The WHO UV index is a 0–11+ scale that condenses the burning intensity of solar ultraviolet radiation reaching the surface. It accounts for sun angle, altitude, cloud cover, and ozone layer thickness. Below 2 is "low" (minimal protection needed); 3–5 "moderate" (sunglasses + SPF 30+ if outdoors for extended periods); 6–7 "high" (cover, shade, hat, SPF 30+); 8+ "very high to extreme" (avoid midday sun, all protections).
Approximate burn times for unprotected fair (Fitzpatrick II) skin: UVI 2 ~60 minutes; UVI 5 ~25 minutes; UVI 8 ~15 minutes; UVI 11 ~10 minutes. Darker skin tones take longer to burn but still accumulate UV damage at the same per-photon rate: the long-term DNA damage that drives skin-cancer risk is largely independent of immediate redness.
Terrestream is an indoor sensor; the OPT3001 lux reading captures the visible-light dimension of daytime exposure but not the UV component (modern windows block most UV-B). The dashboard pulls UV-index forecast from Open-Meteo and surfaces it alongside lux for a combined "outdoor exposure" picture: bright midday lux indoors usually means you should also be aware of outdoor UV intensity.
The dashboard's "Outdoor activity now?" widget combines current UV index with PM2.5, ozone, pollen, and apparent temperature into a single go/no-go heuristic for sensitive household members. The full EPA UV index reference covers daily-life protection guidance.
References
- WHO - UV index Q&A www.who.int
- EPA - UV index scale www.epa.gov
- Open-Meteo - Forecast API documentation open-meteo.com
- NOAA CPC - UV index introduction www.epa.gov
