UV index: how to read it and protect yourself

What the UV index measures

The UV index expresses the intensity of solar ultraviolet radiation at the Earth’s surface, weighted for its ability to cause sunburn. It is an international risk-communication tool, not a thermometer or a measure of perceived brightness. Higher values mean greater potential for harm during unprotected exposure.

Forecasts may show the expected daily maximum or hourly values: check which information you are reading. The daily maximum does not remain constant from sunrise to sunset. The index does not automatically account for your medicines, skin sensitivity or time already spent in the sun.

The scale: low to extreme

0–2, low. Risk is generally limited during ordinary exposure, but “low” does not mean absent. Prolonged exposure, snow, high altitude or particular photosensitivity may require precautions even at these values.

3–5, moderate. This is already the level at which WHO recommends protection: seek shade around midday, cover the skin and apply sunscreen to uncovered areas. Describing it simply as “little risk” can lead to underestimating a day outdoors.

6–7, high. Increase protection and reduce time in direct sunlight around solar noon. A hat, sunglasses and clothing help protect frequently overlooked areas.

8–10, very high. Take particular care: favour shaded or indoor activities during peak hours, especially for children. Sunscreen alone is not sufficient for prolonged exposure.

11 and above, extreme. Avoid direct exposure during peak hours and organise activities to minimise it. The scale does not stop at 11: the plus sign indicates that higher values can occur.

Season, clouds, altitude and reflective surfaces

The index varies with time, season, latitude, altitude and atmospheric conditions. Clouds do not necessarily block all radiation; snow and other surfaces can increase received exposure. Cool air and wind may make sunlight feel less uncomfortable without making it less harmful.

Use an updated local forecast and check hourly changes when planning sport, work or activities with children. A value for a distant city or a different day does not describe current exposure. Index guidance needs to be combined with practical sun protection and the duration of the activity.

Why not calculate “safe” minutes

Charts converting UV index and phototype into minutes before sunburn can create false precision. Individual responses vary, and the absence of redness does not exclude damage. The old minutes-to-damage chart has therefore been replaced by behavioural guidance, rather than used as a countdown for staying in sunlight.

The same applies to SPF: do not multiply an assumed tolerance time by the sunscreen factor. Use the index to decide when and how to protect yourself, not to test your skin’s limits. If you have a photosensitive condition or take medicines that increase sensitivity, agree specific precautions with your clinician.