Cadmium emissions arise from two major source categories, natural sources and man-made or anthropogenic sources. Emissions occur to the three major compartments of the environment - air, water and soil, but there may be considerable transfer between the three compartments after initial deposition. Emissions to air are considered more mobile than those to water which in turn are considered more mobile than those to soils.
2.1 Natural Cadmium Emissions
Even though the average cadmium concentration in the earth's crust is generally placed between 0.1 and 0.5 ppm, much higher levels may accumulate in sedimentary rocks, and marine phosphates and phosphorites have been reported to contain levels as high as 500 ppm (Cook and Morrow 1995, WHO 1992). Weathering and erosion of parent rocks result in the transport by rivers of large quantities, recently estimated at 15,000 metric tonnes (mt) per annum, of cadmium to the world's oceans (WHO 1992, OECD 1994). Volcanic activity is also a major natural source of cadmium release to the atmosphere, and estimates on the amount have been placed as high as 820 mt per year (WHO 1992, OECD 1994, Nriagu 1980, Nriagu 1989). Forest fires have also been reported as a natural source of cadmium air emissions, with estimates from 1 to 70 mt emitted to the atmosphere each year (Nriagu 1980).
2.2 Anthropoqenic Cadmium Emissions
2.2.l Cadmium-Containing vs. Non-Cadmium Con Products
Man-made cadmium emissions arise either from the manufacture, use and disposal of products intentionally utilising cadmium, or from the presence of cadmium as a natural but not functional impurity in non-cadmium containing products. In the former category of cadmium-containing products are included:
· Nickel-Cadmium Batteries
· Cadmium Pigmented Plastics, Ceramics, Glasses, Paints and Enamels
· Cadmium Stabilised Polyvinylchloride (PVC) Products
· Cadmium Coated Ferrous and Non-ferrous Products
· Cadmium Alloys
· Cadmium Electronic Compounds
In the latter category of non-cadmium containing products are included:
· Non-ferrous Metals and Alloys of Zinc, Lead and Copper
· Iron and Steel
· Fossil Fuels (Coal, Oil, Gas, Peat and Wood)
· Phosphate Fertilisers
2.2.2 Factors in Anthropoqenic Emissions Analyses
There are many studies which attempt to present a comprehensive overview of anthropogenic cadmium emissions to air, water and soil and their specific sources (Cook and Morrow 1995, WHO 1992, OECD 1994, Nriagu 1989, ERL 1990, Jackson and MacGillivray 1993, Jensen and Bro-Rasmussen 1992, Jones et al. 1993, Van Assche and Ciarletta 1992, Nriagu and Pacyna 1988). Examination of these analyses immediately indicates that there are three factors of primary importance in determining the levels of cadmium emissions. First, cadmium emission factors which are the amounts of cadmium emitted to the environment per unit of cadmium processed are generally much lower in the more technologically advanced regions of the world such as North America, Western Europe and Japan than in other regions (WHO 1992, Jackson and MacGillivray 1993, Nriagu and Pacyna 1988). Secondly, many countries have only partial data and often do not include significant cadmium emission sources particularly those where cadmium is not intentionally added. Third and most significantly, cadmium emissions have declined dramatically over the past thirty to forty years and are still declining today. Studies in Europe, Japan and the United States have all shown dramatic decreases in cadmium emissions in the time period from approximately 1960 to 1995 (Cook and Morrow 1995, Elgersma et al. 1992, Mukunoki and Fujimoto 1996, U.S. EPA 1996, Van Assche and Ciarlefta 1992). Some of the comprehensive analyses cited earlier present data from the late 1970s and early 1980s and thus appear not to reflect the significantly decreased cadmium emissions during recent years.
2.2.3 Point Sources vs. Diffuse Sources
Cadmium emissions may be considered as arising from either point sources such as large manufacturing or production facilities or from diffuse sources such as may occur from the use and disposal of products by many consumers over large areas. Emissions from point sources have been stringently regulated over the past twenty years, and cadmium emissions from point sources have decreased dramatically during that time period (Elgersma et al. 1992, Mukunoki and Fujimoto 1996, U.S. EPA 1996, Van Assche and Ciarletta 1992) as a result of these regulations and markedly improved emission control technology. Considerable progress is now being made in reduction of diffuse contamination from cadmium products through collection and recycling programs of cadmium-containing products (Morrow 1997, Cook and Morrow 1995, Morrow and Keating 1997, Mukunoki and Fujimoto 1996). Cadmium emissions from products where cadmium is present as an impurity have not been reduced as significantly (Cook and Morrow 1995, Elgersma et al. 1992, Van Assche and Ciarletta 1992) and appear to be the one remaining area where additional reductions might be achieved.
