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Scientifically, This Is How You Choose SPF

What is the best sunblock to use

With summer approaching, we need to be aware of how we can protect our skin from harmful UV rays caused by the sun. Before you plan your next trip to the beach or spend time outdoors this summer, lather up with some sunblock to prevent getting a painful (and embarrassing) sunburn. Buying the highest SPF at the store should do the trick to provide more protection, right? According to our expert, that might not be the case.

Q: Does a higher SPF give you better protection?
A: The number of SPF does make a difference, but only measures how long it takes to get burned by the sun.

Expert explanation:

The first answer is that any SPF is better than no SPF! SPF stands for “sun protection factor” and it’s important to know that it is a term that only applies to a scale of protection from ultraviolet B rays and not a measure of ultraviolet A protection. Since both types of rays can cause deadly skin cancer, wrinkles, spots, and aging, it’s important to always make sure to look for “Broad Spectrum” on the label, since for now that is our best indicator of good quality protection from UVA rays as well as UVB. To understand how SPF works in thinking about UVB protection, it helps to know that the number is not a percentage of UV blocked.

It’s supposed to represent a multiple of the number of minutes you can stay in the sun before getting the same sunburn you’d get with no protection at all. In other words, SPF 8 means it will take 8 times as long to get burned. SPF 15 protects against 93% of UVB rays, SPF 30 against 97%, SPF 70 against 98.5%. No sunscreen protects against 100% of UVB, even SPF 100. That said, the numbers do make a difference, because thinking about it a different way, SPF 70 only lets 1.5% of UVB through, while SPF 30 lets 3% through, meaning that SPF 30 lets through twice as many damaging rays. With a lot of exposure, that can make a difference.

Q: Which SPF should I use?
A: If you keep applying, the level should not matter too much.

Expert explanation:

Unfortunately, there are several reasons that number is not easy to rely on literally in real life and that it does matter which you choose. First, it comes out of a lab, under strict research conditions, where a thick layer of the sunscreen is applied and UV rays are applied in a set, predictable fashion for a known amount of time. The thickness of sunscreen and frequency of reapplication (still properly thick) are carefully regulated, and pretty much largely unrelated to how people use sunscreen in the real world.

We tend to apply a thin layer of sunscreen, too thin to achieve the labeled SPF, and we do not diligently reapply a new thick layer every two hours or less when in sun or water, as recommended. So my tip for buying sunscreen is always to “cheat upwards” and buy a higher SPF than you intend to end up with, to comfortably achieve your goal. The baseline SPF recommended for daily use in moisturizer by the American Academy of Dermatology, and the Skin Cancer Foundation is SPF 30. There are many cosmetically elegant moisturizers made for daily use that contain SPF 30, broad spectrum sunscreen, and to be extra diligent (and extra wrinkle free!), I generally suggest SPF 50 in a moisturizer for daily use in summer, and at least SPF 50 on the beach or by the pool.

Next step: How to apply sunscreen correctly >

Expert Profile

jessicaName: Dr. Jessica Krant, MD, MPH

Capacity: Dermatology

Campus: SUNY Downstate Medical Center

Expertise: Dr. Krant is a board-certified dermatologist and Assistant Clinical Professor of Dermatology at SUNY Downstate Medical Center in NYC. She is also the owner of Art of Dermatology, LLC.

Olivia Santo

Written by Olivia Santo

Olivia is a former student assistant in the Office of New Media for the State University of New York. She is an undergraduate direct and interactive marketing major with a minor in economics at the Fashion Institute of Technology in New York, NY.

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