What Does the SPF Number on Your Sunscreen Really Mean?

Understanding the Sun Protection Factor

Woman applying sunscreen
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SPF stands for Sun Protection Factor. Simply put, SPF tells you how effectively a sunscreen will protect you from UV rays. The SPF rating tells you how long you can stay in the sun without getting burned while wearing that sunscreen, compared to 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 experimentally indoors by exposing human subjects to a light spectrum meant to mimic 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. It is a measure of UVB protection that ranges from 1 to 45 and above.

In other words, a sunscreen with an SPF of x allows you to stay out in the sun x times longer without burning.

Measuring SPF Strength

A higher SPF doesn't indicate superior sun protection. An SPF 2 protects your skin just as effectively as an SPF 30. However, an SPF 2 will need to be applied more frequently because it's only doubling the amount of time you can stay in the sun before burning. However, both SPF 2 and SPF 30 need to be thoroughly applied and reapplied when the protection timeframe runs out and after swimming or sweating.

Broad Spectrum Sun Protection

The SPF only measures the UVB component. 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 that UVA rays also pose risks. They age the skin and contribute to skin cancer.

 SPF does not predict UVA protection, so in order to get the best sun protection possible, look for a sunscreen or sunblock that indicates both UVA and UVB protection, or broad spectrum protection.

SPF Clothing

Clothing is perhaps the most basic form of sun protection, but it's not all created equal. Sure, a long-sleeved shirt covers more skin and therefore 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, like denim, the less UV rays can get through. The less tightly knit a fabric, like linen, the more UV rays can get through.

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

UPF clothing is clothing made with an ultraviolet protection factor. UPF clothing is becoming increasingly more common, especially for kids' clothing and beachwear items like rash guards and swim shirts. The SPF of regular clothing, however, can be measured. For example, nylon stockings have an SPF of 2. Hats, depending on the size and shape of the brim, have an SPF of 3 to 6. Lightweight, summer clothing typically has an SPF of 6.



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