Cosmetics are normally used daily and over long periods. That is why good skin compatibility is particularly important – especially if they are used on sensitive skin areas. The following aspects are important from a dermatological point of view: skin irritation (local toxicity), contact allergy, photosensitization and comedogenicity. Moreover, subjective reactions such as stinging, burning or itching can occur after application of a cosmetic and make acceptance questionable (from: K. Schrader (1989) Grundlagen und Rezepturen der Kosmetika. Hüthig, Heidelberg).
The single application occlusive patch test (Epicutaneous test) is a dermatologically controlled study that tests for skin irritation (local toxicity). The test product is fixed in place on the skin under occlusive conditions with a patch, usually on the back, where it is left for up to 48 hours. After removal of the patch 24 hours or 48 hours later, a dermatologist checks for and assesses adverse skin reactions.
Redness, scaling, blisters or pustules indicate an irritancy potential of the test product. The risk of developing an allergy cannot, however, be determined with this test.
Even more sensitive than the single patch test is the repeated patch test, where the test product is applied repeatedly at defined intervals to the skin with an adhesive patch and evaluated.
Besides the occlusive application commonly used for leave on products, open application allows continuous observation of adverse skin reactions and is used mostly for rinse off products as well as for hair colors, bleaches or depilatories.
Photopatch Test / Flex Wash Test
Other models for studying the safety of cosmetics are the photopatch test and the flex or forearm wash test.
To study phototoxic reactions to a cosmetic the skin is exposed to a defined dose of UVA and UVB radiation after application of the test product. The further procedure is analogous to that of the single occlusive patch test.
The flex wash test is used primarily for surfactant products to test the skin compatibility.