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.

What is the UV Index Reference Table?

Reference for the WHO Global Solar UV Index from 0 to 11+, with the five exposure categories from Low to Extreme, colour codes, protection advice, and a rough fair-skin burn-time estimate. It runs free in your browser on Gera Tools, with nothing uploaded.

UV Index Reference Table

Name: UV Index Reference Table
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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.

What the five exposure categories mean in practice

Low (0–2): Minimal risk for most people. Sun protection is generally not needed for short outdoor exposure. People with very fair skin or certain medications that increase photosensitivity may still want protection.

Moderate (3–5): This is where sun protection advice begins for the general population. Sun-safe habits — sunscreen, hat, sunglasses — are recommended, especially during peak hours. A fair-skinned person can begin to burn in under an hour.

High (6–7): Reduce time in the sun between 10am and 4pm. Seek shade at midday, wear protective clothing, and apply SPF 30+ sunscreen and reapply every two hours.

Very High (8–10): Common in summer at mid-latitudes and year-round in tropical regions. Take extra precautions. Unprotected fair skin can burn in well under 30 minutes. Shade is a primary defense, not just sunscreen.

Extreme (11+): Typical near the equator, at high altitude, and with snow or water reflection. Sun protection is essential and avoiding direct sun during peak hours is strongly advised. Values above 11 are common in tropical destinations and can be reached in temperate zones on summer days at altitude.

Factors that raise your effective UV exposure

The UV index number describes outdoor conditions in an open, unshaded area facing the sky. Several factors can increase your actual exposure above what the index alone suggests:

Altitude. UV intensity increases roughly 10% per 1,000 metres of elevation gain. At 3,000 metres (a typical alpine ski resort), you receive about 30% more UV than at sea level for the same index reading.
Reflective surfaces. Snow reflects up to 80% of UV, effectively doubling your exposure compared to standing on grass. Water reflects about 10%, and sand about 15%. This is why skiers and beachgoers at high-index values burn especially quickly.
Water. Wet skin may offer slightly less UV protection than dry skin, and UV penetrates shallow water — you can burn while swimming in clear water, especially at high elevations.
Latitude and season. The sun sits higher in the sky at low latitudes and in summer, shortening the path UV must travel through the atmosphere and increasing intensity.
Cloud cover. Light clouds have little effect — up to 80% of UV can pass through an overcast sky. Only dense, thick cloud cover significantly reduces UV.

UV protection — what actually helps

Sunscreen does not change the UV index but slows how quickly your skin accumulates a burning dose. Broad-spectrum SPF 30 blocks about 97% of UVB rays; SPF 50 blocks about 98%. The difference is smaller than many people assume — consistency of application matters more than choosing SPF 100 over SPF 50. Reapply every two hours and after water or sweat.

Clothing is highly effective. A standard cotton T-shirt offers roughly SPF 15 protection. Tightly woven, darker, or UV-rated fabrics (marked UPF 50+) offer considerably more.

Shade reduces but does not eliminate UV exposure, as reflected and scattered light still reaches you. It is most effective when combined with sunscreen and clothing.

Sunglasses protect the eyes and surrounding skin, which are particularly susceptible to UV damage. Look for lenses that block 99–100% of UVA and UVB.

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.
The burn-time estimates in this tool are approximate fair-skin guidance only and vary substantially by individual skin type, sunscreen use, and other factors. Consult a dermatologist for personalised advice.