SPF 101: What It Means, How to Use It, and Choosing the Right Formula

It may have taken decades, but it seems that more and more people are appreciating the importance of protecting their skin from the harmful ultraviolet (UV) rays emitted from the sun. Applying sunscreen is the most important part of any skincare routine, and a product containing SPF will be your best bet at achieving sun safety and maintaining healthy skin (aside from protective clothing and hats). And while we know the term SPF, do we really know what it means? How much should we be applying? Which sunscreen innovations are worth buying? And does the number on the bottle really matter? Ahead, board-certified dermatologists Dr. Anna Guanche and Dr. Bradley Glodny explain exactly what SPF is, why we need it, and how to use it effectively.

Decoding Sunscreen Terminology

SPF

SPF stands for sun protection factor, "which is a measure of of how long a sunscreen will protect you from ultraviolet rays," says Guanche. “For example, using SPF 30 means it will take 30 times as long for your skin to burn than it does with no sunscreen applied. This is a carefully measured number and requires lab testing for what we call MED—minimal erythema dosing.”

Sunscreen

"SPF" is commonly used interchangeably with "sunscreen," which is a bit of a misnomer. Sunscreen is the tangible lotion or formula you apply to your skin. You probably remember this stuff as the thick, white lotion your parents slathered on your skin before you hit the beach as a kid, a habit that’s hopefully stuck with you over the years. Like long-sleeved shirts, wide-brimmed straw hats, and beach umbrellas, sunscreen is a highly effective way to keep our skin protected from potential sun damage. “Sunscreen is used to help protect the skin from the sun's harmful ultraviolet (UV) rays. It is safe for all skin types and ethnicities,” says Guanche.

Broad-Spectrum

This is the label given to a sunscreen formula that provides both UVB *and* UVA protection.“Broad spectrum protects against both UVB and UVA rays,” explains Guanche, whereas SPF not labeled as broad-spectrum only protects from UVB rays. “UVB causes burning of the skin, whereas UVA causes photoaging such as wrinkles, loss of collagen, and promotes the formation of brown spots (lentigos) on the skin.” Guanche is a fan of Elta MD UV Clear, Broad-Spectrum SPF 46 ($39), which is a broad spectrum sunscreen that also comes with an array of additional benefits “such as niacinamide (vitamin B3) that helps to reduce redness, hyaluronic acid that attracts and retains moisture within the skin and vitamin E that functions as an antioxidant to reduce free radical formation and subsequent skin damage,” she says. 

Water-Resistant

According to the American Academy of Dermatology, water-resistant labels on sunscreen bottles means the sunscreen will remain effective for up to 40 minutes of swimming or heavy sweating. Meanwhile, "very water-resistant" labels mean the sunscreen will remain effective for up to 80 minutes of swimming or heavy sweating.

Reef-Safe

Some environmental scientists have theorized that chemical sunscreens may have adverse effects on the health of the oceans and coral reefs. That being said, far more research is needed to prove this hypothesis. In the meantime, if you'd like to play it on the safe side, sunscreens labeled as "reef-safe" don't contain any of the ingredients suspected to contribute to coral reef bleaching.

Chemical vs Mineral Sunscreen

While any sunscreen is better than no sunscreen, there are varieties within these products which offer variations both in active ingredients used, and the look of the application. In short, both formulas work effectively and you can choose based on your personal preference, but physical sunscreens work by blocking the sun's rays and chemical sunscreens work by absorbing the rays.

• Physical Sunscreens: “Physical sunscreens contain mineral ingredients such as titanium dioxide and zinc oxide. These ingredients function to block and scatter the UV rays before they penetrate the skin,” says Guanche. Mineral sunscreens are known for often leaving a white or gray-looking cast on the skin, and are usually a little less thin than the alternative, which can make application feel a little tougher.
• Chemical Sunscreens: “On the other hand, chemical sunscreens contain ingredients such as avobenzone and octisalate that function to absorb UV rays before they can damage the skin," notes guanche.

What SPF Level Should You Use?

Low protection is considered anything with an SPF below 15, while medium protection ranges from SPF 15 to 29. An SPF measuring between 30 and 49 offers high protection, and anything over SPF 50 is considered to be “very high protection” according to Guanche. “50 should block out 99 percent of UV when applied correctly,” she explains.

Is SPF 15 Enough?

SPF 15 is low protection sunscreen that protects your untanned skin from UV rays 15 times longer than if your skin did not have sunscreen applied. Generally, Glodny does not recommend SPF 15 when used alone, especially if it is not being used properly. "Many people will not apply enough sunscreen to actually reach the SPF described on the bottle. Also, reapplication is a super important concept that people often fail to perform often enough," he says.

