Corrosive agents are chemicals that cause irreversible damage and destruction of the skin, often burning through several layers of tissue. Corrosive reactions are typified by ulcers, bleeding, bloody scabs, and discoloration.
Corrosivity data are mainly collected by regulatory agencies concerned with the transportation of hazardous substances, in the event of a highway accident and chemical spill. In the U.S., the Department of Transportation requires the submission of skin corrosion data consistent with the standards of the United Nations Transport Authority. Corrosion is also an endpoint in skin irritation studies mandated by the Environmental Protection Agency in its assessments of pesticide formulations and ingredients. In this case, corrosivity represents the most extreme form of skin irritation, in which the skin is literally destroyed beyond the body’s ability to heal.
Rabbits are locked into full-body restraints and a test chemical is applied to the shaved skin on their back. The wound site is then covered with a gauze patch for the duration of the exposure period, normally four hours, after which the patch is removed and the degree of skin damage is read and scored at specified time intervals. Untreated skin areas serve as the control.A chemical is considered to be corrosive if, by the end of a 14-day observation period, the chemical has burned through the outer layer of the skin of one or more animals, leaving visibly dead tissue in its wake. No painkillers are provided.
Despite their years of use, animal-based skin corrosion studies have never been properly validated. In fact, evidence exists that animal studies are highly variable, of limited reliability, and generally poor predictors of human skin reactions.
For example, a comparison of data from rabbit tests and four-hour human skinpatch tests for 65 substances found that 45 percent of classifications of chemical irritation potential based on animal tests were incorrect. (MK Robinson et al., Food Chem Toxicol 40, 573-592, 2002)
Human skin equivalent tests such as EpiDerm™ and EpiSkin™ have been validated and accepted in Canada, the European Union, and virtually all other member countries of the Organization for Economic Cooperation and Development (OECD), as total replacements for animal-based skin corrosion studies. These methods consist of normal, human-derived skin cells, which have been cultured to form a multi-layered model of human skin.
The reliability and relevance of human skin equivalent models has been established through rigorous, inter-laboratory validation studies overseen by the European Centre for the Validation of Alternative Methods (ECVAM), and these methods have been accepted as an official OECD test guideline. However, their acceptance as stand-alone replacements in the U.S. has been undermined by several members of the Interagency Coordinating Committee on the Validation of Alternative Methods (ICCVAM), most notably the Environmental Protection Agency, the Food and Drug Administration, and the Consumer Product Safety Commission, which insists that “confirmatory” testing still be carried out using animals.
Corrositex™ is another non-animal method of assessing skin corrosion. Using a protein membrane instead of skin, Corrositex™ can measure whether, and at what rate, a chemical is capable of penetrating the simulated skin barrier according to a color-change reaction. Corrositex™ was pioneered in the U.S., assessed by ICCVAM to confirm its validity, and subsequently accepted by both the U.S. Department of Transportation and European Union as a partial replacement for animal-based skin corrosion studies.