Level of cadmium in the environment
Cadmium levels in the environment vary widely. As stated above, cadmium emissions to the environment are normally transported continually between the three main environmental compartments, air, water and soils, but a steady state flux is probably achieved and the general levels can reasonably well be established.
3.1 Cadmium in Air
Three distinct categories may be recognised with respect to cadmium-in-air concentrations - cadmium in ambient air, cadmium air levels in occupational exposure situations, and cadmium in air from the smoking of tobacco. Cadmium in ambient air represents, by far, the majority of total airborne cadmium. Inputs from all three categories may affect human cadmium intake and human health, but the levels and the transfer mechanisms to humans are substantially different for the three. Whereas cadmium from occupational environments and cadmium from cigarette smoke are transferred directly to humans, cadmium in ambient air is generally deposited onto waters or soils, then eventually transferred to plants and animals, and finally enters the human body through the food chain.
3.1.1 Cadmium in Ambient Air
Ambient air cadmium concentrations have generally been estimated to range from 0.1 to 5 ng/m³ in rural areas, from 2 to 15 ng/m³ in urban areas, and from 15 to 150 ng/m³ in industrialised areas (Elinder 1985, WHO 1992, OECD 1994) although some much lower values have been noted in extremely remote areas and some much higher values have been recorded in the past near uncontrolled industrial sources. There are generally little or no differences noted in cadmium levels between indoor and outdoor air in non-smoking environments. Smoking, however, may substantially affect indoor ambient air cadmium concentrations.
3.1.2 Cadmium in Occupational Environments
Cadmium air concentrations may be elevated in certain industrial settings, but these exposures are closely controlled today by national occupational exposure standards. Historically, average exposure levels and regulatory permissible exposure limits have decreased markedly in the past 40 years in recognition of the importance of cadmium inhalation to human health and with the significant improvements in air pollution control technology over that period (Elinder 1985, WHO 1992). Occupational exposure standards which were formerly set at 100 to 200 µg/m³ are now specified at 2 to 50 µg/m³ along with requirements to maintain biological indicators such as cadmium-in-blood and cadmium-in-urine below certain levels to assure no adverse human health effects from cadmium occupational exposure (International Labour Organisation 1991, ACGIH 1996, OSHA 1992).
3.1.3 Cadmium in Tobacco Smoke
Tobacco leaves naturally accumulate and concentrate relatively high levels of cadmium, and therefore smoking of tobacco is an important source of air cadmium exposure for smokers. It has been reported that one cigarette contains about 0.5 - 2 µg of cadmium and that about 10% of the cadmium content is inhaled when the cigarette is smoked (Elinder 1985, WHO 1992). Smokers generally exhibit significantly higher cadmium body burdens than non-smokers.
3.2 Cadmium in Water
The average cadmium content in the world's oceans has variously been reported as low as <5 ng/L (WHO 1992) and 5-20 ng/L (OECD 1994, Jensen and Bro-Rasmussen 1992) to as high as 110 ng/L (CRC 1996), 1 00 ng/L (Cook and Morrow 1995) and 10 to 100 ng/L (Elinder 1985). Higher levels have been noted around certain coastal areas (Elinder 1985) and variations of cadmium concentration with the ocean depth, presumably due to patterns of nutrient concentrations, have also been measured (WHO 1992, OECD 1994). Even greater variations are quoted for the cadmium contents of rainwater, fresh waters, and surface waters in urban and industrialised areas. Levels from 10 ng/L to 4000 ng/L have been quoted in the literature depending on specific location and whether or not total cadmium or dissolved cadmium is measured (Elinder 1985, WHO 1992, OECD 1994).
