The bands and what to do
| UV index | Level | EPA advice |
|---|---|---|
| 1 to 2 | Low | No protection needed; you can stay outside with minimal sun protection |
| 3 to 7 | Moderate to high | Protection needed: seek shade from late morning through mid-afternoon; when outside, use broad-spectrum sunscreen of SPF 15 or higher, protective clothing, a wide-brimmed hat and sunglasses |
| 8 and above | Very high to extreme | Extra protection needed: take extra care outdoors, above all from late morning through mid-afternoon; if your shadow is shorter than you, seek shade, cover up and apply SPF 15 or higher broad-spectrum sunscreen |
The scale is open at the top, and the EPA's advice stays the same from 8 upwards: extra protection, above all around the middle of the day.
The shadow rule
No forecast to hand? The EPA's check is your shadow. If it is shorter than you are, the sun is high and UV exposure is higher: seek shade and protect your skin and eyes. A shadow longer than you, the usual picture in early morning and late afternoon, suggests lower exposure. A short shadow means a high sun.
When the sun is strongest
The EPA's advice for both upper bands centres on late morning through mid-afternoon, the hours around solar noon, when the sun is highest. Solar noon is rarely 12:00 on the clock: it shifts with your position in the time zone and with daylight saving time. The sunrise and sunset calculator shows solar noon for any place and date, so you can plan a hike or a beach visit around the strongest hours instead of guessing.
UV is not heat
The UV index rates ultraviolet radiation from the sun, not how warm the air is, so a mild day is not automatically a low-UV day. The heat index calculator handles the other risk of a summer day, how hot the air feels once humidity is counted, and the dew point calculator shows how muggy it is.
A sunny-day checklist
- Look up the UV index for the day; at 3 or more, plan protection.
- Schedule time outdoors outside late morning to mid-afternoon where you can.
- Use broad-spectrum sunscreen of SPF 15 or higher on exposed skin, and reapply as the label directs.
- Add a wide-brimmed hat, sunglasses and clothing that covers arms and legs.
- When in doubt, check your shadow.
Frequently asked questions
What does the UV index measure?
The strength of the sun's ultraviolet radiation at a place and time, as a single number that rises with the risk of sunburn. The US Environmental Protection Agency groups it into three bands with matching advice: 1 to 2 needs no protection, 3 to 7 needs protection, and 8 and above needs extra protection.
At what UV index do I need sunscreen?
From 3. For the 3 to 7 band, the EPA advises shade from late morning through mid-afternoon and, when you are outside, broad-spectrum sunscreen of SPF 15 or higher on exposed skin together with protective clothing, a wide-brimmed hat and sunglasses.
What is the shadow rule?
A quick check without a forecast. The EPA says that if your shadow is shorter than you are, UV exposure is higher, so seek shade and protect skin and eyes; a shadow taller than you, typical of early morning and late afternoon, suggests lower exposure.
When is UV strongest?
Around the middle of the day, when the sun is highest. The EPA's advice speaks of late morning through mid-afternoon; the sunrise and sunset calculator shows solar noon for any place, which is the centre of that window.
Is the UV index the same as temperature?
No. It rates the sun's ultraviolet radiation, not how warm the air feels, so check it on mild days too. The heat index calculator covers the other summer risk, how hot humid air feels.
Sources
General information based on the sources above, not medical advice.