The skin is the largest organ of the body and it provides an efficient barrier regulating the movement of water and electrolytes (Elias, 2006), essential for maintaining overall homeostasis. In addition, the skin barrier protects the body against potentially harmful stimuli, e.g. exposure to antigens, ultraviolet light, microorganisms, toxins, small particles. In addition to such natural and potentially noxious stimuli, the skin may be exposed to a wide range of chemicals, i.e. through occupational exposure or consumer products (e.g. solvents, detergents).
Local (=topical) exposure to chemicals can lead to adverse effects on the skin. The extent of severity and reversibility of effects distinguishes skin irritation from skin corrosion (=skin burns). Irritant substances lead to a reversible local inflammatory reaction caused by the innate (non-specific) immune system of the affected tissue, while corrosive substances irreversibly damage the skin through the epidermis and into the dermis, beyond repair. As a consequence, the affected area can be regenerated only from the healthy skin surrounding the necrotic patch.
According to the OECD Test Guideline (TG) 431 on "In vitro Skin Corrosion: Human Skin Model Test", skin corrosion is further defined as the production of irreversible damage of the skin; namely, visible necrosis through the epidermis and into the dermis, following the application of a test substance for up to four hours. Corrosive reactions are include ulcers, bleeding, bloody scabs, and, by the end of observation at 14 days [in an animal experiment], by discoloration due to blanching of the skin, complete areas of alopecia, and scars.
Corrosivity is not a risk factor that usually occurs with cosmetics, but could occasionally arise after a manufacturing error or misuse by the consumer. However, a cosmetic ingredient that has the intrinsic property to be corrosive is not necessarily excluded for use in cosmetics. It is very much depending on its final concentration in the cosmetic product, the presence of “neutralising” substances, the excipients used, the exposure route, the conditions of use, the amounts applied, etc. (Zuang et al., 2005).
Because chemicals may pose even severe risks, there is the need to have data on skin corrosivity potential in order to ensure a high level of protection to human health and occupational safety on the one hand and a free transport of chemical substances and mixtures, on the other hand with the goal of achieving sustainable development while enhancing competitiveness and innovation. These requirements are reflected in the following legislation:
Legislation requiring data on possible adverse skin effects
Data on skin irritation/corrosion effects are required by several pieces of legislation, notably
- the Classification, Labelling and Packaging (CLP) Regulation (1272/2008)
- the Cosmetics Directive (76/768/EEC, which will be repealed, from July 2013 onwards, by the EU regulation on cosmetic products (EC 1223/2009)
- the REACH Regulation (1907/2006)
Brief outline of Classification and Labelling for skin corrosion
The EU CLP Regulation implements the UN Globally Harmonized System (GHS) for classification and labelling (C&L). The C&L categories used are based on visually observable effects in rabbit skin following exposure (Draize skin corrosion test).
- Corrosive substances are labelled 'Category 1' in agreement. This category contains three further optional subcategories which correspond to the UN Packing Groups I, II and III for the transport of goods. The subcategories are implemented in the EU. They differ with regard to the exposure times required to elicit skin corrosion in the rabbit and are referred to as 1A ("strong corrosive"), 1B ("moderate corrosive") and 1C ("mild corrosives").
Regulatory acceptance of alternative methods
Currently, internationally accepted test methods for skin corrosion testing include the traditional animal test (Draize rabbit test) as well as in vitro test methods, including test methods based on reconstructed human epidermis technology (RhE) validated by ECVAM. RhE models use normal (e.g. non-transformed) human keratinocytes that, during culturing, form a multi-layered epidermis including a stratum corneum at the top, functioning as a barrier. The same RhE test methods, however using a different exposure protocol, are employed for skin corrosion testing. In addition, alternative methods for skin corrosion include the Transcutaneous Electrial Resistance (TER) assay, based on excised animal skin and the CORROSITEX assay, based on an in vitro protein matrix.
Table 1 provides an overview of the available traditional and alternative test methods in the area of skin corrosion that have gained international regulatory acceptance.
