It likely comes as no surprise that many common household chemicals and medical products as well as industrial and agricultural chemicals, may irritate human skin temporarily or, worse, cause permanent, corrosive burns. In order to prevent undue harm regulators in the U.S. and beyond require safety testing of many substances to identify their potential hazards and to ensure that the appropriate warning label appears on a product. Traditionally, such skin tests have been done on live animals—although in recent decades efforts to develop humane approaches, along with ones that are more relevant to people have resulted in new models based on laboratory-grown human skin.
The most recent chapter of this ongoing effort was written on July 22 when the Organization for Economic Cooperation and Development (OECD)—an international group that, among other things, provides guidelines to its 32-member countries on methods to assess chemical safety—officially approved three commercially available in vitro models of human skin for use in chemical testing. Specifically, the new guideline (OECD Test No. 439) stipulates that the models can serve as an alternative to animals in tests for skin irritation, one of several human health endpoints for which chemicals are tested. Similar 3-D models were approved for corrosion tests in 2004, leaving many hopeful that soon it may be possible to the assess the full spectrum of a chemical's effects on human skin—from irritation to corrosion—without using live animals.
Recent legislation in the European Union (E.U.) has made the need for nonanimal test methods an urgent matter. Since 2009 the E.U.'s Cosmetics Directive has banned animal testing of cosmetics ingredients and finished products (with a few exceptions for certain health endpoints). What's more, the ban applies to cosmetics products marketed in the E.U. As a result, manufacturers in countries outside the union must comply with the regulation to sell their products in E.U. markets. Importantly, however, E.U. law prohibits experiments on animals if alternative methods are "reasonably and practicably available."
At the same time, manufacturers are slated to use millions of animals in the coming decade to comply with a 2007 European Community program called Registration, Evaluation, Authorization and Restriction of Chemicals (REACH), which has requested more extensive safety evaluations for approximately 30,000 chemicals.
Although the new in vitro skin models will certainly reduce the need for, and suffering of, animals in some chemical tests, these methods are not yet ready to completely replace animal-based skin tests. To understand why, it is necessary to consider how toxicity testing is conducted and some of the regulatory constraints in place.
Since the mid-1940s researchers have tested the skin-irritancy potential of chemicals primarily on albino rabbits. In a procedure called the Draize rabbit skin test, a patch of the animal's fur is shaved and the test substance is applied to the bare skin for up to four hours. A trained technician then monitors the skin for as many as 14 days for signs of an adverse reaction and subjectively scores the severity of the reaction. The Globally Harmonized System of Classification and Labeling of Chemicals (GHS) classifies a substance as an irritant if it causes reversible damage to the skin or a corrosive if it causes burns or permanent scarring (irreversible damage).