Start protecting at UV index 3. That is the WHO threshold, not a comfort rating. At 3, unprotected skin can take damage in about half an hour around midday—often before the air feels hot.

Check today’s peak on the live UV index, not only the number when you walk out the door. A mild 9 a.m. reading can still sit under a High peak.

What to do at each band

The EPA UV index scale matches the WHO bands:

• 0–2 Low: Short errands are usually fine for most people. Sunglasses still help in open glare.
• 3–5 Moderate: Broad-spectrum SPF 30+, shirt, hat. Limit long midday sits in open sun.
• 6–7 High: Same kit, shorter unprotected stretches. Reapply if you sweat.
• 8–10 Very High / 11+ Extreme: Unprotected skin can burn in minutes. Shade, clothing, and sunscreen together—not one of them.

Sunscreen is the last layer, not the only one

The American Academy of Dermatology still wants a generous coat: about a shot-glass for the body, 15 minutes before you go out, then every two hours. SPF 30 blocks about 97% of UVB in the lab; SPF 50 about 98%. Most people apply too little to hit the label.

If you want a number for your skin type and today’s index, use the SPF calculator. For what the SPF digit actually measures, see what SPF stands for.

Clouds and cool air do not reset the index

UV passes through thin cloud. Some of the worst burns happen on overcast days because people skip cover. Altitude and travel toward the tropics push the same clock higher than a sea-level forecast at home. More on that split: UV peaks before the heat.