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July 21, 2022

Is a 30 SPF double the protection of a 15 SPF?

Is a 30 SPF double the protection of a  15 SPF?

Q: My esthetician told me to wear an SPF 30 sunscreen every day, regardless of my activities and time of year. I believe it gives double the protection as an SPF 15. Is this correct?


A: Although it makes sense that an SPF 30 would offer twice the protection as an SPF 15, the laws of physics prevent this from being true. In reality, an SPF 30 screens less than 4 percent more UV radiation than an SPF 15, but those extra few percentage points protect from the higher range of UVA, the primary ray involved in triggering photosensitivity and photo toxicity as well as advancing signs of aging, including age spots. An SPF 50 offers one percent more protection, screening out 98 percent of UVA and UVB. SPF 30-50 sunscreens are highly recommended for those taking photo-reactive medications, such as the antibiotic Cipro1, people who are HIV-positive or individuals with UV-sensitive diseases such as lupus erythematosus or rheumatoid arthritis. If you’re working on lightening your skin, be it age spots or larger areas of darkening, always be sure to wear an SPF 50 or you’ll swear the product or procedure you’re using doesn't work. Better yet, avoid the sun.

For everyone else applying sunscreen for daily protection, an SPF 30 might be overkill, and, depending upon the formula, be more harmful than it’s worth. That’s because the amount of sunscreen needed to achieve the extra few percentage points of protection can be more than double the amount of sunscreen needed to produce the SPF 30 rating. Organic sunscreens such as octinoxate can be irritating when used at higher percentages. Some researchers believe this irritation poses problems for skin independent of those created by UV exposure.

‘New’ formulation techniques, however, can enhance the spreadability of the sunscreen ingredients, delivering more uniform coverage that results in a higher SPF without increased irritation. In fact, when sunscreen ingredients are spread evenly over the skin, there is less risk that “pockets” of protection will form. This means coverage is consistent, and irritation is reduced—no more blotchy tans or ‘spotty’ burns. Other technologies involve micro-pulverizing the non-irritating particulate sunscreens, titanium dioxide, and zinc oxide, causing them to become transparent a minute or two after application. When aiming for higher SPF ratings, formulators can include these irritant-free sunscreens with lower percentages of other sunscreen ingredients or may rely on the particulate sunscreens alone to produce the higher SPF without increasing irritation.

Finally, it’s important to remember that SPF ratings are not the same for everyone. Instead, they’re based upon the time it takes for a person to turn red when exposed to UV radiation. Multiply the time it takes to turn red—what commonly is perceived as a “burn”—by the SPF number, and you should have an idea of how long it will take to turn red after just one application of sunscreen without exposure to water, sweat or excessive rubbing unless that’s been calibrated into the SPF claim by the product’s manufacturer.

SPF ratings are not only different for each person, but because burn time varies according to the individual’s circumstances, the SPF can change when certain conditions change. Climate, altitude, time of year, medications, disease, cloud cover, and even exposure to reflective surfaces—water, sand, snow, and cement—can double the amount of UV hitting the skin. So, if a person in Los Angeles burns in the backyard in June in 15 minutes, this same person would be able to stay out in that same backyard in June for three and a half hours with an SPF 15. But if that same person goes to the high altitude region of Aspen in June, and has been taking diuretics for several days, the length of protection for the same SPF 15 sunscreen will shorten. The person will burn faster. In fact, the SPF could be cut in half, allowing the person to stay in the sun for less than two hours.

Even with all the mitigating circumstances, most dermatologists agree that a daily application of SPF 15 is a safe bet for the average person exposed to a wide variety of conditions. Switch up to SPF 30 if you’re going to be in direct sunlight for any length of time. Just remember to reapply an SPF 30 every couple of hours during full sun exposure. And always reapply after swimming or sweating—regardless of whether or not the sunscreen is water or sweat-resistant.