What is the UV Index?

The Global Solar UV Index, defined by the WHO and WMO, is a linear measure of the strength of sunburn-causing ultraviolet radiation at the Earth's surface. It runs from 0 upward with no fixed top; a value of 6 represents twice the UV of a value of 3.

What UV index is dangerous?

Protection is recommended from a UV index of 3 (Moderate) upward. Values of 6 to 7 are High, 8 to 10 are Very high, and 11 or above is Extreme, where unprotected fair skin can burn within minutes. The risk rises steadily, so there is no single hard cut-off.

When is the UV index highest?

UV peaks around solar noon when the sun is highest, roughly 10am to 4pm in summer. It also increases with altitude, thinner ozone, and reflective surfaces like snow, water and sand, which can nearly double exposure. Cloud cover reduces but does not eliminate UV.

How long until I burn at a given UV index?

Burn time depends heavily on skin type, but as a rough fair-skin guide it falls inversely with the index: high values mean faster burning. The estimate here is approximate guidance only; darker skin tolerates more UV, and some medications increase sensitivity.

Does sunscreen change my UV exposure?

Sunscreen does not change the UV index, which describes the environment, but it greatly slows how fast your skin accumulates a burning dose. Broad-spectrum SPF 30 or higher, reapplied every two hours, combined with shade and clothing, is recommended from index 3 upward.

UV Index Reference Table

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The WHO UV Index, explained

This reference covers the Global Solar UV Index, the international standard for reporting the strength of sunburn-causing ultraviolet radiation. It lists the five exposure categories from Low to Extreme, each with its standard colour code and the sun-protection advice issued by health authorities. A lookup tool maps any value to its category and a rough burn-time estimate.

How it works

The UV index is a linear scale anchored so that each unit corresponds to a fixed amount of erythemal (sunburn-weighted) UV irradiance. Because it is linear, the categories are simple ranges:

0 - 2   Low         3 - 5   Moderate
6 - 7   High        8 - 10  Very high
11+     Extreme

The classifier finds the band containing your value. The burn-time estimate scales inversely with the index as rough fair-skin guidance: higher index means a burning dose accumulates faster. It is not a medical figure and varies widely by skin type.

Tips and notes

UV peaks near solar noon and is strongest in late spring and summer.
Altitude adds roughly 10 percent more UV per 1,000 metres of elevation.
Snow can reflect up to 80 percent of UV, sand around 15 percent and water 10 percent, raising your effective exposure.
Up to 80 percent of UV passes through light cloud, so cloudy days still warrant protection at moderate or higher index values.