2.2.4 Cadmium Emissions from Municipal Solid Waste Incineration
One of the concerns expressed by some is that increasing incineration of cadmium-containing products will eventually lead to increased cadmium emissions to the environment and increased risk to human health and the environment. While there are large differences from country to country in the amounts of municipal solid waste incinerated, from 10% to 90%, it is clear that modem state-of-the-art emission control devices on these incinerators result in a capture efficiency of better than 99% compared with previous estimates as low as 50% to 75% (Chandler 1996, OECD 1996). Furthermore, voluntary and mandatory programs have been undertaken in some juridictions to reduce the input of cadmium into the municipal solid waste stream. In addition, source separation for recycling or hazardous waste landfall should also result in continually decreasing inputs of cadmium into municipal solid waste incinerators.
Finally, it must also be pointed out that there are many sources of cadmium in municipal solid waste, not just products to which cadmium has intentionally been added. Other sources include products such as iron and steel; gypsum, cement and other construction debris; non-ferrous (Zn, Pb, Cu) metal and alloy products; fossil fuels and fossil fuel residues; and natural substances such as grass, plants and foods, all of which may contain cadmium (Chandler 1996). Recent studies have determined that incineration accounts for only about 1% of the total sources of human cadmium exposure (OECD 1994, ERL 1990, Van Assche and Ciarletta 1992, Owens 1994, Van Assche 1998). The human exposure resulting from the disposal of cadmium-containing products by incineration is even less and is generally considered insignificant for all of these reasons (OECD 1994, ERL 1990, Owens 1994, Van Assche 1998).
2.2.5 Partitioning of Cadmium Emissions to Compartments
Most of the studies cited above indicate that the vast majority of cadmium emissions, approximately 80% to 90%, partition initially to soils. While some transfer does occur from soils back to the air or water compartments, the net flux into the soil is generally regarded as positive since there is deposition from both air and water onto soils. Thus, most cadmium emissions eventually return to soil. In soils, cadmium is largely bound to the non-exchangeable fraction, e.g. on clays, manganese and iron oxides. For this reason, its mobility and transfer into the animal and human food chain is limited. The remaining 10% to 20% of anthropogenic cadmium emissions partition between air and water and depend largely on the type of source. For example, cadmium electroplating results in no air emissions but, in the past, did result in some water emissions. Today, water effluent regulations ensure that even cadmium electroplating water emissions are negligible. Any cadmium waste from cadmium electroplating is now present in the electroplating sludge which may be recycled to recover valuable metals.
2.2.6 Anthropoqenic Sources of Cadmium Emissions to Air Water and Soil
Cadmium emissions to air arise, in decreasing order of importance, from the combustion of fossil fuels, iron and steel production, non-ferrous metals production and municipal solid waste combustion (Cook and Morrow 1995, ERL 1990, Jackson and MacGillivray 1993, Jones et al. 1993, Van Assche and Ciarletta 1992). Cadmium emissions to water arise, in decreasing order of importance, from phosphate fertilisers, non-ferrous metals production, and the iron and steel industry (OECD 1994, ERL 1990, Van Assche and Ciarletta 1992). Cadmium emissions to soils must be considered in three distinct categories, as inputs to agricultural soils, as emissions to non-agricultural soils and as depositions in controlled landfills. In the first case, the main inputs to agricultural soils which are of primary relevance to human exposure to cadmium arise from atmospheric deposition, sewage sludge application, and phosphate fertiliser application (Jensen and Bro-Rasmussen 1992, Van Assche and Ciarletta 1992). In the second case, inputs to non-agricultural soils arise mainly from the iron and steel industry, non-ferrous metals production, fossil fuel combustion, and cement manufacture (OECD 1994, ERL 1990, Jackson and MacGillivray 1993). In the case of cadmium present in controlled landfills, these amounts can arise from disposal of spent cadmium-containing products, non-cadmium containing products which may contain cadmium impurities, and naturally-occurring wastes such as grass, food and soil which inherently contain trace levels of cadmium (Chandler 1996). Cadmium input to agricultural soils is of far greater relevance to human health than cadmium input to non-agricultural soils, and input to controlled landfalls is of even less importance because cadmium is largely immobilised in controlled landfills. For example, numerous studies of the leachate from municipal solid waste landfalls has conclusively demonstrated that, even after long periods of time, the leachate from these landfills contains only very low cadmium levels, sufficiently low enough to meet the world's cadmium drinking water standards (Eggenberger and Waber 1998, NUS 1987).
2.3 Anthropoqenic Cadmium Emissions vs. Natural Cadmium Emissions
Earlier estimates of anthropogenic cadmium emissions vs. natural cadmium emissions had indicated approximately 8,000 to 10,000 mt per year for anthropogenic emissions compared to 800 to 1,000 mt per year for natural cadmium emissions (Nriagu 1980, Nriagu 1989, WHO 1992). Anthropogenic cadmium emissions, however, have decreased by approximately 90% or more in the past twenty to thirty years. Thus, natural cadmium emissions, expressed in terms of contained cadmium, which in the past have been stated to be only about 10% of the level of anthropogenic cadmium emissions, may, in fact be more nearly equal to man-made emissions today. This parity is becoming increasingly likely in view of the rapidly decreasing anthropogenic cadmium emissions to the environment discussed above compared to the relatively stable natural cadmium emissions.