The transition metal chromium is commonly found in either its +3 [Cr (III)] or +6 [Cr(VI)] oxidation state. Cr(VI) compounds are strong oxidizing agents, forming the Cr(III) species in most oxidation-reduction reactions.

Chromium compounds affect humans in many ways. There is increasing evidence that Cr³⁺ is an essential trace element in animals. An insufficient level of chromium in the diet results in impaired glucose and lipid metabolism. Some research scientists have suggested that a long-term dietary insufficiency may lead to maturity-onset diabetes and/or cardiovascular diseases.

The first evidence of an essential role for chromium in humans was demonstrated in hospital patients who were receiving all nutrients via a feeding tube. Such persons can become insensitive to insulin and unable to metabolize glucose. However, both of these conditions are almost immediately restored to normal when up to 5 µmol/day of chromium(III) chloride are administered intravenously. The Food and Nutrition Board of the U.S. National Research Council, the World Health Organization, and the International Program on Chemical Safety have reported that the normal dietary intake of Cr(III) should be from 1 to 4 µmol/day. As with many micronutrients, a trace amount is necessary, but too much can lead to liver, kidney, and other health problems.

Aqueous solutions of chromium compounds in which the oxidation number of the metal is +3 [in Cr(NO₃)₃ (violet) and CrCl₃ (green)] and +6 [in K₂CrO₄ (yellow) and K₂Cr₂O₇ (orange)]. (C. D. Winters)

In recent years, uses of chromium in industry have been significantly reduced as the toxic effects became more apparent. Chromium(III) nitrate is used, for example, in the preparation of catalysts and in textile printing operations. However, it is known to affect humans when inhaled into the lungs. It creates skin and eye irritation and may cause mutations that can lead to birth defects. Skin contact can cause an allergic skin rash similar to the rash many people develop from wearing nickel-plated jewelry. The OSHA legal airborne permissible exposure limit is 0.5 mg/m³, averaged over an 8-hour work shift.

Chromium(VI) oxide, used in chrome plating and dye manufacturing, is a carcinogen in humans. Chromium(VI) is considered a carcinogen because it does enter cells where it converts to Cr(III) through a series of steps. Cr(III) can not easily leave the cells. Researchers believe that this conversion damages the user’s DNA, resulting in tumor formation (cancer). Further study is needed to determine the validity of this hypothesis.

A solid sample of Cr₂O₃ is green. Although historically chemists have used this compound extensively, knowledge of its toxicity has curtailed its use in recent years. (Photo by Charles D. Winters)

In addition to affecting human health in adverse ways, chromium compounds can affect plant and animal life. Consequently, EPA has designated chromium wastes as hazardous wastes. Wastes that contain more than 5 mg of total chromium per kilogram are classified as hazardous wastes and must be sent to hazardous waste treatment, storage, and disposal (TSD) facilities.

As with other hazardous wastes, chromium solutions should not be placed directly into landfills, since all free liquids have the potential to migrate through the soil and into the groundwater. Current treatment includes binding the chromium into cement and then disposing of it in a secure hazardous waste landfill. Because of this requirement, chromium plating facilities are continually investigating ways to remove the chromium from their wastewater and, whenever possible, to effectively reuse the chromium.

Reger/Goode/Mercer:  Chemistry: Principles and Practice,  2/e,  pp. 112–113