Most People Should Use SPF 30 at Minimum

For most individuals who are not at increased risk for skin cancer, the Skin Cancer Foundation advises to use a water-resistant, broad-spectrum SPF 30 or higher.

Is Higher SPF Better?

Even if you're using SPF 30, keep in mind that to get that level of sun protection, you need to apply 1/4 teaspoon to your face and neck, plus a shot glass amount of sunscreen for your body. You also need to reapply every two hours, or immediately after swimming or sweating. Very few people apply the correct amount of sunscreen, and even fewer people reapply diligently and on time. Therefore, dermatologists often recommend using a sunscreen of 50+ to create more room for human error. If you do follow the rules of application (and re-application) to a tee, derms say you likely aren't getting any extra protection once you go over SPF 50.

Use SPF 70-100 If You Have Risk Factors Like Fair Skin

If you have fair skin, a family history of skin cancer, or any other sun exposure risk, you should use a much higher SPF level. According to Glodny, SPF 100 is the highest SPF possible and it blocks out about 99 percent of UVB rays. "No sunscreen will ever block out 100 percent of UV light and, therefore, there will never be an SPF higher than 100. I would only recommend my patient use an SPF of 100 if they have very, very fair skin and a significant history of sun damage or even skin cancers." Additionally, Glodny notes that there are "certain genetic disorders that prevent the skin from protecting itself from UV radiation adequately, for example, Xeroderma pigmentosum, and these people should use the highest SPF available."

Pair Sunscreen With Other Protective Measures

Whenever possible, you should pair your sunscreen with other sun protection strategies, such as avoiding the sun during peak hours, wearing UPF clothing, wearing hats, and seeking shade when you are going to be outside in the sun.

How Much Sunscreen to Use

As a general rule of thumb, Guanche recommends using three-fingers' length of sunscreen for your face and neck, plus a shot glass worth of sunscreen for your body.

It is always better to be generous when applying SPF, which according to Guanche, is "measured based on the application of two milligrams (mg) of sunscreen for each square centimeter (cm) of skin surface." This comes out to be equivalent to "six full teaspoons to cover the body of an average adult...a lot more than the average person applies," she says.

In addition to applying the recommended amount, it’s important that we treat our skin to SPF frequently enough to get the most out of that product. “Sunscreen should be applied every morning, and reapplied every two hours—no matter the SPF, sunscreen should always be reapplied,” she explains. “If you’re wearing makeup and don’t want to reapply a cream to your face, Colorescience Sunforgettable Brush-On Sunscreen, Broad-Spectrum SPF 50 ($43), has a fabulous broad-spectrum brush on sunscreen that makes reapplying even easier.”

UPF vs. SPF

UPF (ultraviolet protection factor) is the scale used to describe the amount of UV radiation that can penetrate through fabrics such as sun shirts, bathing suits, hats, and other clothing. "The higher the UPF, the less harmful rays will penetrate your clothing to reach your skin. This measure of UV light accounts for both UVA and UVB," says Glodny.

SPF (sun protection factor) is the scale based on the time it takes for "untanned skin to become red when exposed to UV light," says Glodny. For example, he says that a sunscreen with an SPF of 50 will "allow your skin to be exposed to the sun 50 times longer before it turned red compared to your skin without sunscreen." Glodny also notes that SPF differs from UPF in that it "only accounts for UVB while UPF measures both UVA and UVB."

Do You Really Need to Wear SPF Every Day?

Yes⁠ you need to wear sunscreen every single day—even in the winter and even when you'll be inside all day. "Sunscreen is for every day!” emphasizes Guanche. That means overcast days, rainy days, partially sunny days… you name it. As a general rule of thumb: If it’s daytime, your exposed skin should be covered in SPF, period. This includes often overlooked areas like the tops of the ears, eyelids, neck, and the tops of your hands and feet. “While clouds can block the sunshine, they can’t block UV rays, and you are still at risk for sunburn and skin damage while in cloudy weather,” Guanche states. 

If you’re indoors, and especially seated near a window, or even in front of a computer screen, you should be protected. “UVB can’t penetrate glass, but UVA can. This means that while you’re indoors, you are still at risk of the damaging effects of UVA, which include accelerating skin aging and formation of unwanted pigmentation," says Guanche.

The Takeaway

Sunscreen is the closest thing we have to a miracle beauty product—not only does it drastically reduce your risk for skin cancer, but it also prevents sun damage that manifests itself as fine lines, wrinkles, and sagging skin. Regardless of whether you use chemical, mineral, or reef-safe, just be sure to use a broad-spectrum, water-resistant SPF 30 or higher every single day to keep your skin protected. It should be worn every single day and, ideally, reapplied every two hours.