How long can your skin take the sun today?

A 10-second check — your personal safe-exposure window and vitamin-D window, from your skin type and today's UV index. Conservative and science-based. No account, no tracking.

How this works

No camera, no AI guess — just two facts (your skin type and today's UV index) run through published photobiology reference values. One UV Index point equals 25 mW/m² of erythemal (sunburn-causing) irradiance, the World Health Organization's own definition. From that, standard minimal-erythema-dose reference tables for each Fitzpatrick skin type give a conservative time-to-burn — and, above UV Index 3 (below that, vitamin-D-effective UVB is negligible), a rough vitamin-D synthesis window from published models (Webb; Engelsen's FastRT).

What's a Fitzpatrick skin type?

A six-point scale (Fitzpatrick, 1988) describing how skin typically responds to strong sun — from always burns, never tans (Type I) to never burns (Type VI). It's the standard reference dermatology and photobiology have used for decades. Self-rating it is approximate, which is exactly why the window we show is conservative, not a precise countdown.

Where does the UV Index come from?

You enter today's local UV Index — the same number your weather app already shows, defined by the WHO as the ground-level intensity of sunburn-causing UV radiation. We don't fetch it ourselves in this version; you paste in the number you already have, so the math has nothing to get wrong about your location.

Is this medical advice?
No. It's general sun-safety and vitamin-D-education guidance based on public photobiology reference values, not a diagnosis, a prescription, or a substitute for a doctor or dermatologist. If you have a skin condition, sun sensitivity, or are on a photosensitizing medication, this tool isn't calibrated for you.
How accurate is the window?
Minimal-erythema-dose reference values vary roughly ±30-50% within a single Fitzpatrick type, and self-rating your own type is imprecise — so the window is intentionally conservative, never a false-precision countdown. Treat it as a sensible starting point, not a stopwatch.
Why not just check the UV Index directly?
The UV Index alone is the same number for everyone in a location — it doesn't account for how your specific skin type responds. This turns that one shared number into a personal estimate.