What Does SPF Mean?

SunscreenEvery summer I’ve put on sunscreen and have seen the three letters, SPF, yet I had no idea what they meant until a few days ago. I also thought that the higher the number is, the better protection I was getting (which is partly true).

SPF stands for “Sun Protection Factor.” SPF only measures protection against UVB and tells nothing about UVA protection. As a side-note, UVA is the range of wavelengths that generally cause wrinkling while UVB is generally responsible for sunburn. However both UVA and UVB can cause cancer.

How is SPF measured and what do the numbers mean? SPF is measured by shining UV light on a region of skin which is partly unprotected and partly protected by sunscreen. The time it takes for redness (erythema) to develop is measured for both the unprotected and sunscreen protected regions. The actual SPF number is calculated by dividing the time it took for redness to appear on the region with sunscreen by the time it took for redness to appear on the region lacking sunscreen.

For example, say it took 10 minutes for redness to appear on the unprotected region while it took 150 minutes for redness to appear on the region with sunscreen. The resulting SPF would be 150/10 = SPF 15.

The higher the SPF number, the better the protection, right? Well, kind of. SPF 30 doesn’t offer twice the protection as SPF 15. It’s a case of diminishing returns; as the numbers increase, the amount of protection increase becomes smaller and smaller. SPF 15 blocks about 93%, SPF 30 blocks about 96.7%, and SPF 40 blocks about 97.5% of UVB. In fact, any SPF above about 50 to 60 can’t currently be labeled as such with great confidence. There simply aren’t any methods that can determine such high SPF values with reproducibility. Generally, SPF 30 provides adequate sun protection for most people.

I hope you found this interesting; I did. Who knows, maybe this bit of trivia will lend itself useful one day if you ever find yourself on Jeopardy.

[Source of all information] Lehne, Richard A. Pharmacology for Nursing Care. St. Louis, MO: Saunders/Elsevier, 2010. Print.

[Photo Credit] Joe Shlabotnik

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