Chromium is an odorless and tasteless metallic element with the chemical symbol Cr and an atomic weight of 24. It is found naturally in the earth's crust, where it is the twenty-second most common element and in soil, water, air, and food. In its pure metallic form, or Cr (0), it is a hard, steel-grey brittle metal that takes a high polish and resists tarnishing, which is why it is often used in plating jewelry, automobile trim, and other tarnish-resistant objects. Its name is derived from the Greek word for color, because many compounds of chromium are brightly colored. Hexavalent chromium is abbreviated as Cr (VI) or as Cr 6+. It is also sometimes referred to as chromium hexavalent ion.
Chromium can occur in liquid and gaseous forms as well as solids. It can combine with oxygen to form such compounds as trivalent chromium or Cr (III), which is considered an essential nutrient in the human body because it helps the body use protein, sugar, and fat; or hexavalent chromium or Cr (VI), a large group of compounds used in various industries but recognized as carcinogens. The term hexavalent means that chromium in this oxidation state has a valence of six. There are a number of different chemicals that are classified as hexavalent chromium; they include chromium trioxide (CrO3), potassium dichromate (K2Cr2O7) and sodium dichromate (Na2Cr2O7). Other forms of hexavalent chromium commonly encountered in industry are calcium chromate, sodium chromate, potassium chromate, lead chromate, strontium chromate, and zinc chromate. In general, it is the chromate forms of Cr (VI) that are the most harmful to workers' health when inhaled or touched.
Hexavalent chromium compounds vary widely in appearance. Some, like sodium chromate and potassium chromate, are water-soluble, whereas lead chromate is not. Most hexavalent chromium compounds are dark red, orange, or lemon yellow at room temperature and are crystalline, powdery, or grainy. In addition to these compounds, hexavalent chromium can be formed and given off when chromium in a nonhexavalent form is being welded, melted, cast, or otherwise handled during hot work.
Chromium in its elemental Cr (0) form was not isolated until the late eighteenth century. Various minerals containing chromium were discovered in Russia in the 1760s and used as pigments to create bright red and yellow paints. It was not until 1798 that French chemist Louis Nicolas Vauquelin (1763–1829) discovered that he could isolate metallic chromium by heating chromium trioxide in a charcoal oven. Vauquelin also found that chromium is the trace element responsible for the red color of precious rubies.
Hexavalent chromium was not used in significant quantities until the twentieth century, as it occurs in nature only rarely and usually requires some type of industrial process for its production. Virtually all chromium ores, which contain chromium (III), are processed by the use of hexavalent chromium.
Most people worldwide are exposed to chromium primarily in its trivalent form, CR (III). This is the most stable form of chromium and is found in soil, in foods and wine, and in the human body where it serves as a trace element that enhances the action of insulin in the metabolism and storage of fats, protein, and carbohydrates. Chromium (III) deficiency is rare in the general population but is known to occur in patients who require intravenous feeding for long periods of time.
Several million adults worldwide are exposed to hexavalent chromium in their workplaces through inhalation, ingestion, or direct skin contact with Cr (VI) compounds. According to the U.S. Occupational Safety and Health Administration (OSHA) and the International Agency for Research on Cancer (IARC), there are about 560,000 workers in the United States with occupational exposure to Cr (VI); 785,000 workers in the European Union; and 85,000 workers in Canada. Figures for China and other Asian countries were not available as of 2018.
In the United States, workers most likely to be exposed are those doing hot work involving stainless steel; those working with spray paints, textile pigments, and chromate coatings; and those operating chrome plating equipment. In Canada, welders are the largest single occupational group exposed to hexavalent chromium—19,100 men and 800 women. In 2017, the U.S. Army undertook a study of military personnel exposed to hexavalent chromium in the course of duty; 34 Army installations reported the presence of Cr (VI) in their facilities, with the highest levels associated with aircraft maintenance and repair; metal treatment and metal machining; and painting and coating procedures.
The compounds classified as hexavalent chromium are widely used in industry as corrosion inhibitors, in pigment and paint manufacturing, in metal finishing and chrome plating, in stainless steel production, in leather tanning, and in wood preservatives. According to the Agency for Toxic Substances and Disease Registry (ATSDR), about half of all chromium compounds used in U.S. chemical industry are used to produce wood preservatives; the rest are used in leather tanning (13%), metals finishing (13%), pigments (12%), refractories (linings for ceramic kilns and other high-temperature industrial furnaces) (3%), and miscellaneous uses (7%). According to the National Library of Medicine TOXNET, hexavalent chromium compounds are also used in making textile dyes, printing inks, drilling muds, fireworks and safety matches, and in water treatment and chemical synthesis.
