Cadmium

Table of Contents

Cadmium is a heavy metal used in the manufacturing of many electronic devices. Cadmium’s use is limited by the European Union’s RoHS directive due to cadmium’s high toxicity.

Properties of Cadmium

Cadmium is a heavy metal used in electronics. Heavy metals are metals with relatively high density that are toxic at low concentrations, meaning a small amount of the metal is toxic.

cadmium restriction of hazardous substances rohs
A bar of cadmium
Source: Alchemist-Hp, Wikipedia

Cadmium is naturally occurring and can be found in mineral deposits in the earth’s crust. The chemical symbol for cadmium is Cd and the atomic weight (the total weight of the atoms making up an element) is 112.41 g/mol.

Cadmium is often produced as a byproduct of zinc mining. Around 14,000 tonnes of cadmium are produced annually, mainly by top global suppliers Canada, the U.S., Australia, Mexico, Japan, and Peru.

Characteristics that make cadmium popular in electronic manufacturing include its malleability, high conductivity (ability to conduct electricity), and its resistance to corrosion.

Cadmium in Electronics

Cadmium is used in electronics either as pure cadmium, or, more often, as a compound with another element. 

Rechargeable Batteries

About 75% of cadmium used in manufacturing is used to produce cadmium-nickel rechargeable batteries, used in electronics like cell phones. Batteries are not restricted by the RoHS directive.

Semiconductors

cadmium restriction of hazardous substances rohs
Semiconductor

Cadmium selenide, a cadmium compound, is an easily shaped metal that is most often used as semiconductors. Semiconductors can conduct electricity in high heat, but not in low heat. Cadmium is a good semiconductor because it is highly resistant to high temperatures. Semiconductors like cadmium selenide have a conductivity (ability to conduct electricity) that lies in between a conductor (like metal) and a non-conductive insulator (like ceramic). As temperature rises, the conductivity of the semiconductor increases, and the cadmium material can act as a conductor. At low temperatures, the same material blocks the flow of electricity. 

Semiconductors are found in most electronic devices today. Semiconductors have an extremely wide variety of functions, but generally they work to amplify signals in a circuit, convert energy, and pass current in a specific direction.

Plating

Cadmium’s resistance to corrosion makes it a popular metal to use as a protective shield against corrosion in electronics and other metals. Cadmium plating, also known as cad plating, offers low levels of electrical resistance, protects against rust, and can be soldered like tin to attach metals.

Toxicity of Cadmium

When products containing cadmium are not disposed of properly, particularly when they are burned, cadmium is released into the air. While the main source of airborne cadmium is the burning of fossil fuels, disposal of electronic waste can also release cadmium. 

Another route of cadmium exposure is through working in a factory that produces electronics or electronic components containing cadmium. Inhalation of cadmium is an occupational hazard.

Long term exposure to cadmium can cause serious harm to human and animal health.

Exposure to cadmium is most famous for causing a degenerative bone disease, Itai-itai disease. This disease was caused by cadmium poisoning contracted as a result of mining. Cadmium is highly water soluble, and so as mining byproducts that contained cadmium were released into a nearby river, cadmium pollution occurred. Eventually, cadmium was absorbed into crops irrigated by the river, causing widespread disease.

Cadmium can also cause lung irritation, kidney disease, and lung cancer. The U.S. Environmental Protection Agency has classified cadmium as a probable human carcinogen.

Cadmium also has negative environmental impacts. For example, in aquatic organisms, long term exposure to cadmium can cause death and harm to growth, reproduction, immune systems, and development.

RoHS: Restriction of Hazardous Substances

RoHS stands for Restriction of Hazardous Substances. The RoHS directive, issued in the European Union, restricts the use of several hazardous materials in electronic and electrical equipment (EEE). All EEE products sold in the EU must comply with RoHS.

RoHS restricts the use of ten hazardous substances in electronics, including cadmium. Electronic devices may only contain these substances in amounts lower than 1000 ppm. The allowable amount for cadmium is 100 ppm.

The materials include:

Any manufacturer, importer, or distributor of electronic and electrical equipment (EEE) sold on the EU market must be compliant with RoHS. 

EEE regulated under RoHS includes a wide variety of electronic products separated into eleven categories, ranging from large household appliances to medical devices. The eleventh category is all-encompassing, as it includes any EEE not covered in the previous ten categories. 

Read more about how to ensure compliance with RoHS: What is ‘RoHS Compliant’?

RoHS Cadmium Exemptions

While alternatives to cadmium in electronics exist and are in the process of being refined, exemptions do exist under RoHS for the use of cadmium in certain applications and electronic categories.

Visit the European Chemicals Agency website for a full list of cadmium exemptions.

RoHS and Cadmium: A Reduction of Risk

RoHS works to reduce the risk of cadmium exposure by limiting the amount of cadmium used in electronic devices. While a product that is RoHS compliant may still contain cadmium, it is present in such small quantities that it poses a lower risk if humans are exposed.

Cadmium is most toxic at high levels of exposure. Lower amounts of cadmium in electronics means there is a smaller chance of high exposure, both for users if a cadmium-containing device breaks, and also for manufacturers of cadmium-containing devices.

While cadmium may still be used in electronics, it is limited to amounts less than 100ppm. Ppm stands for “parts per million.” In other words, in an electronic device, cadmium may only make up .01% or less by weight. This is a fairly small amount of cadmium. 

Because of this limit, some manufacturers no longer use cadmium. Additionally, RoHS pushes industry to innovate substitutions for restricted hazardous materials. Many manufacturers and scientists are researching alternatives to cadmium, such as zinc alloys or gold.

Cadmium-Limiting Legislation

RoHS is the main legislation that limits cadmium in electronics. However, other regulations exist that limit the use of cadmium in consumer products. For example, the United States ASTM Standard F 963-17: Requirements Standard Consumer Safety Specification for Toy Safety requires surface coatings used in toys to contain less than 75ppm cadmium. 

In addition to its RoHS legislation, the state of California passed Proposition 65, which limits the use of cadmium in certain products, including several electronic devices.

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