Transmittance of UVB, UVA, and visible light (blue-violet) among the main Brazilian commercial opaque sunscreens

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geographic factors should be weighed in the indication of the photoprotection strategy, such as sunscreens (SSs), mechanical photoprotection (coverings, glasses, clothing), and behavioral aspects. 1 SSs use compounds that interfere with the penetration of solar radiation into the skin, minimizing its biological tissue effects. Such substances can be organic or inorganic, and pigments are used to potentiate visible light (VL) blockage. 1,2 As different types of radiation trigger characteristic pathological processes, knowledge of SS photoprotection patterns is essential for their indication. The prevention of sunburn is associated with the sun protection factor (SPF), and the persistent pigmentation prevention is associated with persistent pigment darkening (PPD) and the maintenance of immunological surveillance of the skin by the immune protection factor. 1,3   The VL spectrum (400---780 nm) is below the UVA range, and represents 40% of the incident solar energy, which can promote persistent pigmentation in higher phototypes and free radicals in the stratum corneum. However, VL promotes less tissue interaction and its effects are dozens of times less intense than those promoted by UVA and thousands of times smaller than those induced by UVB. 1,4 The most biologically active fraction of VL comprises the blue-violet range (400---500 nm), and may be relevant in pre-venting aging and dyschromia, such as melasma, as well as UVA. However, there is still no universally accepted method to evaluate the photobiological protection against VL, nor any reference to this protection in the SS. 5 Topical protection against VL is promoted by opaque SS. In Brazil, there are SSs marketed with a proposed VL block; however, there is no clear picture of the simultaneous protections against the other radiation ranges offered by these products.
A cross-sectional study was conducted to evaluate the in vitro transmittance of UVB, UVA, and blue-violet light (400---500 nm) among the major Brazilian commercial SSs. There were 41 opaque SSs with SPF > 30 evaluated between September 2017 and September 2018. The characteristics of the SS tested are provided in Table 1.
Additionally, three pigment-free SSs were evaluated as controls of the experiment ( Table 1).
The calculated transmittance was the percentage of radiation that passes through each SS, being complementary to the value of the sum of the absorbance. For its calculation, multiple measurements were taken on the surface covered with SS, and the mean value was calculated, divided by the irradiation of each source through the transparent film, without SS.
All SS tested showed UVB transmittance <0.1%. The UVA and VL transmittances are shown in Table 2. In general, opaque SSs had higher UVA coverage than the controls. It is noteworthy that, of the opaque SSs, 63% (26/41) blocked >99.9% of UVA and 63% (26/41) blocked >99.9% of blue-violet light. However, this blockade was not concurrent, since 31% (8/26) of the opaque SSs that blocked >99.9% of the VL did not have the same performance for the UVA.
Opaque SSs marketed in Brazil showed great variability in UVA and VL transmittance, despite excellent UVB performance. Interestingly, opaque SSs of the same commercial line, but of different shades, did not demonstrate differences in transmittance of VL.
The transmittance of SS is not perfectly parallel to the biological effect of radiation; however, it is a reasonable way to compare SS performance in vitro. Even the critical wavelength, an indicator of UVA protection, is based on the transmittance curve within the UVA spectrum. 1 Moreover, the biological effect of UVA and VL can be reduced by the use of adjuncts such as antioxidants, present in several SSs tested.
Our results do not deprecate the studied opaque SSs, but highlight their intrinsic differences. These results should be confirmed with in vivo methodologies.
In conclusion, SS labels should provide detailed information on SPF and PPD (or another UVA standard) to favor the indication of SS in different clinical settings. This is especially relevant in pigmented dermatoses, more sensitive to UVA, because different opaque SSs with good performance against VL allow a significant passage of UVA, albeit inferior to the transparent SSs tested.

Financial support
None declared.