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Writer's pictureMargarida Lindo

Safety tests: phototoxicity and photosensitization


The safety of cosmetic products is extremely important, as it constitutes a fundamental requirement for protecting public health, maintaining consumer confidence, and complying with regulatory standards. When it comes to cosmetic products with possible exposure to ultraviolet (UV) radiation, it is important to assess their potential to cause phototoxicity or photosensitization.



Phototoxicity: Corresponds to increased skin reactivity to UV radiation with no immunological basis. Clinically, it manifests as initial erythema after a latency period of hours or days, pigmentation and sometimes the formation of vesicles or blisters. The intensity will depend on the amount of radiation, skin type, place of exposure and product concentration. It only manifests itself at the irradiated site, occurring on first exposure.


Photosensitization: Corresponds to increased skin reactivity to UV radiation on an immunological basis. Photosensitivity reactions develop in previously exposed individuals and are not dependent on the dose of the product. The reaction usually appears as an exaggerated sunburn or rash after sun exposure and can occur hours or even days after exposure to the sensitizing substance.


Photosensitizing and phototoxic cosmetic products contain ingredients that can react with UV radiation. These ingredients include essential oils, especially citrus oils, and alpha-hydroxy acids such as glycolic acid and lactic acid.


In order to assess the phototoxic or photosensitizing potential of cosmetic products, safety tests are carried out, the method of which differs slightly in the assessment of these 2 parameters:


Phototoxicity assessment: Volunteers are exposed to the product through a patch which is later removed. The exposed area is then irradiated with UVA. The product is applied only once. Finally, the formation of erythema and oedema is assessed in the areas where the product and control were applied, at different observation times. Phototoxic effects generally appear within 24 hours of irradiation and disappear after 72 hours.


Photosensitization assessment: This involves a modified Human Repeat Insult Patch Test (HRIPT) using UVA radiation. A sensitization induction phase is carried out during which the subjects are regularly exposed, over several weeks, to the cosmetic product under patches and subsequently to UVA radiation. After a period of rest, they are subjected to a challenge phase where they will be exposed to the product and UVA radiation again. Photosensitization is assessed collectively based on the differences in skin reaction between the control and the test product.


Safety tests must be carried out in accordance with guidelines and standards for clinical trials, such as the ICH E6 (R2) Good clinical practice - Scientific guideline, an international ethical and scientific quality standard for the designing, conducting, recording and reporting trials that involve the participation of human beings, and the Declaration of Helsinki by the World Medical Association (WMA) establishes ethical principles for medical research involving human beings.


Phototoxic reactions are more common than photosensitizing reactions. As mentioned above, unlike photosensitivity, which requires prior sensitization to the product/ingredient, phototoxic reactions can occur in anyone, regardless of prior exposure, and do not involve an immunological mechanism.


Identifying photosensitivity and phototoxic reactions allows manufacturers to reformulate products or adjust labeling to include relevant warnings so that consumers use these products correctly and at the right time in their daily care routine. This proactive approach helps to reduce the risks associated with adverse reactions and demonstrates a commitment to consumer safety.


Are you planning on carrying out phototoxicity or photosensitization tests on your cosmetic products? Contact us!

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