Cadmium is a naturally occurring minor element, one of the metallic components in the earth’s crust and oceans, and present everywhere in our environment. It was first discovered in Germany in 1817 as a by-product of the zinc refining process. Its name is derived from the Latin cadmia and the Greek kadmeia.

Industrial applications for cadmium were developed in the late 19th and early 20th Century. Cadmium-sulfide based pigments were used as early as 1850 and appeared prominently in the paintings of Vincent Van Gogh in the late 1800s. Thomas A. Edison in the United States and Waldemar Junger in Sweden developed the first nickel-cadmium batteries early in the 20th Century. However, the most significant early use of cadmium was as a corrosion-protection coating on steel.

Releases of cadmium to the environment from the manufacture of cadmium products are well controlled and, today, are insignificant contributors to human exposure to cadmium. Similarly, consumer use and disposal of cadmium-containing products, such as batteries, pigments and coatings, have been shown not to contribute significantly to increased human exposure to cadmium.

Cadmium is recognized to produce toxic effects on humans. Long-term occupational exposure can cause adverse health effects on the lungs and kidneys. Under normal conditions, adverse human health effects have not been encountered from general population exposure to cadmium. Potential risks have been extensively studied and are now tightly controlled.

Cadmium is natural

Cadmium occurs naturally in the environment from the gradual process of erosion and abrasion of rocks and soils, and from singular events such as forest fires and volcanic eruptions. It is therefore naturally present everywhere in air, water, soils and foodstuffs.

Cadmium metal is produced as a by-product from the extraction, smelting and refining of the nonferrous metals zinc, lead and copper. Rather than disposing of it as a waste, engineers have been able to utilize its unique properties for many important industrial applications.

Cadmium metal exhibits excellent resistance to corrosion, particularly in alkaline and seawater environments, possesses a low melting temperature and rapid electrical exchange activity, and has both high electrical and thermal conductivity. Cadmium compounds possess outstanding resistance to high stresses and high temperatures, and deter ultraviolet light degradation of certain plastics. Some cadmium electronic compounds exhibit semi-conducting properties and are utilized in solar cells and many electronic applications. Cadmium pigments produce intense yellow, orange and red colours, and are widely used in plastics, glasses, ceramics, enamels and artists’ colours.

Because of this wide variety of unique properties, cadmium metal and cadmium compounds are used as pigments, stabilizers, coatings, specialty alloys, electronic compounds, but, most of all (more than 80% of its use), in rechargeable nickel-cadmium batteries. Trends in Western World cadmium consumption patterns from 2000 to 2005 are shown in Figure 1 below.

Cadmium levels in the environment reached a peak in the 1960's. Since then, these levels have been constantly decreasing due to improved technology for the production, use and disposal of cadmium and cadmium-containing products (Figure 2).

Figure 2
Changes in Cadmium concentrations of Greenland ice and snow from the 1800's to present (after Boutron et al. 1995) pg = 10E-12 g

Industrial emissions are now tightly controlled due to the significant improvement in pollution control technology and to strict regulation and legislation, particularly in the metals industry.

The problems of incinerating waste containing cadmium can be solved using existing best available technology to capture more than 99% of incinerator fume emissions.

With regard to end-of-life disposal of products containing cadmium, it should be emphasised that, in many of its applications, cadmium is embedded in a product matrix and hence not directly bioavailable. In the very long term, the limited traces of cadmium eventually released from waste products will transform to a stable chemical form (oxide or sulphide) and so return to the original state found in nature.

Cadmium-containing batteries, coatings, and alloys are totally recyclable; the techniques have been mastered and existing capacities are available throughout the world. The costs involved are economically acceptable, and hence viable. Increased recycling of cadmium products using modern technologies will further decrease its dispersion into the environment resulting from human activity.

The health effects of cadmium are well
known and controlled

Beginning in the 1950s, the attention of the scientific community has been focused on the potential toxicity of cadmium and on the risks presented by its accumulation in man.

In the human body, cadmium accumulates mainly in the kidneys. At high levels, it can reach a critical threshold and can lead to serious kidney failure. Recent studies (Buchet & Bernard 1998) have shown that kidney effects may be reversible at low exposures once cadmium exposure is reduced or removed.

