Burning question

Dear Lucy: I’m a redhead and get sunburns very easily. For that reason, I always buy the highest SPF sunscreen I can find. But is a higher number really better? SPF 100 costs much more than SPF 50. Do higher numbers really provide extra protection? Or at least enough extra protection to make them worth the much higher cost? —Better Red Than Dead?

Illustration of two containers of sunscreen, one with a label reading 'SPF 30' and one with a label reading 'SPF 75'

Dear Red: What a terrific and timely question! For an answer—and, as it turned out, a math and physics lesson—Lucy paid a visit to her source for all things skin-related, MIT Medical Dermatologist Allison Larson.

Larson began by showing Lucy a nifty graph illustrating the relationship between SPF (sun protection factor) and percentage of UV (ultraviolet) rays blocked. “As you can see,” she pointed out, “it’s a logarithmic function. With an SPF 2, you’re blocking 50 percent of UV rays. At SPF 4, it jumps to 75 percent. It’s 93 percent with SPF 15, and, as you go higher, the curve flattens further, meaning that relatively large gains in SPF provide only small increases in UV protection.” 

The answer seemed clear. Between SPF 30 and 100, the gain in UV protection was just 2 percent. And since SPF 30 already provides 97 percent protection against UV rays, why would anyone spring for a pricier option? But as Lucy headed for the office door, all set to dash off a “don’t bother with the more expensive stuff” answer, Larson stopped her in her tracks. “And now I’m going to tell you why your redheaded reader should consider shelling out for a higher SPF,” she said.

“It’s because most people don’t use enough,” she continued. “SPF values are based on 2 milligrams of sunscreen per centimeter of skin (2 mg./cm.2). That may not sound like a lot, but, on average, it works out to about an eighth of a cup per application — at least double what most people actually use.”

“And does that change the level of protection?” Lucy asked.

“It does,” Larson answered. “Based on the Beer-Lambert law, the traditional hypothesis has been that UV protection decreases exponentially with inadequate application. Or, in other words, that half the recommended amount of sunscreen equals less than half the protection,” she explained. But, Beer-Lambert assumes an even surface, she noted, when skin is actually uneven, adding that recent research indicates a more linear relationship.

“In either case, when you don’t use enough sunscreen, actual SPF level drops significantly,” she continued. “And when you combine that with forgetting to reapply sunscreen every two to three hours as recommended, your actual protection ends up being far less than your sunscreen’s SPF value might suggest. 

“This is why it can be worthwhile to spring for the sunscreen with a higher SPF,” she concluded. “And, the greater tendency a person has to burn, the more that extra two or three percent of UV rays matters.” 

Lucy hopes this answers your question, Red, and to conclude, she would like to remind you and all her other readers that sun protection isn’t only about sunscreen. Avoid sun exposure when possible. But when you can’t, use “wearable protection” as well. You can’t underapply a shirt, hat, or sunglasses; nor do you need to remember to put it on again in a couple of hours! —Lucy

Back to Ask Lucy Information contained in Ask Lucy is intended solely for general educational purposes and is not intended as professional medical advice related to individual situations. Always obtain the advice of a qualified healthcare professional if you need medical diagnosis, advice, or treatment. Never disregard medical advice you have received, nor delay getting such advice, because of something you read in this column.