[Research] TIL salicylic acid has UV protectant properties, and one of its salt forms used to be an FDA approved sunscreen

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Consider my mind blown. I've always thought of salicylic acid (BHA) as a photosensitizer like glycolic acid and AHAs, but apparently the opposite is true.The use of salicylic acid is contraindicated prior to UVB therapy (where a patient's skin is purposefully exposed to UVB light, usually to treat psoriasis) because it blocks UVB radiation. (1, 2, 3, 4)In vivo studies also show that salicylic acid blocks UVA radiation (1, 2) suggesting it is also contraindicated before PUVA therapy (combination of UVA light and the oral drug psoralen for treatment for various skin disorders, graft-versus-host disease, and cutaneous T-cell lymphoma).I would guess it does still increase photosensitivity if the skin is overexfoliated. This does make me feel less worried about using SA-containing products in the morning under sunscreen, though!Also, regarding trolamine salicylate, a salt formed by combining salicylic acid and triethanolamine (a base which neutralizes the pH of salicylic acid):It's mainly used as an analgesic agent in Aspercreme-type muscle rubs. It is sometimes used as an additive to increase the photostability of topical medications and cosmetics, though.Trolamine salicylate was also used as a UV filter in some sunscreens, but as of 2019 the FDA explicitly does NOT recommend its use in sunscreens any longer because of safety and efficacy concerns. Basically, sunscreens have to sit on the surface of the skin in order to actually work. Trolamine salicylate, however, absorbs deeper into the skin and therefore is not as effective as other sunscreen filters. (1)Additionally, liberal reapplication of trolamine salicylate can cause "serious detrimental health effects, including bleeding, caused by the anti-coagulation effects of salicylic acid." We know this because there have been cases where people have repeatedly slathered on large amounts of analgesic creams containing trolamine salicylate prior to surgery or while taking blood thinners and had adverse effects as a result. If people were to apply a sunscreen with trolamine salicylate to their whole body every 2 hours, that could be quite risky. (1)tl;dr: Salicylic acid has UVA and UVB filtering/protectant properties shown in vivo and in vitro. In addition, its derivative trolamine salicylate was previously used in sunscreens but is no longer FDA approved due to risks outweighing the benefits.Also, if you just skimmed this post and think this means you should toss out your sunscreen for some Stridex... no. Just no.
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