What does the SPF number mean?

What does SPF 30 actually mean?

Under test conditions, skin covered with SPF 30 needs about 30 times the dose of ultraviolet radiation to develop sunburn compared with unprotected skin. The test uses a sunscreen layer of 2 milligrams per square centimetre. Applying less reduces the protection you get. The SPF number is a protection rating, not a score for how impressive a bottle looks on a bathroom shelf.34

How much difference does a higher SPF make?

Choose at least SPF 30, and SPF 50 or higher for extended time outdoors. Higher numbers filter slightly more UVB, but no sunscreen filters 100%. A higher SPF can help compensate for applying too little, but you should still apply the full amount. Shade and protective clothing remain part of sun protection at every SPF.125

Does SPF tell you how long you can stay outside?

No. Do not multiply your usual time to sunburn by the SPF number. SPF compares doses of radiation, and the dose reaching your skin changes with the sun's intensity, location and cloud cover. Midday sunlight delivers the same dose faster than early morning sunlight. Your skin type, the amount applied and how often you reapply also affect protection. Do not extend your time in the sun because you are wearing sunscreen.46

What else should you check on the label?

Look for broad spectrum protection, which covers both UVA and UVB. UVA contributes to premature skin ageing, while UVB is the main cause of sunburn; both contribute to skin cancer. On UK sunscreens, look for at least 4 UVA stars. A UVA symbol inside a circle indicates that the sunscreen meets the EU standard. The useful label details are SPF, UVA protection and water resistance, which leaves the more dramatic packaging with very little to contribute.145

Choose water resistance if you will swim or sweat. US labels specify 40 or 80 minutes of protection in water; neither means waterproof. Reapply after leaving the water and after towel drying, following the bottle's directions. A high SPF does not make sunscreen last longer on your skin.124

How do you get the protection on the bottle?

1. 1Apply generously to dry skin before going outdoors, allowing at least 15 minutes. If you expect enough exposure to risk burning, apply 30 minutes before leaving and again just before going out.
2. 2Cover all exposed skin, including your face, neck, ears and tops of your feet. Most adults need about 1 ounce to cover skin not covered by clothing; body size changes the amount needed.
3. 3Reapply about every 2 hours outdoors, and after swimming, sweating or towel drying. Follow the product's directions even when the SPF is high.

A sunscreen you like and can afford is easier to use generously, because expensive packaging is not the part that needs protection. SPF moisturizers can help with small amounts of sun exposure, but a thin morning application can give less protection than expected and wears off. They still need reapplication outdoors.27

If you get sunburn, stay out of the sun until redness has gone. Seek medical help if you feel unwell or your skin blisters or swells badly.4