Chromium (Cr) exists in several forms, which are described by different numbers in parentheses. The most common forms are elemental chromium (0), chromium (III), and chromium (VI). Chromium (III) occurs naturally in the environment and is an essential nutrient for the human body. Chromium (0) and chromium (VI) are generally produced by industrial processes.
Chromium VI, also called Hexavalent Chromium, is recognized as a human carcinogen by the National Toxicology Program; The International Agency for Research on Cancer (IARC) has determined that chromium VI is carcinogenic to humans. Chromium compounds are linked to lung cancer. Chromate-dyed textiles and chromate-tanned leather can cause or exacerbate contact dermatitis.
Chromium VI is used in textile manufacturing as a catalyst in the dyeing process and as a dye for wool (chrome dyes)(1). But you may know much more about it through its use in tanning leather.
Before the advent of synthetic dyes, all dyes came from natural sources such as minerals and plants. Often these dyes faded quickly if the dyed material was laundered. To fix or stabilize the color, chemical agents called mordants were used. Chemically, the mordant binds with the dye and the fibers of the material, preventing bleeding and fading. As early as 1820 the cotton and wool industries were using large amounts of chromium compounds (such as potassium bichromate) in the dyeing process. Red and green pigments developed from chromium compounds were also used for printing wallpaper during this period.
In 1822, a man named Andreas Kurtz moved to England and began producing potassium bichromate and selling it to the English textile industry at 5 shillings a pound. Competition soon drove the price down to 8 pence, about an eighth of the original price. This did not give Kurtz a satisfactory profit, so he began producing other chrome compounds, specifically chrome pigments. His chrome yellow achieved cult status when Princess Charlotte, daughter of George IV, used it to paint her carriage. This was perhaps the origin of the “yellow cab,” an idea exemplified today in New York City taxis. Kurtz left his mark on the world of color; “Kurtz yellow” is still available in British color catalogues.
In the film Erin Brockovich (2001, Universal Studios) Pacific Gas and Electric is portrayed as a corporate giant that poisoned the water of the small town of Hinkley, California. The movie, which is based on a real lawsuit, suggests that high levels of chromium-6 in the groundwater were responsible for an eclectic range of diseases among residents there, including various cancers, miscarriages, Hodgkin’s disease and nosebleeds. In 2010, the Enironmental Working Group studied the drinking water in 35 American cities. The study was the first nationwide analysis measuring the presence of the chemical in U.S. water systems. The study found measurable hexavalent chromium in the tap water of 31 of the cities sampled, with Norman, Oklahoma, at the top of list; 25 cities had levels that exceeded California’s proposed limit of Chromium VI and it’s less toxic forms.
It is leather tanning for which chromium is perhaps best known, because the animal skins are first given a chrome bath to prevent decomposition. This step is the most environmentally harmful of the entire tanning process, generating 90% of the water pollution associated with tanning leather (3). And that’s saying a lot, because tanning is an environmental nightmare: skins are transferred from vat to vat, soaked and treated and dyed. Chemicals used include alcohol, coal tar , sodium sulfate, sulfuric acid, chlorinated phenols (e.g. 3,5-dichlorophenol), chromium (trivalent and hexavalent), azo dyes, cadmium, cobalt, copper, antimony, cyanide, barium, lead, selenium, mercury, zinc, polychlorinated biphenyels (PCBs), nickel, formaldehyde and pesticide residues. At the same time, toxic gases like ammonia, hydrogen sulfide, and carcinogenic arylamines are emitted into the air(4). The smell of a tannery is the most horrifyingly putrid smell on earth.
According to the results of a three year study to address health impacts of pollution from the Blacksmith Institute, which works to solve pollution problems in the developing world, the tanning of leather is in the top 10 of the world’s worst pollution threats, at #5, directly affecting more than 1.8 million people (5).
(1) Duffield, P.A., et al, “Wool dyeing with Environmentally Acceptable Levels of Chrome in Effluent”, IWS Development Centre, West Yorkshire, England
(2) “EPA’s recommendations for enhanced monitoring for Hexavalent Chromium (Chromium-6) in Drinking Water: http://water.epa.gov/drink/info/chromium/guidance.cfm
(3) Blackman, Allen, “Adoption of Clean Leather-Tanning Technologies in Mexico”, discussion paper, Resources for the Future, August 2005
(4) Barton, Cat, “Workers pay high price at Bangladesh tanneries”, AFP, Feb. 2011