Along with the benefits of sunlight on physical and mental health, the role of ultraviolet radiation in inducing skin, ocular and systemic damage is getting clearer. This requires a prudent and controlled liaison with sun exposure.
In the past, the attention of the dermatologist and of the formulator has been focused on protection from UVB rays responsible for erythema and biological damage. In recent years the importance of adequate protection from UVA responsible for photosensitization and phototoxicity has been understood. To meet these requirements, there is an evolution of the solar product towards formulations with higher and wider protection and towards more complete and understandable consumer information.
Filters
The photoprotective action of sunscreen products is carried out by filters, which can be chemical or inorganic. Chemical filters act by absorption of UV radiation, the electronic configuration is changed, giving rise to higher-energy excited compounds. Inorganic filters act by absorption and by affixing a physical barrier to the penetration of radiation. The trend in recent years is to use a combination of physical and chemical filters, especially for higher protection, in order to achieve broad-spectrum protection.
Labelling and consumer information
The prerequisite for consciously choosing a sun product is that the product information is comprehensive and easy to understand. According to the Colipa Recommendation 2009 sunscreen products must protect against both UVA and UVB, the minimum SPF value to be stated on the label is 6 and the maximum is 50+. Absolute terms such as sunblock, total shield or 100 per cent protection should not be used. The SPF values to be stated on the label are 8:
Low protection: 6/10
Medium protection: 15/20/25
High protection: 30/50
Very high protection: 50+
In vivo SPF evaluation methods
The SPF (Sunburn Protection Factor) expresses numerically the protective capacity of a product against sunburn. Developed more than 60 years ago, the SPF has been universally adopted as an indicator of sun protection against UVB radiation and is stated on the label of all sunscreen products and various cosmetic products for daily use. In 2010, the International Standard ISO 24444: 2019/Amd.1:2022 method was published, which is accepted and used practically all over the world except in the USA where the FDA method is used. The method consists of inducing the formation of an erythema on the back of informed volunteers with a Xenon arc sun lamp. The SPF is calculated from the average of the individual SPFs of the test participants.
In-vitro SPF evaluation
The in-vitro test is not an alternative to the in-vivo test but can be used as a screening for the selection of products with good filtering capacity. The calculation of the sun protection factor was theorized and applied by Brian L. Diffey and J. Robson in 1989 and it is based on the spectrophotometric evaluation of the UV transmittance of a product layer spread on a substrate.
Water resistance evaluation
In 2020 the International Organization for Standardization published two new ISOs for the assessment of water resistance of sun protection products: ISO 16217: 2020 and ISO 18861: 2020. They describe the procedure of immersion and the calculation of the percentage of water resistance. For products declared "water resistant", the SPF is measured after 40 minutes of immersion, while to support the claim "very water resistant" the immersion time is 80 minutes. A solar product is defined as "water resistant" or "very water resistant" if the SPF value after immersion is greater than or equal to 50% of the SPF before immersion.
In-Vivo UVA evaluation
The in-vivo method currently used is ISO 24442: 2011 which consists in inducing the formation of a pigmentation on the back of at least 10 volunteers with a Xenon arc lamp with emission spectrum between 320 and 400 nm (UVA)
In-Vitro UVA evaluation
The in vitro method ISO 24443:2021 is based on the spectrophotometric evaluation of the UV transmittance of the product applied to polymethylmethacrylate plates before and after UV exposure. At the end of the test, there are obtained the UVA protection factor, the ratio between SPF and UVAPF and the critical wavelength of the tested solar product.
Evaluations of harmlessness
Open and/or occlusive patch test on sensitive and/or normal skin
Human Repeated Insult Patch Test - HRIPT
Photopatch Test
Predictive test for hypoallergenicity
Evaluation of comedogenic power
Sensory evaluations
Sensory analysis is a scientific discipline that identifies, measures and interprets the stimuli evoked by the use of a product and transmitted to the sense organs. It uses a group of panelists trained to perceive and quantify the sensory stimuli generated by the interaction between skin and product. In addition, the user's judgement is collected by filling out a questionnaire evaluating the perceived performance and pleasantness of the product.
Other tests and evaluations
In vitro friction resistance
In vitro sweat resistance
In vivo sand-repellent properties
In vitro sand-resistant properties
Photostability
Determination of sun protection factor in vivo on wet skin
