Answer: SPF stands for Sun Protection Factor and refers to the theoretical amount of time you can stay in the sun without getting sunburned. For example, an SPF of 15 would allow you to stay in the sun 15 times longer than you could without protection. So, if your skin starts to redden in 20 minutes without sun block, applying a product with SPF 15 increases that time by a factor of 15, meaning you could stay in the sun for 300 minutes. In addition, a higher SPF blocks out more rays—a product with an SPF of 15 will filter out approximately 93 percent of UVB rays; SPF 30 filters out about 97 percent.
It’s also important to know that SPF only applies to UVB rays. There is no SPF equivalent for UVA. You can look for additional ingredients in your sunscreen to protect you from UVA—such as zinc oxide and titanium dioxide—but there’s no standard measurement for how long these ingredients will keep you protected.
For all of these reasons, it’s not a good idea to solely rely on SPF to gauge how much time you can spend in the sun. For most people, SPF 15 will suffice, if you keep in mind the following tips:
Select the Right Sunscreen
Choose a sunscreen that protects against both UVA and UVB. Look for products containing zinc oxide or titanium dioxide because they protect against the full spectrum of UVA rays.
Lotion Up Liberally
Reapply sunscreen at least every two hours—sooner if you swim or exercise because water, sweat, and clothing can remove it from the skin. Don’t skimp! You should use at least an ounce with every application, so in a full day at the beach you’d go through half of an eight ounce bottle.
Try Sun-Protective Clothing
Choose your clothes wisely before you go out in the sun. Dark clothing can block nearly all UV radiation and tightly woven fabrics are more protective than looser weaves. If you’re wondering how well your clothing will protect you, just hold it up to the sun. If you can see light passing through it, UV rays can get through, too.
You might also consider purchasing special sun-protective clothing. A relatively new rating for sun protective textiles is UPF, or Ultraviolet Protection Factor. UPF blocks both UVA and UVB rays. (You can find UPF-rated clothes online at coolibar.com.)
Special laundry detergents such as Rit Sun guard (sunguardsunprotection.com) can increase the UPF of most clothing for about 20 washings. Keep in mind that no matter what you’re wearing, all fabrics block less UV light when they’re stretched or wet.
Keep an eye on your skin: Look for new moles or changes in old moles and report any concerns to your health care provider. We recommend annual skin exams for anyone over 40, or for anyone with fair skin or a history of multiple sunburns.
For more information on how to stay safe in the sun, check out skincancer.org and the American Cancer Society.