Cadmium is a natural, usually minor constituent of surface and groundwater. It may exist in water as the hydrated ion, as inorganic complexes such as carbonates, hydroxides, chlorides or sulphates, or as organic complexes with humic acids (OECD 1994). Cadmium may enter aquatic systems through weathering and erosion of soils and bedrock, atmospheric deposition direct discharge from industrial operations, leakage from landfalls and contaminated sites, and the dispersive use of sludge and fertilisers in agriculture. Much of the cadmium entering fresh waters from industrial sources may be rapidly adsorbed by particulate matter, and thus sediment may be a significant sink for cadmium emitted to the aquatic environment (WHO 1992). Some data shows that recent sediments in lakes and streams range from 0.2 to 0.9 ppm in contrast to the levels of generally less than 0. 1 ppm cited above for fresh waters (Cook and Morrow 1995). Partitioning of cadmium between the adsorbed-in-sediment state and dissolved-in-water state is therefore an important factor in whether cadmium emitted to waters is or is not available to enter the food chain and affect human health.
Rivers containing excess cadmium can contaminate surrounding land, either through irrigation for agricultural purposes, dumping of dredged sediments or flooding. It has also been demonstrated that rivers can transport cadmium for considerable distances, up to 50 km, from the source (WHO 1992). Nonetheless, studies of cadmium contamination in major river systems over the past twenty to thirty years have conclusively demonstrated that cadmium levels in these rivers have decreased significantly since the 1960s and 1970s (Cook and Morrow 1995, Elgersma et al. 1992, Mukunoki and Fujimoto 1996, Van Assche and Ciarletta 1992). For example, studies on the Rhine River Basin from 1973 through 1987 indicated that the point source cadmium discharges to the Rhine River decreased from 130 to 11 mt per year over that 14-year time span, a reduction of over 90% (Elgersma et al. 1992). Similarly, data on total cadmium and dissolved cadmium at the Dutch/German border over the period from 1971 to 1987 have shown comparable reductions (Van Urk and Marquenie 1989).
3.3 Cadmium in Soil
Cadmium in soils is derived from both natural and anthropogenic sources. Natural sources include underlying bedrock or transported parent material such as glacial till and alluvium. Anthropogenic input of cadmium to soils occurs by aerial deposition and sewage sludge, manure and phosphate fertiliser application. Cadmium is much less mobile in soils than in air and water. The major factors governing cadmium speciation, adsorption and distribution in soils are pH, soluble organic matter content, hydrous metal oxide content, clay content and type, presence of organic and inorganic ligands, and competition from other metal ions (OECD 1994). The use of cadmium-containing fertilisers and sewage sludge is most often quoted as the primary reason for the increase in the cadmium content of soils over the last 20 to 30 years in Europe (Jensen and Bro-Rasmussen 1992). Atmospheric cadmium emissions deposition onto soils has generally decreased significantly over that same time period (Cook and Morrow 1995, Mukunoki and Fujimoto 1996). Indeed, recent studies in Europe have documented that atmospheric emissions do not presently have a significant impact upon the cadmium content of soils (Bak et al. 1997).
3.3.2 Cadmium Levels in Soils
The average natural abundance of cadmium in the earth's crust has most often been reported from 0.1 to 0.5 ppm, but much higher and much lower values have also been cited depending on a large number of factors. Igneous and metamorphic rocks tend to show lower values, from 0.02 to 0.2 ppm whereas sedimentary rocks have much higher values, from 0.1 to 25 ppm. Naturally, zinc, lead and copper ores, which are mainly sulphides and oxides, contain even higher levels, 200 to 14,000 ppm for zinc ores and around 500 ppm for typical lead and copper ores. The raw materials for iron and steel production contain approximately 0.1 to 5.0 ppm, while those for cement production contain about 2 ppm. Fossil fuels contain 0.5 to 1.5 ppm cadmium, but phosphate fertilisers contain from 10 to 200 ppm cadmium (Cook and Morrow 1995).
