Glossary

UV Index

Written by Reviewed by the SYNC editorial team Published Last reviewed Editorial standards

UV Index — a standardized scale that forecasts the strength of ultraviolet radiation from the sun at a given place and time, typically running from 0 upward. It helps people gauge daily sun exposure risk and plan protective measures, with higher numbers signaling a greater potential for skin damage.

Key points

  • The UV Index is an international scale, developed with guidance from the World Health Organization, that estimates UV radiation strength.
  • Values are generally grouped as low (0-2), moderate (3-5), high (6-7), very high (8-10), and extreme (11 and above).
  • UV Index levels are influenced by factors such as time of day, season, latitude, altitude, cloud cover, and reflective surfaces.
  • Sun protection like sunscreen, shade, and protective clothing is typically recommended when the index reaches 3 or higher.

Frequently asked

At what UV Index should I wear sunscreen?

Sun protection is commonly advised when the UV Index is 3 or above, though many people apply daily sunscreen regardless because UV exposure accumulates over time.

Can the UV Index be high on a cloudy day?

Yes, clouds do not block all UV radiation, so meaningful exposure can still occur on overcast days, especially when the forecast index is high.

Related topics

This is a foundational entry in the SYNC Skin Encyclopedia and is expanded over time. Educational information only — not medical advice.

Encyclopedia

Further reading

This entry was written and checked against the sources below. They are published by clinical and scientific bodies, they are listed most readable first, and each one opens in a new tab. They are background for the whole entry rather than footnotes to individual sentences.

  1. Ultraviolet radiation and human health DermNet NZ Clinical guidance dermnetnz.org
  2. Ultraviolet (UV) Radiation U.S. FDA Regulatory and safety fda.gov
  3. Comparison of the Biological Impact of UVA and UVB upon ... PubMed Central Peer-reviewed, open access pmc.ncbi.nlm.nih.gov
  4. The impact of ultraviolet radiation on skin photoaging — review of in vitro studies PubMed Central Peer-reviewed, open access pmc.ncbi.nlm.nih.gov
  5. UV Radiation and the Skin PubMed Central Peer-reviewed, open access pmc.ncbi.nlm.nih.gov
  6. Ultraviolet Radiation Biological and Medical Implications PubMed Central Peer-reviewed, open access pmc.ncbi.nlm.nih.gov

SYNC does not publish medical advice. Nothing here replaces a consultation with a doctor or a pharmacist about your own skin.