Sunscreen and SPF Ratings

Sunscreen and SPF Ratings

Updated Oct 6, 2019

Warmer weather is on its way and people will soon be spending more time outdoors and at the beach. Many of us will be flocking to the stores for some sunscreen, too, but confronted with such a wide range of SPF ratings, how do you know you are buying the right one?

SPF: What it is and isn’t

SPF, or Sun Protection Factor, is a factor rating system for sunscreens related to how long it takes someone to get a sunburn. For example, let’s say you know that you typically get a sunburn after being in the sun for 20 minutes with no sunscreen. If you buy sunscreen with SPF 10, this means you can stay out in the sun for 10 times as long (200 minutes) without getting a sunburn. Buy SPF 15 and you can stay out in the sun 15 times as long and so on. This, however, is not the full explanation.

When choosing a sunscreen, you may be inclined to think that since SPF 30 is twice SPF 15, it must be twice as effective as SPF 15 in blocking harmful rays from the sun (specifically UVB rays which are the main cause of sunburns). Unfortunately, this is misleading. According to the Skin Cancer Foundation,

  • SPF 15 blocks 93% of UVB light
  • SPF 30 blocks 97% of UVB light
  • SPF 50 blocks 98% of UVB light

As you can see, higher SPF numbers do provide additional protection against UVB light. However, as the SPF number increases, the amount of additional benefit decreases. While going from SPF 15 to 30 provides 4% more protection, the jump from SPF 30 to 50 only provides an additional 1% protection. Higher SPFs like SPF 100 will provide marginally more protection than SPF 50. Importantly, no matter what, no SPF rating will block 100% of rays from getting through to your skin.

Why it Matters

Turning our attention back to what an SPF rating is, you may think that using a sunscreen with a high SPF means you can apply it in the morning and be safe for, say, the next 6-10 hours. However, experts agree that to maintain a safe level of protection from the sun, sunscreen should always be reapplied every two hours, or after activities like swimming or drying off with a towel. This rule applies to all sunscreens whether they have an SPF rating of 10 or 100. So, even though using SPF 100 implies you stay in the sun 100 times longer without getting a sunburn, the efficacy of the sunscreen will still drop after a couple of hours and still need to be reapplied.

Given this, unless you have a specific need for the maximum protection possible from the sun, a sunscreen with SPF 30—blocking 97% UVB light—will generally keep you safe through a two-hour period. If you really want that extra 1% protection, go for the SPF 50. Anything higher than that and you may be paying a premium for minimal added benefit.

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