Role of Sunscreens in Photoaging
The concept of a topical photoprotective product has been around since the times of the ancient Egyptians in 4000 BC, but the first commercial sunscreens were not available until the 1920–1930s.[27,28] At that time, understanding of UV radiation was limited and focused mainly on UVB protection. With the increasing popularity of sunscreen over the years, the concept of standardization of photoprotection against UVB was introduced. SPF was recognized by the FDA in 1978 as the standard for measuring sun protection.
UV-induced erythema is mostly attributed to UVB, with a minor contribution by UVA2. The concept of SPF, an assessment using UV-induced erythema as an endpoint, as a sole measurement of sun protection persisted for many decades despite advances in the study of UVR suggesting that UVA may play a significant role in photoaging.[27,29,30] In 1992, the UVA star rating system was created by The Boots Company in the UK but was not widely implemented. Although other methods of evaluating the efficacy of UVA filters have been proposed, the FDA currently uses critical wavelength (CW) determination. With this method, sunscreen products whose 90% UV absorbance occurs at ≥ 370 nm are allowed to be labeled as "broad spectrum". In Europe, the International Organization Standardization 24443 guidelines use a minimum ratio of UVA protection factor to SPF of 1:3 for all marketed sunscreens. In a study of 20 sunscreens tested against the FDA guidelines and the ISO 24443 guidelines, 19 of 20 sunscreens met the CW requirements set by the FDA, whereas only 11 of 20 sunscreens met the ISO 24443 standard. To address this disparity, the FDA proposed a new rule on sunscreens in 2019 that specifically highlighted a requirement for a UVA1 (340–400 nm) to UVA and UVB (290–400 nm) ratio of ≥ 0.7; however, the FDA has not yet made a final decision. Clearly, there exists further need for global standardization to help protect and guide consumers.
In recent years, tinted sunscreens have become more prevalent as a means of protection against VL. Most FDA-approved compounds for UV protection do not adequately protect against VL because compounds must be opaque to filter VL. Zinc oxide and titanium dioxide can protect against VL but only when they are pigmentary grade and not micronized. Tinted sunscreens incorporate combinations of iron oxides and pigmentary titanium dioxide to offer VL protection and utilize the different colors of iron oxides and pigmentary titanium dioxide to improve color match on people of all Fitzpatrick skin types.[34,35] It should be noted that iron oxides are not considered to be UV filters so are listed under "inactive ingredients" on sunscreen product packages, whereas pigmentary-grade titanium dioxide and zinc oxide are FDA-approved inorganic filters. However, the exact efficacy of specific tinted sunscreens for VL protection has been largely unregulated as no standards or guidelines for VL protection yet exist. A method for VL protection factor has been recently suggested using in vivo assessment in melano-competent subjects.[22,36]
There is good evidence that daily photoprotection and daily sunscreen use plays an important role in the prevention of photoaging.[37,38] In a study of 46 patients randomly selected to use vehicle or sunscreens with UVA and UVB protection daily for 24 months, a significant histological difference in solar elastosis was observed in the vehicle versus treatment group. Furthermore, in a study of 12 subjects in which each subject was exposed to one minimal erythemal dose of simulated solar radiation to three areas of buttock skin (unprotected skin, vehicle, and day cream with UVA and UVB protection) and control (no exposure), the unprotected skin demonstrated significant melanization, increased stratum corneum and stratum granulosum thickness, elevated expression of tenascin, reduced type I procollagen, and slightly increased lysozyme and alpha-1 antitrypsin, which were all mitigated by the day cream–sunscreen combination. Not only have sunscreens been shown to prevent photoaging but evidence also suggests that they may play a role in the reversal of extrinsic aging. In a prospective study, 32 subjects were asked to apply daily broad-spectrum photostable sunscreen (SPF 30) for 52 weeks. At the end of the study, significant improvements in skin texture, clarity, and mottled and discrete pigmentation were observed, with 100% of subjects showing improvement in skin clarity and texture. However, further research into the molecular mechanism of sunscreen's effects on the reversal of chronologic aging must be performed.
Am J Clin Dermatol. 2021;22(6):819-828. © 2021 Adis Springer International Publishing AG