Sun Protection Factor (SPF) and Sunscreen

Woman sunbathing on the beach

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SPF stands for Sun Protection Factor, which is an indication of how much protection a sunscreen offers against UVB rays and sunburn.

In general, a sunscreen with:

  • SPF 2 blocks 50 percent of UVB rays
  • SPF 4 blocks 75 percent of UVB rays
  • SPF 8 blocks 87 percent of UVB rays
  • SPF 15 blocks 93 percent of UVB rays
  • SPF 30 blocks 97 percent of UVB rays
  • SPF 50 blocks 98 percent of UVB rays
  • SPF 100 blocks 99 percent of UVB rays

As you can see, once you get to SPF 30, you don't get that much extra protection by going higher.

While you can certainly use a sunscreen with SPF 50+ to SPF 100+, keep in mind that they don't offer that much extra protection. Using a high SPF sunscreen might be a good idea for those parents who don't use enough sunscreen and don't reapply it often enough though.

Also, remember that SPF is only an indication of effectiveness against UVB rays. That is why it is important for parents to for a broad-spectrum sunscreen with at least SPF 15 to SPF 30, which should block most UVA and UVB rays when applied properly. You should avoid a low SPF sunscreen or suntan lotion, which don't provide enough sun protection.

UVA Protection Ratings

There is currently no easy way to know how effective a sunscreen is against UVA rays. New FDA sunscreen labeling, in addition to possibly placing a cap of SPF 50+, was supposed to introduce a new UVA four-star system so that parents could easily figure out if a sunscreen offered low UVA protection (one star) or the highest UVA protection (four stars). The new labels would also clearly state if a sunscreen offered 'no UVA protection.'

Unfortunately, the Final Rule on new sunscreen labeling eliminated the star system, thinking it would be too confusing. If a sunscreen is now labeled Broad Spectrum, then it does protect against UVA rays.

SPF for Clothing

Clothing has a separate rating system that is similar to the SPF rating of sunscreen.

However, instead of an SPF rating, some clothing has an Ultraviolet Protection Factor (UPF) rating, which can range from 15 (good sun protection) to 50+ (excellent sun protection) to indicate the percentage of UVA and UVB rays it can block.

So What Does SPF Really Mean?

Even if you understand that a sunscreen with SPF 15 blocks 93 percent of UVB rays, what does that really mean in the real world?

A practical way to think about it is that a sunscreen with SPF 15 allows you to stay in the sun 15 times longer than if you were unprotected before you would get a sunburn. Similarly, a sunscreen with SPF 30 means that you can be in the sun 30 times longer.

Since it is really difficult to say exactly how long it will take for any one person get a sunburn without any sun protection, the whole 15 times longer and 30 times longer doesn't necessarily mean much. You have to factor in a person's skin type, where they live, the time of year, and even the time of day to know how long they might be able to stay in the sun and not get a sunburn. For example, someone with a light complexion in the sun in Texas at 2 pm in the middle of summer on a day with a high UV index is going to burn a lot faster, maybe in less than 15 minutes, 6 pm and certainly faster than someone with a darker complexion in Idaho 6 pm in the winter.

Other factors that will increase your child's risk of getting a sunburn more quickly include taking certain medications, including many medications used to ​treat acne, being at a high altitude, and being near surfaces that might reflect the sun, such as snow and sand.

And remember that sunscreen becomes less effective after about two hours and needs to be reapplied, so an SPF 50 sunscreen is not really going to allow you to stay in the sun 50 times longer than if you were unprotected before getting a sunburn.

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Article Sources

  • Wang, Steven Q. Current status of the sunscreen regulation in the United States: 2011 Food and Drug Administration’s final rule on labeling and effectiveness testing. J Am Acad Dermatol. August 5, 2011.