production almost doubled between 1950 and 1990. Since 1990, global consumption has remained constant, at about 20,000 tons per year, although major changes have occurred in its geographical distribution. Production in Asia has increased sharply, whereas the production in Europe has decreased. Major shifts and improvements in smelting and refining technologies have led to significant decreases in releases of cadmium to the environment. Recycling accounts for 18 percent of the cadmium production worldwide.
Most of the cadmium produced is used in the production of Nickel -Cadmium batteries, which in 2004 represented already 81 per cent of the total production of cadmium. Other major uses of refined cadmium are: pigments for plastics, ceramics and enamels; stabilizers for plastics; plating on iron and steel; and as an alloying element of some lead, copper and tin alloys. Since 1990, consumption for pigments, stabilizers, alloys and other uses has decreased significantly.
Cadmium is released by various natural and anthropogenic sources to the atmosphere, aquatic and terrestrial environments, mostly in the form of particles of cadmium oxides.
Natural sources of cadmium result from mobilization of naturally occurring cadmium from the Earth's crust and mantle, by volcanic activity and weathering of rocks. Human activities that are a source of cadmium are, for instance, the release of cadmium impurities from raw materials such as phosphate minerals, fossil fuels and other metals, particularly zinc and copper. Cadmium is also released as a result of use, disposal, recycling, reclamation of various products, of open burning or incineration and of the mobilization and releases of cadmium previously deposited in soils, sediments, landfills and waste or tailings piles.
Once in the environment, cadmium can be transported from one setting to another through transport of particles that are either blown by the wind of washed away by water. The long-term sinks are deep-sea sediments and, to a certain extent, controlled landfills. It is generally considered that there is little long-distance transport of cadmium through aerosol particles, but in oceans, cadmium can accumulate and be transported over long distances.
The major natural sources for emission to air are volcanoes, airborne soil particles, sea spray, biogenic material and forest fires. Very different estimates of total releases of cadmium to the atmosphere by natural processes have been reported. A study estimated the total emissions in 1983 between 150 and 2 600 tonnes per year, whereas a more recent study estimated the total emissions from natural sources between 15 000 and 88 000 tonnes per year. The large discrepancy is mainly due to different estimates of the amount of cadmium released to air with soil particles. Because of the limited data and huge differences between the findings of these two studies, there is uncertainty about the relative magnitude of natural emissions as compared to anthropogenic emissions. The more recent study suggests that natural emissions might be between 5 and 30 times higher than anthropogenic emissions.
The most recent study of global human emissions of cadmium estimated the total in the mid-1990’s at 2 983 tonnes. Available data indicate, that anthropogenic emissions of cadmium have decreased by an average of about 50 per cent from 1990 to 2003 in developed countries. The main sources of emissions are non-ferrous metal production and fossil fuel combustion. Other sources include iron and steel production, waste incineration and cement production. In some developing countries, open burning of cadmium-containing products and indiscriminate dumping contribute to local and regional exposure.
As awareness of the adverse impacts of cadmium has increased, many uses have been reduced significantly in industrialized countries, and waste management systems have been put in place to limit the release of cadmium into the environment. In contrast, in some developing countries, some used are increasing, and products containing cadmium are not typically collected separately from the general waste stream. Therefore cadmium discards will end up in municipal waste and disposed of in landfills, incineration, open burning or indiscriminate dumping. Some of the cadmium in these products will be released to the environment.
The International Agency for Research on Cancer (IARC) classifies cadmium in Group 1: carcinogenic to humans and the US Environmental Protection Agency (EPA) has determined that cadmium is a probable human carcinogen by inhalation. Epidemiological data from occupational settings confirm lungs being the primary target organ. Cadmium is not considered a carcinogen by ingestion.
The critical health effect of ingested cadmium is on the kidneys where it damages the blood filtration system, which results in proteins being excreted in urine. The severity of the effect depends on duration and magnitude of exposure. Skeletal damage is another critical effect of long-term exposure to cadmium exposure at levels somewhat higher than those for which kidney problems occur.
Cadmium is mainly stored in the liver, kidneys and bones. Excretion is slow, with a very long half-life (decades) in the human body. Cadmium concentrations in most tissues increase with age.
There are different ways by which people are exposed to cadmium:
The level of dietary exposure can exceed the guidelines set by the United Nations Food and Agriculture Organization (FAO) and the World Health Organization (WHO). According to available data, the average weekly intake of cadmium from food in most countries is within the range of 0.7–2.8 μg/kg body weight per week, which is below the provisional tolerable weekly intake (PTWI) (7 μg/kg body weight per week). In some groups, like smokers and some vulnerable groups like people with kidney problems, their intake can reach or be above the tolerable limit.
Cadmium is a non-essential heavy metal, meaning that it is not used by biological systems. It tends to accumulate in vertebrates, and it also accumulate in aquatic invertebrates and algae. Effects on birds and mammals are mainly due to kidney damage. Aquatic fresh water and marine invertebrates are the most sensitive organisms to cadmium.
The dissolved cadmium concentrations measured in some European waters (mainly rivers) are exceeding the threshold concentration that produces adverse effects of cadmium in the aquatic ecosystem.
With regard to vertebrates, despite relatively high levels of cadmium in seabirds and marine mammals in Greenland, no evidence has been found of effects in ringed seals with very high cadmium levels in their kidneys.
In terrestrial ecosystems, micro-organisms and plants are more sensitive to cadmium than invertebrates). Accumulation of cadmium by plants and predators feeding on soil invertebrates introduce cadmium into the food chain, which suggests a risk of secondary poisoning through the food chain, from worms to higher trophic levels (birds or mammals). In some European areas, the cadmium concentrations, both measured and estimated, are exceeding the threshold concentration for adverse effects on terrestrial ecosystems.
A number of data gaps and needs are still identified, for instance:
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