Generic description of test method
Standardised description of test method in EU legislation
OECD Test Guidelines
In vivo skin corrosion and irritation
In vivo Draize rabbit test for skin corrosion/irritation testing
Acute toxicity: dermal irritation/corrosion
In vitro skin corrosion
Transcutaneous electrical resistance test (TER)
In vitro skin corrosion: Transcutaneous electrical resistance test (TER)
RhE- Human Skin Models:
EpiskinTM, EpidermTM, SkinEthicTM
In vitro skin corrosion: human skin model test
No EU test method available
Six methods have been validated for the assessment of skin corrosion. Four of them (the EpiDerm™, EPISKIN™, SkinEthic™ and epiCS® (formerly known as EST1000 RhE test method) models) are reconstructed human epidermis (RhE) models. In their overall design (use of human derived keratinocytes, representative tissue architecture), they closely mimic the biochemical and physiological properties of the outer layers of human skin, i.e., the epidermis (the EpiDerm™ model) and the epidermis and underlying dermis (the EPISKIN™ and SkinEthic™ models). In these model systems, the impairment of cell viability, as measured by vital dyes (e.g., MTT), is used as a predictor for corrosivity. These three methods and the TER assay (based on the change of transcutaneous electrical resistance of explanted rat skin due to chemical challenge) were validated as full replacements, although the provisions in the respective OECD guideline may demand the generation of additional data to support negative predictions.
During the ECVAM Validation Study on Skin Corrosion, conducted between 1996 and 1997, the Rat Skin Transcutaneous Electrical Resistance (TER) test, the EpiSkin™ test and the CORROSITEX" test, have been assessed in a full prospective validation study, coordinated and sponsored by ECVAM.
- The Transcutaneous Electrical Resistance Test (TER) was scientifically validated to distinguish between corrosive and non-corrosive chemicals (ESAC, 1998). This assay replaces the classical animal test but still uses ex-vivo organs from sacrificed animals. It was adopted in the EU in 2000 and by the OECD in 2004 (OECD TG 430).
- The EpiSkin™ RhE test method was scientifically validated to distinguish between corrosive and non-corrosive chemicals of different physical forms, and also to distinguish between known R35 (UN packing group I, corresponding to GHS category 1A) and R34 (UN packing group II & III, corresponding to GHS categories 1B and 1C) chemicals (ESAC, 1998). It was adopted in the EU in 2000 and by the OECD in 2004 (OECD TG 431).
- The CORROSITEX™ method was scientifically validated for the identification of corrosive properties for acids, bases and their derivatives (ESAC, 2000). Although it received an ESAC recommendation, the test method was not taken up by EU legislation. It was adopted by the OECD in 2006 (OECD TG 435).
- The EpiDerm™RhE test method, a similar/me-too method, was scientifically validated to identify corrosive and non-corrosive chemicals following a formal ECVAM-coordinated catch-up validation study performed during 1999-2000 (ESAC, 2000). It was adopted in the EU in 2000 and by the OECD in 2004 (OECD TG 431).
- The SkinEthic™ RhE test method, a similar/me-too method, underwent an external validation study. The method was scientifically validated as sufficiently similar with regard to its structural and functional characteristics and its performance in reference to the Performance Standards as required by OECD TG 431 (ESAC, 2006).
- The epiCS® test method, a similar/me-too method, underwent a non-ECVAM coordinated catch-up study. The method was scientifically validated as sufficiently similar with regard to its structural and functional characteristics and its performance in reference to the Performance Standards as required by OECD TG 431 (ESAC, 2009). The epiCS® test method was formerly known under the name EST-1000 and was recently renamed.
More details on the validation studies and download of relevant study documents can be found in Table 2 below.
