Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:5:p1
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 5 (pt 1/9)
Character Range: 2144855–2147583

5                   Chromium VI

5.1              General
Several comprehensive reviews of chromium VI (Cr VI) in the environment and its toxicity to humans are available and should be consulted for more detailed information not presented in this summary (ATSDR 1997 and 2008; DEFRA and EA 2002; APVMA 2005). The following provides a summary of the key aspects of Cr VI that are relevant to the derivation of a soil HIL.

Cr VI is less stable than the commonly occurring trivalent chromium but can be found naturally in the rare mineral crocoite. Cr VI typically exists as strongly oxidising species such as CrO3 and CrO42-. Some Cr VI compounds, such as chromic acid and the ammonium and alkali metal salts (e.g. sodium and potassium) of chromic acid are readily soluble in water. The Cr VI compounds are reduced to the trivalent form in the presence of oxidisable organic matter. However, in natural waters where there is a low concentration of reducing materials, Cr VI compounds are more stable (ATSDR 2008).

Chromium is of fundamental use in a wide range of industries, including the metallurgical (to produce stainless steels, alloy cast irons and nonferrous alloys), refractory (to produce linings used for high-temperature industrial furnaces) and chemical industries. In the chemical industry, Cr VI is used in pigments, metal finishing and in wood preservatives (ATSDR 2008).

The soil chemistry and toxicity of chromium is complex and hence the form of chromium in soil is of importance. In general, soil chromium is present as Cr III but the distribution of Cr III and Cr VI depends of factors such as Redox potential, pH, presence of oxidising or reducing compounds and formation of Cr complexes and salts.

Cr VI can readily pass through cell membranes and be absorbed by the body. Inside the body, Cr VI is rapidly reduced to Cr III. This reduction reaction can act as a detoxification process when it occurs at a distance from the target site for toxic or genotoxic effect. Similarly, if Cr VI is reduced to Cr III extracellularly, this form of the metal is not readily transported into cells and so toxicity is not observed (ATSDR 2008). However, if Cr VI is transported into cells, and close to the target site for toxic effect, under physiological conditions it can be reduced. This reduction reaction produces reactive intermediates, which can attack DNA, proteins, and membrane lipids, thereby disrupting cellular integrity and functions (ATSDR 2008).

5.2              Previous HIL
The derivation of the previous HIL (HIL A = 100 mg/kg) for Cr VI is presented by Soong & Emmett (1993). In summary, the HIL was derived on the basis of the following:
    * Intakes of Cr VI from other sources were estimated