Chromium is a lustrous, brittle, hard metal. Its colour is silver-grey and it can be highly polished. It does not tarnish in air, when heated it burns and forms the green chromic oxide. Chromium is unstable in oxygen, it immediately produces a thin oxide layer that is impermeable to oxygen and protects the metal below.
The most common forms of chromium in the environment are trivalent chromium (chromium-3), hexavalent chromium (chromium-6) and the metal form of chromium (chromium-0).
Hexavalent chromium occurs naturally but it is usually produced by industrial process.
It can easily gain electrons from other elements, meaning that it can easily react with them. This ability to react can produce hard coatings.
However, its ease of reaction with other elements is the main reason why hexavalent chromium is considered a major health hazard.
What is it used for?
Its properties include corrosion-resistance, durability and hardness. It is used in chrome plating and as an alloy in the production of stainless steel, as well as in anti-corrosion and conversion coatings.
Hexavalent chromium is used to produce CCA (chromated copper arsenate) that is applied as a preservative in the treatment of structural timber.
The use of chromium in welding produces hexavalent chromium as a by-product.
Hexavalent chromium in the environment
Hexavalent chromium finds its way into the environment naturally or through manufacturing activities. Most industrial output is to water but coal burning also increases air concentration.
Most of the chromium in air will eventually settle and end up in waters or soils. Chromium in soils strongly attaches to soil particles and as a result it will not move towards groundwater. In water chromium will absorb on sediment and become immobile. Only a small part of the chromium that ends up in water will eventually dissolve.
Hexavalent chromium is a danger to human health, mainly for people who work in the steel and textile industry. People who smoke tobacco also have a higher chance of exposure to chromium.
The level of chromium in air and water is generally low. In drinking water the level of chromium is usually low as well. Levels of chromium in these conditions are usually well-below those that would be considered dangerous.
Effects on health
Hexavalent chromium is classified as a human carcinogen based on excess lung cancer found in heavily exposed workers through inhalation in chrome plating and chromate pigment production.
Other adverse health effects associated with chromium-6 exposure, according to the US National Institute for Occupational Safety and Health (NIOSH), include skin irritation or ulceration, allergic contact dermatitis, occupational asthma, nasal irritation and ulceration, perforated nasal septa, rhinitis, nosebleed, respiratory irritation, nasal cancer, sinus cancer, eye irritation and damage, perforated eardrums, kidney damage, liver damage, pulmonary congestion and edema, epigastric pain, and erosion and discoloration of one's teeth.
The US Environmental Protection Agency (EPA) applies an emissions limit on chrome 6 from hard chrome plating facilities.