What Does the SPF Number on Your Sunscreen Mean?

Understanding the Sun Protection Factor

Woman applying sunscreen
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SPF stands for Sun Protection Factor. Simply put, an SPF rating tells you how long you can stay in the sun without getting burned while wearing that sunscreen, compared with how long you can stay in the sun before you burn without wearing that sunscreen. For example, if it typically takes you 15 minutes to burn without sunscreen and you apply an SPF 10, it will take you 10 times longer to burn, or 2.5 hours.

How SPF Is Determined

The SPF number is determined through indoor experiments, by exposing human subjects to a light spectrum that's meant to mimic the noontime sun—when the sun's rays are at their most intense. Some subjects wear sunscreen and others do not. The amount of light that induces redness in sunscreen-protected skin, divided by the amount of light that induces redness in unprotected skin is the SPF. 

What the SPF Number Means

A higher SPF doesn't indicate superior sun protection—it indicates that you will remain protected in the sun for a longer amount of time. For example, an SPF 2 protects your skin just as effectively as an SPF 30. However, an SPF 2 will need to be applied more frequently. To be safe, no matter what SPF you choose, it's best to reapply sunscreen at least every two hours, as well as after swimming or sweating. In fact, the American Academy of Dermatology recommends daily application of SPF 30 to all exposed skin.

Broad-Spectrum Sun Protection

The SPF measures only protection against the sun's ultraviolet B rays, which are called UVB for short. Initially, UVB rays were thought to be the only UV rays to worry about, since they are shorter in length and cause sunburn. However, it became understood later that UVA rays also pose risks. They age the skin and contribute to skin cancer. SPF alone does not protect against UVA rays, so in order to get the best sun protection possible, look for a sunscreen or sunblock that provides both UVA and UVB protection—it should say "broad spectrum" or "full spectrum" on the label and it should have an SPF of at least 30.

SPF Clothing

Wearing sun-safe clothing is a great additional measure that you can take, beyond wearing sunscreen, to help protect your skin from the sun. It's especially a great idea for young, active children who may have trouble sitting still for a few minutes while a parent tries to apply sunscreen, especially those who are often in and out of a pool, lake, or ocean.

But be picky about which type of sun-safe clothing you buy. Sure, a long-sleeved shirt covers more skin and provides more sun protection than a tank top, and a floppy, wide-brimmed hat shields the face better than a baseball hat. But even if you're covered in clothing from head to toe, that clothing isn't of much use if the sun can make its way right through the fabric fibers.

Fabric is made from fibers that are knitted or woven together, and when a fabric is examined under a microscope, the spaces between those fibers are visible. UV rays can penetrate through those spaces and reach the skin. The more tightly knit a fabric is, like denim, the less likely it is that UV rays can get through. The less tightly knit a fabric is, like linen, the more likely it is that UV rays can get through.

Unfortunately, lightweight, summery fabrics like linen and cotton feature open weaves that allow more UV rays to penetrate. So if you're wearing items like those, you should still apply SPF to parts of your body that are covered up, depending on the fabric you're wearing.

UPF clothing is clothing made with something that's called an ultraviolet protection factor. One example of a brand that specializes in this type of UPF clothing is Coolibar, which makes clothing and accessories for men, women, boys, and girls. UPF clothing is becoming increasingly more common, especially for kids' clothing and beachwear items like rash guards. 

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Article Sources
  • http://www.skincancer.org/prevention/sun-protection/clothing/protection