What Do the SPF Numbers Mean? | Northwestern Medicine

When you go outside to soak up the sun, you’re also soaking up two types of ultraviolet (UV) radiation: UVA and UVB rays.

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UVB rays primarily contribute to sunburn through DNA damage, while UVA rays are more associated with tanning and aging. Both types of UV rays can lead to all types of skin cancer. That's why everyone, regardless of age, gender or skin color, should protect their skin from the sun. This includes using sunscreen.

Here’s what to know about SPF and protecting your skin from the sun.

Assessing SPF

If you look on a bottle of sunscreen, you'll see a number for the sun protection factor (SPF).
SPF measures how well a sunscreen protects against UVB rays (burning rays), but does not apply to UVA rays (aging rays).

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However, SPF numbers can be tricky to interpret and don’t always tell the full story. For example, SPF 30 blocks 97% of UVB rays, compared to SPF 50, which blocks 98% of UVB rays.

You may think that a higher SPF number will give you more sun protection, but as the numbers increase, the difference in protection becomes smaller. No sunscreen can block 100% of the sun — even SPF 100 sunscreen blocks only 99% of UVB rays.

As a general rule, everyone should use an SPF of at least 30.

Skin cancer doesn’t discriminate. People with darker skin tones need to use sunscreen (SPF 30 or higher) just as much as people with lighter complexions. But what’s in your sunscreen may be just as important as the SPF number.

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What to Look for in a Sunscreen

• To ensure full protection against the sun, look for broad spectrum protection sunscreen, which protects against both UVA and UVB rays.
• Choose a physical blocker instead of a chemical blocker. Chemical sunscreens absorb UV photons before they can damage your DNA. Physical sunscreens sit on your skin and reflect UV rays away from the cells in your skin. Physical blockers are especially good for people with sensitive skin types. To tell chemical sunscreens and physical sunscreens apart, check their active ingredients:
• Chemical blockers contain avobenzone, octinoxate and oxybenzone.
• Physical blockers contain zinc oxide and titanium dioxide.

 7 quick tips to help reduce your risk of developing skin cancer from UV exposure:

• Apply sunscreen 15 minutes before going outdoors.
• Reapply often, about every two hours, and after swimming and sweating. No sunscreen is truly water resistant.
• Thoroughly coat your exposed skin with sunscreen, including around the ears, lips and neck.
• Protect yourself with your wardrobe. Wear light clothing that covers your skin, and accessorize with a wide-brimmed hat and sunglasses.
• Seek shade when possible.
• Check your sunscreen’s expiration date. Most sunscreens expire after three years.
• Avoid tanning beds.