What does SPF mean in sunscreen?

What does the number actually measure?

SPF compares the amount of ultraviolet radiation needed to cause sunburn on protected skin with the amount needed on unprotected skin. SPF 30 means it takes 30 times as much UV exposure to cause sunburn under the test conditions. That comparison depends on applying enough sunscreen evenly. It does not mean you can stay outside 30 times longer: sunlight intensity changes with the time of day and your location.13

How much protection do different SPFs give?

Higher SPFs filter out slightly more UVB, but no sunscreen blocks 100%. SPF 30 is not twice the protection of SPF 15, despite the tidy arithmetic on the bottle. Higher SPF also lasts the same amount of time on your skin as lower SPF, so the reapplication schedule stays the same.2

Does SPF cover UVA too?

SPF primarily describes UVB protection. Look for "broad spectrum" as well: that means protection against both UVB and ultraviolet A, or UVA, rays. UVA contributes to premature wrinkles and age spots; UVB is the main cause of sunburn. Both can cause skin cancer. A high SPF number alone is not a substitute for the broad spectrum wording.12

Which SPF should you choose?

Choose broad spectrum SPF 30 or higher for everyday use, and SPF 50 or higher for extended stays outdoors. Apply sunscreen to uncovered skin every day you will be outside, including cloudy days. UV rays can damage skin in any season or at any temperature, so cool weather does not remove the need for protection.235

How do you get the protection on the label?

Using too little sunscreen reduces the protection you get. The labeled SPF requires approximately two milligrams of sunscreen per square centimeter of skin. In practical terms, allow about two tablespoons for the exposed face and body, with more needed depending on body size and the area exposed.26

1. 1Apply sunscreen evenly to dry skin 15 minutes before going outdoors.
2. 2For your face, use at least one teaspoon, approximately the amount that covers the length of your index and middle fingers.
3. 3Cover all uncovered areas, including your ears, neck, tops of feet and exposed scalp. Use SPF 30 or higher lip balm for your lips.
4. 4Reapply approximately every two hours outdoors, and after swimming, sweating or toweling off. Follow the bottle's directions.

Does water resistance change the SPF?

Water resistance is a separate label claim. It tells you whether sunscreen remains effective for 40 or 80 minutes while swimming or sweating; no sunscreen is waterproof. Check that duration rather than assuming a high SPF stays on longer.14

What else do you need besides sunscreen?

Use shade, protective clothing, a wide brimmed hat and UV protective sunglasses alongside sunscreen. Limit sun exposure especially between 10 a.m. and 2 p.m., when the sun's rays are strongest. Sunscreen is one part of protection, not a replacement for covering up.56