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). For UV, the web dashboard's outdoor section shows the UV index from Open-Meteo, and the mobile apps offer a UV map layer; a bright room tells you little about the UV outside, so check the index before heading out.
For sensitive household members, weigh the UV index together with outdoor air quality, pollen, and heat before planning time outside; Terrestream shows these outdoor readings separately rather than rolling them into a single verdict. 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