Table 2: ECVAM-validated in vitro test methods for skin corrosion testing, DB-ALM method protocols for the prediction of skin corrosion and ESAC statements
Test method name DB-ALM Protocols/SOP Download documents Transcutaneous Electrical Resistance Test (TER) CORROSITEX™ EpiSkin™ EpiDerm™ SkinEthic™ RHE epiCS®
epiCS® Skin Corrosion Test **
Test method name
Transcutaneous Electrical Resistance Test (TER)
* This statement was revised on 21 September 2010 in order to correct a mistake in the Specificity and Overall Accuracy values expressed in the document's Table 1 (please see note on page 5 of the statement)
** Provides the latest updates of 2 DB-ALM protocols and 2 SOPs related to: EpiSkin™, EpiDerm™ SCT, SkinEthic™ RHE and epiCS® (formerly known as EST-1000), respectively. These updates (Dec 2011, Feb 2012, Jan 2012 and Nov 2012, respectively ) include information on adapted controls for encountered MTT reducers and colour interfering chemicals.
There are currently no methods for skin corrosion under validation by ECVAM.
Having validated all current RhE-based in vitro skin corrosion (Fentem et al., 1998; Barratt et al., 1998), ECVAM is continuing to follow the development of this area with respect to new technologies, assays and emerging concepts of production of reconstructed tissues (see also: Griesinger C, Zuang V, Section "Skin Irritation and Corrosion" in Bouvier d'Yvoire et al., 2012).
ECVAM is actively contributing with its expertise to international projects aiming at the further improvement of current available alternative methods for skin corrosion and irritation. During the recent past most efforts regarding the update and further improvement of test methods for regulatory use have taken place within an OECD expert group on skin corrosion and irritation bringing together scientists from various OECD member countries, regulators, test method developers as well as validation experts (e.g. from ECVAM).
- Barratt MD, Brantom PG, Fentem JH, Gerner I, Walker AP, Worth AP (1998). The ECVAM international validation study for in vitro testes for skin corrosivity. 1. Selection and distribution of the test chemicals. Toxicol. In Vitro 12:471-482.
- Bouvier d'Yvoire M, Bremer S, Casati S, Ceridono M, Coecke S, Corvi R, Eskes C, Gribaldo L, Griesinger C, Knaut H, Linge JP, Roi A, Zuang V. (2012). ECVAM and New Technologies for Toxicity Testing. In: New Technologies for Toxicity Testing, eds Balls M, Combes RD, Bhogal N. Adv Exp Med Biol. 745:154-80. doi: 10.1007/978-1-4614-3055-1_10. Review.
- Elias PM, Feingold KR. Skin Barrier. Taylor and Francis; New York, USA: 2006.
- Fentem JH, Archer GE, Balls M, Botham PA, Curren RD, Earl LK, Esdaile DJ, Holzhütter HG, Liebsch M. (1998). The ECVAM International Validation Study on In Vitro Tests for Skin Corrosivity. 2. Results and Evaluation by the Management Team. Toxicol In Vitro. 12(4):483-524.
- OECD (2002),Test No. 404: Acute Dermal Irritation/Corrosion, OECD Guidelines for the Testing of Chemicals, Section 4, OECD Publishing, doi: 10.1787/9789264070622-en.
- OECD (2004),Test No. 430: In Vitro Skin Corrosion: Transcutaneous Electrical Resistance Test (TER), OECD Guidelines for the Testing of Chemicals, Section 4, OECD Publishing, doi: 10.1787/9789264071124-en.
- OECD (2004),Test No. 431: In Vitro Skin Corrosion: Human Skin Model Test, OECD Guidelines for the Testing of Chemicals, Section 4, OECD Publishing, doi: 10.1787/9789264071148-en.
- OECD (2006),Test No. 435: In Vitro Membrane Barrier Test Method for Skin Corrosion, OECD Guidelines for the Testing of Chemicals, Section 4, OECD Publishing, doi: 10.1787/9789264067318-en.
- Zuang V, Alonso MA, Botham PA, Eskes C, Fentem J, Liebsch M, van de Sandt JJ. (2005) 3.2. Skin irritation and corrosion. Altern Lab Anim. 33 Suppl 1:35-46.