The UV Index Explained — What Each Level Means for Your Skin

The UV index is a number, usually from 0 to 11 or higher, that tells you how strong the sun’s skin-burning ultraviolet radiation is at a given place and time. The World Health Organization groups it into five levels: low (0–2), moderate (3–5), high (6–7), very high (8–10) and extreme (11+). WHO recommends sun protection from 3 upwards and extra protection from 8. The higher the number, the less time it takes for unprotected skin to burn.

What the UV index measures

WHO’s Global Solar UV Index guide defines the index as a measure of the intensity of UV radiation at the Earth’s surface that is relevant to effects on human skin. In other words, it’s weighted towards the wavelengths that cause sunburn, not just a raw count of sunlight. That’s why a hot day isn’t necessarily a high-UV day: the warmth you feel comes from infrared, and WHO points out that you can’t feel UV at all.

Forecasts usually report the day’s maximum, but the UV index changes hour by hour. Apple Weather, which Shade uses for its data, shows both the current value and an hourly forecast.

The five UV index levels and what WHO advises

UV index	Level	WHO advice
0–2	Low	No protection required under normal circumstances.
3–5	Moderate	Protection required: seek shade during midday hours, wear a shirt and hat, use sunscreen.
6–7	High	Protection required: the same messages as moderate. Burn times are shorter, so they matter more.
8–10	Very high	Extra protection: avoid being outside during midday hours, seek shade, and treat shirt, sunscreen and hat as a must.
11+	Extreme	Extra protection: as for very high.

Sources: WHO UV Index Q&A and the WHO Global Solar UV Index guide. The US EPA uses the same breakpoints on its UV Index scale and adds practical detail: broad-spectrum SPF 15+ sunscreen, a wide-brimmed hat and sunglasses from UV 3, reapplying every two hours, and a simple shadow rule: if your shadow is shorter than you are, UV is high.

WHO’s basic sun protection messages are the same at every level where protection is needed: limit midday exposure, seek shade, wear protective clothing, a broad-brimmed hat and wrap-around sunglasses, and use broad-spectrum SPF 15+ sunscreen liberally, reapplying it.

Why UV peaks around solar noon

UV is strongest when the sun is highest, because its rays take the shortest path through the atmosphere. WHO says the daily maximum occurs in the four-hour period around solar noon, and that solar noon falls between local noon and 2 p.m. depending on your location and daylight saving time. The same guide estimates that about 60% of the day’s UV arrives between 10 a.m. and 2 p.m.

That’s why the same 30 minutes outside can mean very different doses. Half an hour at 13:00 in July is not the same as half an hour at 17:00. It’s also why Shade follows the hourly forecast during a session instead of using a single number for the whole afternoon.

What else changes the UV you get

The UV index forecast already accounts for some of these. Others depend on exactly where you are standing.

• Clouds. Light or thin cloud doesn’t block much. WHO’s guide says up to 80% of solar UV can get through light cloud cover, and haze can even increase exposure. UV is highest under clear skies, but a grey sky isn’t a reason to skip protection.
• Altitude. Thinner air absorbs less UV. WHO estimates UV levels rise by 10% to 12% for every 1,000 metres you climb.
• Reflection. Surfaces bounce UV back at you. According to WHO, fresh snow can reflect as much as 80%, sea foam about 25% and dry beach sand about 15%. That’s why people burn on ski slopes in early spring.
• Water. Being in the water doesn’t protect you much. WHO notes that UV at half a metre deep is still about 40% as intense as at the surface.
• Latitude and season. The closer you are to the equator, and the closer to midsummer, the higher the sun climbs and the higher the UV.
• Shade. WHO says shade can reduce UV by 50% or more. Reflection from sand or water still adds to what reaches you.

How the UV index relates to skin type

The UV index is the same for everyone standing in the same spot. What changes is how your skin responds. Dermatologists use the Fitzpatrick scale, from type I (always burns, seldom tans) to type VI (naturally black skin), and WHO’s guide uses the same classification. WHO notes that more than 90% of non-melanoma skin cancers occur in skin types I and II, which is why its basic messages focus on fair skin. Darker skin burns less easily, but it isn’t immune to UV damage, especially to the eyes.

A tan doesn’t change the picture much. WHO says a dark tan on fair skin offers protection roughly equal to SPF 4, and lists “a tan protects you from the sun” among common myths.

From UV index to burn time

Each skin type has a typical UV dose at which it starts to redden, called the minimal erythemal dose. Divide that dose by the UV you’re getting and you get an estimate of how long until your skin is likely to burn. That’s the idea behind Shade’s burn-time estimate, and you can try the same maths in our burn-time calculator.

It’s worth being honest about the limits. WHO’s guide warns that burn times can be misread as permission to stay in the sun unprotected, and stresses that UV damage adds up over the day and over the years even without a burn. Shade treats burn time as an estimate with a margin below the threshold, counts sunscreen conservatively because people under-apply it, and never presents it as a risk-free amount of sun. Use it to decide when to cover up or head inside, not as a target to reach.

Putting it to use

• Check the UV index before you go out, not just the temperature.
• From UV 3, plan for shade, clothing, a hat, sunglasses and sunscreen.
• From UV 8, avoid the midday hours outside where you can.
• Remember reflection, altitude and thin cloud can push your exposure above the forecast.
• If you want the number in front of you without opening an app, a widget or Watch complication helps. See how Shade shows it on the UV index feature page, or compare options in our list of the best UV index apps for iPhone.