Does SPF 100 really work?

What does SPF 100 actually mean?

SPF stands for sun protection factor, which is unusually literal for an industry that names things for a living. It compares the amount of ultraviolet radiation needed to cause sunburn on protected skin with the amount needed on unprotected skin. SPF 100 describes sunburn protection, not the entire sunscreen.13

SPF 100 does not mean you can stay outdoors 100 times longer without burning. Sun intensity changes with time of day and location, while the amount you apply and how often you reapply affect protection. SPF compares protection, not safe hours in the sun.1

Is SPF 100 worth choosing over SPF 30?

SPF 30 blocks about 97% of UVB rays. Higher SPFs block slightly more, so the extra protection is real, but SPF 100 is not complete protection. Many people apply only 20% to 50% of the amount needed to achieve the labelled SPF. A higher SPF helps compensate for applying too little; it is not a reason to deliberately use less.2

Choose SPF 100 if you want that extra margin and will use it consistently. Broad spectrum, water resistant SPF 30 or higher remains a recommended choice. A lower SPF that meets those requirements is still useful; SPF 100 is an option, not a requirement for everyone.3

What else should the label say?

Look for broad spectrum protection against both UVA and UVB. SPF is mainly a measure of UVB protection; a high number alone does not tell you the level of UVA protection. On UK sunscreens, look for at least four stars for UVA protection. A circled UVA mark indicates that the sunscreen meets the EU standard, which is quite a workload for a circle.34

Water resistance is separate from SPF, which makes buying sunscreen surprisingly close to reading a job specification. US water resistance labels specify 40 or 80 minutes of effectiveness in water. No sunscreen is waterproof, and even water resistant sunscreen needs reapplication after toweling off.3

How to use SPF 100 properly

1. 1Apply before going outdoors, following the bottle's directions. General advice ranges from 15 to 30 minutes beforehand. Use sunscreen every day you go outside, including winter and cloudy days.
2. 2Cover all exposed skin generously. Most adults need about one ounce to cover skin not covered by clothing. Include your ears, neck, hands, feet and exposed scalp, and use SPF 30 or higher lip protection.
3. 3Reapply approximately every two hours outdoors, even with SPF 100. Reapply after swimming, sweating or towel drying, and follow the label's water resistance limit.

Who should take extra care?

Extra precautions matter if you burn easily, work outdoors or visit sunnier places or higher elevations. Some medicines, including doxycycline, certain diuretics and retinoids, increase sensitivity to sunlight. Do not treat SPF 100 as your only protection in these situations.467

For sensitive skin, a mineral sunscreen containing zinc oxide or titanium dioxide is a recommended option. Choose by the formula as well as the SPF number.5

Shade and protective clothing remain part of sun protection, because a bottle of sunscreen cannot also be a hat. Wear a wide brimmed hat and UV protective sunglasses, and take extra care around reflective water, snow and sand.2

If you burn despite using SPF 100, stay out of the sun until the burn heals. Seek medical help if you feel unwell or develop marked swelling or blisters.467