The major route of cadmium intake for non-smokers is ingestion through food or water. This is largely due to the presence of trace amounts of cadmium in foodstuffs of natural origin or to the use of phosphate fertilizers or sludge on agricultural soils.

Investigations around the world have shown that, for the general population, the average daily cadmium intake is low compared to the World Health Organization’s (WHO) daily tolerable cadmium intake levels and has been continuously decreasing over the past 20 years. These results are summarized in Figure 3 below.

Figure 3. Daily Cadmium Intake for the General Population

The production of cadmium and cadmium products, their use and final disposal represent only a very small fraction of the total sources of all human cadmium exposure. These results are summarized in Figure 4 below and show that the production, use and disposal of products to which cadmium has deliberately been added account for less than 2% of total sources of human cadmium exposure.

Figure 4. Relative Contributions of Sources of Cadmium Human Exposure

Workers may be occupationally exposed to cadmium in industrial environments. However, occupational exposure standards for cadmium throughout the world have now been reduced to safe levels, workers’ exposure to cadmium is strictly controlled by rigorous industrial hygiene practice, and workers are continuously tested by biological monitoring to prevent any unacceptable health risk to workers occupationally exposed to cadmium. The carcinogenicity of cadmium is still an area of controversy. While some jurisdictions have classified cadmium as a known human carcinogen and others have indicated that it is a possible or probable human carcinogen, many of the earlier studies on which these classifications were based have now been contested by newer studies (Sorahan, 1995, 1997, 2000) which have indicated the importance of confounding exposures.

Cadmium and the Future

NICKEL-CADMIUM RECHARGEABLE BATTERIES ARE VITAL IN DAILY LIFE.

Nickel-Cadmium (NiCd) batteries are one of the major rechargeable battery chemistries in use today in a wide variety of consumer and industrial applications. Consumer uses include cordless power tools, cordless telephones, portable household appliances, and battery-operated toys and hobbies. Industrial uses include aircraft and railroad applications, emergency lighting, remote area power systems, and electric and hybrid electric vehicles. NiCd batteries are employed in these applications because of their long lives, durability and reliability, rapid charge and discharge characteristics, wide temperature operating range, and cost-effectiveness. It is the combination of all these properties rather than one outstanding parameter which make NiCds still so widely used in spite of the development of more advanced battery chemistries.

Nickel-Cadmium batteries are also easy to recycle, and the industry has committed itself to the collection and recycling of all NiCd batteries to minimize any risk which might arise from the disposal of the spent batteries. Studies have shown that more than 99% of the materials present in NiCd batteries may be recycled for subsequent use in the production of new batteries. Collection programs have been organized in Japan, North America and Europe to ensure that the materials in spent NiCd batteries are recycled rather than discarded into the environment. Figure 5 below shows the progress in the collection and recycling of NiCd batteries in Europe in the past ten years. Similar improvements have been established in North America and Japan as well.

Figure 5. Collection and Recycling of Spent Portable NiCd Batteries
In Europe (Metric Tonnes per Year)

PIGMENTS FOR PLASTICS, CERAMICS, GLASSES AND ENAMELS

Cadmium pigments are based on the compound, cadmium sulfide, and may also contain zinc and selenium which gives them a wide range of rich red, orange and yellow colorings. They are highly insoluble and resistant to high temperatures and stresses and are therefore used to colour plastics, glasses, enamels and ceramics in applications exposed to high temperatures or stresses during production or use. Their chemical insolubility results in negligible environmental emissions of cadmium from pigments during their production and use.

CADMIUM COATINGS FOR DEMANDING AND CRITICAL APPLICATIONS.

Cadmium coatings are sacrificial corrosion protection coatings widely used on steel and aluminum especially when exposed to alkaline and salt water environments. They also possess a very low coefficient of friction and low electrical resistivity which makes them ideal for corrosion protection coatings on threaded fasteners or electrical connectors. These are most often used in critical or safety related applications in the aerospace, electrical connector, defense, mining, nuclear, and off-shore oil and gas industries.