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The UV Index: what the numbers mean

Summary

The UV Index shows how strong the sun's ultraviolet radiation is on a given day. The EPA groups it into three bands: 1-2 low, 3-7 moderate to high, and 8 and above very high to extreme. The higher the number, the more protection you need. A quick test is the shadow rule: if your shadow is shorter than you are, UV is strong.

Draft. License: CC BY 4.0 (this summary). Sources: [S1] US Environmental Protection Agency (EPA), "UV Index Scale" (public domain (US government work)).

Facts

• id: F1 claim: The US UV Index scale follows international UV Index reporting guidelines set by the World Health Organization. confidence: medium date: 2026-11-02 sources: [S1] license: CC BY 4.0 checked_on: 2026-11-02
• id: F2 claim: EPA describes a UV Index of 1-2 as low, needing little protection beyond minimal sun protection. confidence: medium date: 2026-11-02 sources: [S1] license: CC BY 4.0 checked_on: 2026-11-02
• id: F3 claim: EPA describes 3-7 as moderate to high: seek shade from late morning through mid-afternoon and use broad-spectrum SPF 15+ sunscreen, protective clothing, a wide-brimmed hat and sunglasses. confidence: medium date: 2026-11-02 sources: [S1] license: CC BY 4.0 checked_on: 2026-11-02
• id: F4 claim: EPA describes 8 and above as very high to extreme, needing extra protection, especially from late morning through mid-afternoon. confidence: medium date: 2026-11-02 sources: [S1] license: CC BY 4.0 checked_on: 2026-11-02
• id: F5 claim: Shadow rule: a shadow taller than you (early morning, late afternoon) means lower UV exposure, and a shadow shorter than you (around midday) means higher exposure. confidence: medium date: 2026-11-02 sources: [S1] license: CC BY 4.0 checked_on: 2026-11-02

Caveats

• The WHO scale splits these bands more finely (moderate 3-5, high 6-7, very high 8-10, extreme 11+); EPA's page groups 3-7 together. Check which scale a forecast uses.
• Daily forecasts depend on location, season, altitude and clouds; check the local forecast for today's value.

Pointers

• id: P1 answers: Daily UV Index forecast by ZIP code where: https://www.epa.gov/sunsafety/uv-index-1 access: free, public web page

@misc{iil_note_3vk6rrf5,
  author = {{iinventedlight Library}},
  title = {{The UV Index: what the numbers mean}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-02},
  url = {https://iinventedlight.com/library/reference/us-gov/uv-index/},
  note = {Permanent ID iil:note:3vk6rrf5. License: CC-BY-4.0. Summary of https://www.epa.gov/sunsafety/uv-index-scale-0.},
}

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