Safe Sun Window: your sun timing, computed from published science — open for testing

Public-health apps tell you when to cover up. Almost nothing tells you the other half: how much sun your skin type can take today before burn risk, and what that means for the daily vitamin-D moment. The science for both halves is published — Fitzpatrick phototype scales, WHO UV guidance, peer- reviewed sun-exposure models — but it mostly lives in papers, not products.

Safe Sun Window puts it in a page. Pick your skin type, enter today’s UV index, and it computes your personal window: roughly how long you can be out before burn risk rises, and the shorter window relevant to vitamin-D synthesis — with the sources cited, so you can read exactly where the numbers come from.

What we sweated:

• It remembers you — on your device. Your skin type is saved locally, so a return visit is one UV entry away from today’s answer. No account. Our server keeps three things: a record of which days a random, first-party id came back, so we can tell whether the tool is worth returning to; one UV reading per hour per ~10 km cell, shared by everyone in that cell; and your email, if you choose to leave one.
• Estimates, honestly labelled. The app computes published models; it doesn’t diagnose skin types, doesn’t promise safety, and says so plainly. UV varies with cloud, altitude, and season — the window is a starting point, not a certificate.
• Zero friction. Free, no card, no sign-up, works on your phone.

There’s also a reference set alongside the app: a page for every UV index from 3 to 11, each with the full Fitzpatrick type I–VI table computed at that level, the conditions that actually produce that reading, the published guidance for its band, and the arithmetic laid out so you can check it. Start with how long it takes to burn at UV 8 if you want to see the shape of it.

Automatic local UV is live: allow location when the check opens and it fills today’s index in for you, so the answer is zero-typing. Your device rounds your position to about 10 km before it leaves your device, and the manual field stays if you decline or the lookup fails.

It’s open for testing now and feedback shapes what we build: try it at safesunwindow.acjlabs.com and tell us what worked and what didn’t at contact@acjlabs.com or @acjlabs.com on Bluesky.