Synopsis

This assessment of cadmium and its compounds focuses on the forms of cadmium that are known to be found in the environment (i.e., several forms of inorganic cadmium). Cadmium (Cd) is present in the Canadian environment as a result of both natural processes (including forest fires, volcanic emissions and weathering of soil, till, and bedrock) and human activities. Of the natural sources of cadmium to the environment, weathering and erosion of cadmium-bearing rocks represent perhaps the most important source. Approximately 1963 tonnes (t) of refined cadmium are produced, 23 t imported, and 1580 t exported each year in Canada (1992 estimates). Anthropogenic sources of cadmium entry to the Canadian environment include metal production (particularly base metal smelting and refining), stationary fuel combustion (power generation and heating), transportation, solid waste disposal, and sewage sludge application.

Although quantitative releases were not identified for all of these sources, the available data indicate that an estimated 159 t of cadmium are released annually to the Canadian environment as a result of domestic anthropogenic activities. Of this total, 92% is released to air and 8% is released to water. Approximately 340 t of cadmium wastes from the metal smelting and refining industry are deposited into landfills, although the amount of cadmium from this source that is potentially available to the Canadian environment is not known. The most recent estimates identified indicate that base metal smelting and refining operations account for 82% (130 t) of the total releases to air and water.

Cadmium does not break down in the environment, but it may be affected by physical and chemical processes that modify its mobility, bioavailability, and residence time in different environmental media. Atmospheric cadmium compounds (e.g., cadmium oxide) are predominantly in particulate form (fine particulates are more easily solubilized and more bioavailable than larger fractions), have relatively short tropospheric residence times, and are removed from air by wet and dry deposition. The mobility and bioavailability of cadmium in aquatic environments are enhanced under conditions of low pH, low hardness, low suspended matter levels, high redox potential, and low salinity. The movement of cadmium in soil and potential accumulation by biota is enhanced by low pH, low organic matter content, large soil particle size, and high soil moisture.

Mean levels of cadmium in some Canadian lakes in the vicinity of known  anthropogenic sources (e.g., base metal smelters) have exceeded the estimated effects threshold for the most sensitive freshwater species (Daphnia magna). Mean levels and concentration ranges of cadmium in marine waters and sediments from two locations in Canada (Belledune Harbour, New Brunswick and Vancouver Harbour, British Columbia) have equalled or exceeded the estimated effects thresholds for the most sensitive marine species (Mysidopsis bahia and Rhepoxynius abronius).

Mean levels of cadmium in soils near known sources (e.g., base metal smelters) from four provinces have exceeded the estimated effects threshold for the most sensitive soil. Species (Aiolopus thalassinus and Spinacia olaracea). Mean cadmium levels in kidney tissue of some Canadian mammalian wildlife and domestic species (e.g., narwhal) have exceeded the estimated effects threshold for renal dysfunction based on critical concentrations determined in laboratory animals.

Atmospheric inorganic cadmium compounds (oxide, chloride, sulphide, and sulphate) have relatively short tropospheric residence times, low tropospheric concentrations, and do not absorb appreciable amounts of infrared radiation. Thus, inorganic cadmium compounds are not expected to contribute to global climate change. Inorganic cadmium compounds are not expected to react with ozone, and therefore, are not expected to contribute to stratospheric ozone depletion.

Based on estimation of the average daily intake of cadmium (total) from air, drinking water, food, and soil for various age groups in the general population, food is likely the most significant source of human exposure in Canada. In several studies of workers, exposure to airborne cadmium compounds has been associated with increased mortality from lung cancer, but the increase may have been due, in part, to concurrent exposure to arsenic. However, principally on the basis of results of inhalation studies in animal species, inorganic cadmium compounds have been classified as "probably carcinogenic to humans", i.e., as substances for which there is believed to be some chance of adverse health effects at any level of exposure. For such substances, where data permit, estimated exposure is compared to quantitative estimates of cancer potency, to characterize risk and provide guidance for further action (i.e., analysis of options to reduce exposure). For inorganic cadmium compounds, such a comparison suggests that the priority for analysis of options to reduce exposure would be high.

In addition, renal tubular dysfunction has been observed in human populations exposed to cadmium in the workplace or in the general environment. While these effects were originally reported in populations having relatively high exposure to cadmium, in recent European studies, mild effects on the kidney have been associated with levels of cadmium at or near those to which a portion of the Canadian general population is exposed. Based on available information, it is concluded that dissolved and soluble* forms of inorganic cadmium are entering the environment in a quantity or concentration or under conditions that are having or may have a harmful effect on the environment. It is concluded that inorganic cadmium compounds are not entering the environment in a quantity or concentration or under conditions that constitute a danger to the environment on which human life depends. Finally, it is concluded that inorganic cadmium compounds are entering the environment in a quantity or concentration or under conditions that may constitute a danger to human life or health in Canada.