Together 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.
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.
The photoprotective action of solar products is carried out by filters, which can be chemical or inorganic. The chemical filters act by absorption of UV radiation, the electronic configuration is modified giving rise to compounds excited at a higher energy level. Inorganic filters work by absorbing and acting a physical barrier against the penetration of radiation. The last trend is to use a combination of physical and chemical filters, especially for the high protections, to obtain a broad-spectrum protection.
The prerequisite for consciously choosing a solar product is that the product information must be comprehensive and easy to understand. According to the 2009 Colipa Recommendation, sunscreen products must protect against both UVA and UVB, the minimum SPF value to be labelled is 6 and the maximum is 50+.
Absolute terms such as sunblock, total screen or 100% protection should not be used. There are 8 SPF values:
The SPF, Sunburn Protection Factor, expresses 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 from UVB rays and is shown on the label of all sun products and various cosmetic products for daily use. In 2010 the International Standard ISO 24444 method was published, approved and used practically all over the world except in the United States where it is uses the FDA method. The method consists in inducing the formation of an erythema on the back of the volunteers with a solar Xenon arc lamp. The SPF is calculated from the average of the individual SPFs of the test participants.
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.
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.
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)
The in vitro method ISO 24443:2012 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.
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