How Does Sunscreen Actually Work?

May 19, 2017
Jessica Khorsandi
By: Jessica Khorsandi | by L'Oréal
How Does Sunscreen Actually Work?

Curious to know how sunscreen works? You’re in luck. We share the answer to that and more popular sunscreen questions, below, just in time for warmer weather! 

Everyone knows­ that daily sunscreen use is one great way to help protect skin from harmful UV rays. Each morning we diligently apply broad-spectrum SPF—and reapply every two hours during the day—to help prevent sunburn. This practice can help reduce your chances of developing visible signs of skin aging. But, in between those daily applications, have you ever stopped to wonder how sunscreen actually works to help protect your skin? After all, sunscreen is a non-negotiable part of every skin care routine. We should—at the very least—be educated on how the product works, right? To that end, we’re providing the answer to how sunscreen actually works, and more of your burning sunscreen questions, ahead!


The answer, unsurprisingly, has a lot to do with the formulation of these products. Simply put, sunscreen works by combining organic and inorganic active ingredients that are designed to help protect your skin. Physical sunscreens are generally formulated with inorganic active ingredients—like zinc oxide or titanium oxide—that sit on the surface of your skin and help to reflect or scatter radiation. Chemical sunscreens are generally formulated with organic active ingredients—like octocrylene or avobenzone—to help to absorb UV radiation at the surface of your skin, change the absorbed UV rays into heat, then release the heat from your skin. There are also some sunscreens which are categorized as both physical and chemical sunscreens depending on their formulation. When reaching for a sunscreen, look for a formula that’s water-resistant and offers broad-spectrum protection, meaning it protects from both UVA and UVB rays.

To learn more about the difference between physical and chemical sunscreens, read this!


By now you’re likely aware that both UVA and UVB rays are harmful. The key difference between the two is that UVA rays—which are not fully absorbed by the ozone—tend to penetrate the skin deeper than UVB rays, and can prematurely age the appearance of your skin by contributing to noticeable wrinkles and age spots. UVB rays—which are partially blocked by the ozone layer—are primarily responsible for delayed tanning and burning.

Did you know that there’s a third type of radiation called UVC rays? Since UVC rays are completely filtered by the atmosphere and don’t reach the Earth’s surface, they’re often not as widely discussed.


SPF—or Sun Protection Factor—is a measure of a sunscreen’s ability to prevent UVB rays from damaging the skin. For instance, if it takes 20 minutes for unprotected skin to start turning red, using an SPF 15 sunscreen should theoretically prevent skin reddening 15 times longer than unprotected skin, so about five hours. It’s important to note, however, that SPF only measures UVB rays—the type that burns the skin—and not UVA rays, which are also harmful. To protect against both, reach for a broad-spectrum sunscreen and take other sun protection measures.

Editor’s note: There’s no sunscreen out there that can fully block all UV rays. In addition to wearing sunscreen, be sure to practice other safe sun protection measures like wearing protective clothing, seeking shade, and avoiding peak sun hours.


According to the Mayo Clinic, most sunscreens are designed to remain at original strength for up to three years. If your sunscreen doesn’t have an expiration date, it’s always good practice to write a date of purchase on the bottle and toss it after three years. This rule should always be followed unless the sunscreen is stored improperly, which can reduce the shelf life of the formula. If this is the case, it should be discarded and replaced with a new product earlier. Take note of any obvious changes in color or consistency of your sunscreen. If anything seems fishy, discard it for another.

Editor’s note: Scan your sunscreen package for an expiration date, since most should include them. If you see one, use the expiration date on the bottle/tube as a guideline for how long the formula can be used before it’s no longer effective.


If your sunscreen bottle is lasting you through years and years, chances are you’re not applying the recommended amount. Generally, a good application of sunscreen is about one ounce—enough to fill up a shot glass—to cover exposed parts of the body. Depending on your body size, this amount could fluctuate. Be sure to reapply the same amount of sunscreen at least every two hours. If you go for a dip, sweat profusely, or towel-dry, reapply immediately.


Despite what you may have heard, there is no safe way to tan. Every time you are exposed to UV radiation—by the sun or through artificial sources like tanning beds and sun lamps—you damage your skin. It may seem harmless at first, but as this damage builds it can cause the appearance of premature skin aging and increase your risk for skin damage. 

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