Another common source of exposure to Cr (VI) compounds is contact with Portland cement contaminated by hexavalent chromium. Portland cement is a basic ingredient used in construction since the 1840s to make concrete, stucco, and mortar; it is made from limestone and clay heated in a kiln then ground and mixed with gypsum. Portland cement is caustic, however, and often contains small amounts of hexavalent chromium as a result of modern production methods.
Hexavalent chromium can enter the human or natural environment in several ways:
As far as was known in 2018, fish do not accumulate hexavalent chromium in their bodies from living in contaminated water, and Cr (VI) was not known to enter the human food chain.
Symptoms of exposure to hexavalent chromium vary widely according to the specific compound involved, the path of exposure, and the duration of exposure. In general, workplace exposure to Cr (VI) results in more severe symptoms than exposure through outdoor air or drinking water:
In regard to cancer specifically, hexavalent chromium is thought to cause cancer in humans by damaging the genetic material within a cell, leading to chromosome breakage, direct damage to DNA, and subsequent mutations. The water-soluble forms of Cr (VI) are particularly likely to cause cancer in humans.
Diagnosis of the effects of exposure to hexavalent chromium in adults is almost always based on a history of occupational exposure; a history of living in close proximity to a plant that produces or uses Cr (VI); or a known industrial accident. The most significant accident involving the release of Cr (VI) into the atmosphere occurred in Australia in 2011 when an ammonium nitrate plant in New South Wales leaked condensate containing Cr (VI) through a vent stack during the start-up procedure after a shutdown for maintenance and overhaul. A major accident involving the discharge of wastewater containing hexavalent chromium occurred in Indiana in April 2017 when the wastewater pipes of a U.S. Steel plant ruptured, discharging the contaminated water into a tributary of Lake Michigan. The steel company paid a heavy fine and reimbursement costs to the U.S. Justice Department in April 2018.
Although the amount of chromium (III) in a person's body can be measured in samples of blood, hair, and urine, this form of chromium is normally found in body tissues. Elevated levels of Cr (III) may indicate that a person has been exposed to hexavalent chromium, but such measurements could not be used as of 2018 to predict the type or severity of health effects that might result from such exposure.
The National Institute for Occupational Safety and Health (NIOSH) recommends the following steps to lower the risk of exposure to hexavalent chromium:
Remediation is the term used to describe the removal of pollutants or contaminants from the soil, air, groundwater, or surface water. Improved control of industrial emissions has significantly reduced the levels of workplace airborne Cr (VI) from what they were in the 1980s.
New techniques to remediate hexavalent chromium were an active area of research as of 2018. One substance under investigation was biochar, or charcoal used as soil conditioner, which appears to be effective in removing Cr (VI) from water as well as soil. Other remediation methods reported to be successful are the use of recycled glass and aluminum from municipal landfills and the use of inorganicorganic clays (IOCs).
Treatment of exposure to hexavalent chromium varies according to the route of exposure, the length of exposure, and the patient's specific symptoms. In some workplace situations, moving the worker to another job within the plant or changing occupations altogether may be the best solution. People exposed to hexavalent chromium through drinking water may want to consider moving if environmental remediation in their town or city is not immediately feasible.
The prognosis of exposure to hexavalent chromium varies widely depending on the route of exposure, the length of exposure, and the specific organs affected. In general, patients who worked in facilities with a history of safety violations; those who worked for long periods of time; those exposed specifically to chromates; and those who worked before the passage of the Clean Water Act (1972) and the issuing of OSHA regulations regarding permissible exposure levels (PELs) have the worst prognoses.
Precautionary regulations in the United States for limiting exposure to hexavalent chromium are as follows as of 2018:
See also Agency for Toxic Substances and Disease Registry .
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International Agency for Research on Cancer, 150 Cours Albert Thomas, Lyon, France, 69372 Lyon CEDEX 08, +33 04 72 73 84 85, http://www.iarc.fr .
International Programme on Chemical Safety, Avenue Appia 20, Geneva, Switzerland, 1202 Geneva, +41 22 791211, http://www.who.int/ipcs/en .
National Institute of Environmental Health Sciences, 111 T. W. Alexander Dr., Durham, NC, 27709, (919) 541-3345, Fax: (301) 480-2978, email@example.com, https://www.niehs.nih.gov .
U.S. Environmental Protection Agency, 1200 Pennsylvania Ave., NW, Washington, DC, 20460, (202) 564-4700, https://www.epa.gov .
Rebecca J. Frey, Ph.D.