UV Index

The short answer

The UV Index tells you how strong sunburn-weighted ultraviolet radiation is expected to be at the Earth's surface. A higher number means more intense UV and less time before unprotected skin may be harmed.

The World Health Organization recommends sun protection when the UV Index reaches 3 or higher^[1]. That means shade, clothing, a hat, sunglasses, and sunscreen according to the exposure - not a reason to stay anxious indoors all summer.

What the numbers mean

• 1-2: low
• 3-5: moderate
• 6-7: high
• 8-10: very high
• 11+: extreme

The scale is open-ended, so 11 is not a maximum. WHO compares the Index to a temperature scale: it gives the public a simple indication of UV level and potential harm^[2].

At higher readings, protection becomes more urgent and unprotected skin can burn faster. Skin tone changes the time to visible redness, but it does not make anyone immune to UV damage.

How it is calculated

Forecast systems estimate ultraviolet radiation reaching the surface, then weight each wavelength by how strongly it contributes to erythema, or sunburn. NOAA's calculation also accounts for factors including ozone, clouds, elevation, aerosols, and reflected radiation from the surface^[3].

That weighting is useful, but it means the UV Index is not a pure UVA meter. People focused on pigmentation should still choose broad-spectrum protection and pay attention to a credible UVA claim.

How to use it

Check the local UV Index before planning a long outdoor period. At 3 or above, use protection. As the number rises, shorten direct exposure, seek shade, cover more skin, and be more disciplined about applying enough sunscreen.

For sustained outdoor exposure, reapply sunscreen at least every two hours and after swimming, sweating, or towelling. Use the Index to plan the day; use the sunscreen label to use the product correctly.

Common misunderstandings

It is not the temperature. A cool spring day can carry meaningful UV, while heat alone does not tell you the ultraviolet level.

Clouds do not make it zero. Cloud cover can reduce UV, but it varies with cloud type and conditions. Bright gaps and reflections can still create substantial exposure.

A low reading is not a personal diagnosis. Photosensitising medication, pigment conditions, recent procedures, or a dermatologist's plan may call for more cautious protection.

Mads's practical read

Use the UV Index as a decision tool, not a score to fear. A low-exposure indoor day needs a different plan from a beach afternoon at 8.

Check the number, look at how long you will be outside, then choose sensible layers of protection. That is calmer - and more accurate - than guessing from how warm the air feels.