# D Fetcher

> Vitamin D synthesis calculator. Given location, time, Fitzpatrick skin type, exposed body area, and age, returns how many minutes of sun exposure are needed to hit a target dose, alongside the sunburn-threshold time (MED). Live UV index from a TEMIS-class satellite forecast, with a clear-sky fallback computed from solar geometry.

The site is a single-page calculator at the root, with supporting pages for the FAQ, the JSON API, the MCP server, and the changelog. Everything is publicly accessible; no signup, no key.

## Key endpoints

- [Calculator](https://dfetcher.com/): the interactive UI.
- [JSON API](https://dfetcher.com/api): `GET /api/calculate?lat=…&lon=…&skin=…&target=…` returns the full computation (sun position, UVI, IU/min, minutes to target, MED, peak window, day + year curves). CORS-open, 60s edge cache.
- [MCP server](https://dfetcher.com/mcp): Model Context Protocol over Streamable HTTP. Three tools: `calculate`, `geocode`, `describe_skin_types`. Listed in the official MCP Registry as `com.dfetcher/vitamin-d`.
- [FAQ](https://dfetcher.com/faq): how the model works, accuracy, limitations, cloud handling, what counts as medical advice.
- [Changelog](https://dfetcher.com/changelog): reverse-chronological notes.

## Model in one paragraph

Solar position is computed in-browser via a simplified NOAA Solar Position Algorithm. Live UV index is pulled from currentuvindex.com (TEMIS-class satellite forecast driven by ozone column and cloud cover); when the live feed is unreachable, the calculator falls back to a clear-sky estimate `12.5 · sin(elev)^1.4`. Vitamin D synthesis is anchored on Holick's rule reference times (Mediterranean basin, 1000 IU at 25% body, UVI 7) scaled linearly by UVI and exposed body fraction, with a quadratic elevation gate that zeroes synthesis below 20° solar elevation and ramps to full at 45°, and a linear age factor (1.0 at age 20 → 0.4 at 70+). Sunburn risk is `SKIN_MED_AT_UVI1 / UVI`. The 12-month chart applies the Foster–Lopez UVB cloud modification factor over a three-year monthly cloud-cover climatology from Open-Meteo's historical archive.

## What it is not

Not medical advice. Order-of-magnitude under best conditions, with self-reported skin type the single largest error source. Does not consider serum vitamin D levels, supplements, medications, glass attenuation, sunscreen, or shade.

## Privacy

No data leaves the user's browser to D Fetcher's servers. Form state is persisted only in `localStorage` under `dfetcher.settings.v1`. Geocoding, UV index, and cloud climatology fetches go from the browser directly to the upstream APIs.