At what UV index do you need sunscreen?

UV itself can't be felt.¹ The warmth of the sun on your skin comes from its infrared rays, so a cool day can still burn you.¹ A cloudy day can too, since up to 80% of UV passes through light cloud.¹ The index tells you what your skin can't: the higher it is, the less time it takes for your skin to be damaged.² UV generally peaks around midday, and a good guide is to take extra care anytime your shadow is shorter than you are.¹

At the low end, below 3, the risk of damage is limited, and in normal conditions most people need no protection.¹ If you burn easily, though, protect your skin at any level, and do the same on snow, which can nearly double your UV exposure.¹ Dermatologists go further and recommend sunscreen on uncovered skin every day you're outside.³

When the index is 3 or higher, shade does the most good between 10 a.m. and 4 p.m., when UV is strongest.⁴ For the skin that a hat and clothes leave bare, most adults need about an ounce of sunscreen, roughly a shot glass full.⁴ Reapply it at least every two hours, and more often if you're swimming or sweating.⁴ For the pool or a hot game, choose a water-resistant one.³ Babies under 6 months are the exception: keep them in the shade and covered, and ask a doctor before putting sunscreen on them.³