3.3.3 Cadmium Emissions to Soils
Cadmium in soils must be distinctly classified in three separate areas with regard to their relative effects on human health and the environment These three areas are agricultural soils, non-agricultural soils, and controlled landfills. Cadmium in controlled landfalls is virtually immobile, and is unlikely to have any effect on human health or the environment simply because it is so well contained (Eggenberger and Waber 1998, NUS 1987). Cadmium in non-agricultural soils will generally not affect human health as it does not enter the food chain readily or may do so only indirectly by transfer from non-agricultural soils to agricultural soils via airborne or water transport. However, the amount thus transferred is considered to be relatively low and is not expected to be a significant proportion of the cadmium in non-agricultural soils. Cadmium in agricultural soils is likewise relatively immobile under normal conditions, but could become more mobile under certain conditions such as increased soil acidity and its cadmium level may be enhanced by the usage of phosphate fertilisers, manure or sewage sludge.
3.3.4 Cadmium in Agricultural Soils
Numerous agencies have focused upon the presence of cadmium in agricultural soils, the means by which agricultural soils may be enriched by cadmium, the degree to which cadmium is taken up by food stuffs, and the subsequent transfer of cadmium to humans via food stuffs. Because cadmium is a naturally occurring component of all soils, all food stuffs will contain some cadmium and therefore all humans are exposed to natural levels of cadmium. Although much attention has been focused upon the cadmium content of agricultural soils, it is important to recognise that the cadmium content of food items varies more as a function of the nature of the crop grown and the agricultural practices followed. Except in cases of extreme contamination, the concentration of cadmium in soils is not the primary determinant of cadmium in the human diet. For example, leafy vegetables and potato tubers naturally accumulate higher levels of cadmium than do fruits and cereals (Mench et al. 1998). Moreover, tillage and crop rotation practices similarly have a greater impact upon the cadmium content of food than does the concentration of cadmium in soils (Mench et al. 1998). Cadmium absorption may also depend on other factors as well as described below.
3.3.5 Cadmium Levels in Foodstuffs
Cadmium levels can vary Widely in various types of foodstuffs. Leafy vegetables such as lettuce and spinach and certain staples such as potatoes and grain foods exhibit relatively high values from 30 to 150 ppb. Peanuts, soybeans and sunflower seeds also exhibit naturally high values of cadmium with seemingly no adverse health effects. Meat and fish normally contain lower cadmium contents, from 5 to 40 ppb. Animal offal such as kidney and liver can exhibit extraordinarily high cadmium values, up to 1,000 ppb, as these are the organs in animals where cadmium concentrates (WHO 1992, ATSDR 1997). The cadmium contents of foodstuffs may vary widely with the agricultural practices utilised in the particular areas such as phosphate fertiliser, sewage sludge and manure application, the types of crops grown, and atmospheric cadmium deposition from natural or anthropogenic sources. Since various studies have shown that man's cadmium intake, as least for non-smokers, comes principally (approximately 95%) from the ingestion of foods rather than from inhalation of cadmium in air, it is the cadmium levels of foods which most affect the general population. There are strong indications that cadmium levels in foodstuffs have substantially decreased during the past several decades due to the progressive control of cadmium emissions to the environment (Van Assche and Ciarletta 1993, Watanabe et al. 1993, Watanabe et al. 1994). Recent studies have further documented that the cadmium content of food crops in Europe and many other countries are now stable and not increasing with time (Chaudri et al. 1995).
3.3.6 Cadmium Contamination of Agricultural Soils
In the past, there have been examples of marked cadmium contamination in areas where food has been grown. This was particularly so for rice crops in Japan in the 1950s and 1960s where cadmium concentrations from 200 to 2,000 ppb were found (Elinder 1985). In general, soils which have been historically contaminated with cadmium from industrial operations are no longer used for agricultural purposes. In those cases where old industrial installations which are cadmium-contaminated are subsequently employed for growing crops, suitable remediation techniques do exist to immobilise the cadmium present in the soil and thus to control the risk to human health. There is, however, no doubt that old sites which are so contaminated do require proper risk management and control by cleaning up or immobilising the existing excess cadmium in the soil.