What Does SPF Actually Mean, Anyway?

In recent years, it seems that more and more people are appreciating the importance of protecting their skin from the potentially harmful ultraviolet (UV) rays emitted from the sun. According to the American Cancer Society, sunlight is the number one source of UV radiation, which can lead to common forms of skin cancer such as basal cell and squamous cell carcinoma. Some easy ways to avoid exposure to UV rays include wearing sun protective clothing, like hats and sunglasses, and just avoiding the sun as best you can. In addition to taking any of these measures, adding a product containing SPF will be your best bet at achieving sun safety and maintaining healthy skin. 

We’ve all likely heard of the term SPF before, but what does that actually mean? Ahead, we spoke to board-certified dermatologist Dr. Anna Guanche to find out what SPF is, why we need it, and how to use it effectively.

“For example, if it takes 30 times longer to burn your skin with a sunscreen on than it does without sunscreen applied, the SPF is 30. This is a carefully measured number and requires lab testing for what we call MED—minimal erythema dosing.”

Dr. Guanche goes on to add that the higher the SPF number, the greater the amount of protection you’ll get from that particular sunscreen. These numbers span over a wide range, which can indicate the level of protection you can expect from a sunscreen. 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 Dr. Guanche. “50 should block out 99% of UV when applied correctly,” she explains. 

SPF vs. Sunscreen

Some people may confuse sunscreen with SPF, using them as almost interchangeable terms. And while you can’t really have one without the other, there is a difference between the two.

• Sunscreen: 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 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 Dr. Guanche.
• SPF: The carefully measured number that is given to a sunscreen formula to describe its UVB protection level. Sunscreen is the actual formula you are using, while SPF is the quantifiable degree of protection that formula provides your skin (if the sunscreen label says SPF 30, that means it would take 30x as long for your skin to burn when you're wearing that sunscreen formula).
• Broad-Spectrum SPF: The carefully measured number that is given to a sunscreen formula to describe its UVB *and* UVA protection level. “Broad spectrum protects against both UVB and UVA rays,” explains Dr. 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.”

How Does SPF Work? 

“SPF works by either blocking out the sun's rays with an opaque coating on the skin (physical sunscreens), or by causing a chemical reaction, whereby the UV rays are absorbed and transmuted into another type of energy (chemical sunscreens),” Dr. Guanche explains. Understanding how SPF works can help you determine the difference between two different types of sunscreen—physical, and chemical.

Chemical vs. Physical Sunscreens

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, physical sunscreens block the rays and chemical sunscreens absorb 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 Dr. 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.”

When it comes to blocking UV rays from her own skin, Dr. Guanche is a fan of Elta MD UV Clear, 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. 

Elta MD UV Clear, Broad-Spectrum SPF 46 $33.00

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How Much SPF Do You Need to Apply? 

So we now know that SPF can not only save our lives but the look and feel of our complexions… how do we use it? According to Dr. Guanche, carefully, generously is the best way to go. “SPF is measured based on the application of two milligrams (mg) of sunscreen for each square centimeter (cm) of skin surface. This is roughly equivalent to six full teaspoons to cover the body of an average adult. This is a lot more than the average person applies.”

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 re-apply a cream to your face, Colorescience has a fabulous broad-spectrum brush on sunscreen that makes reapplying even easier.”

Colorscience Unforgettable Brush-On Sunscreen, Broad-Spectrum SPF 30 $65.00

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Do You Really Need to Wear SPF Every Day?

The short answer: Yes⁠—even in the winter and even when you'll be inside all day. When asked this question, Dr. Guanche offered an answer that cannot possibly be misunderstood: “Yes!!! Sunscreen is for EVERY DAY!” 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,” Dr. Guanche states. 

SPF isn’t just suggested for when you’re heading outdoors, either—if you’re indoors, and especially seated near a window, or driving during the daytime, you should be protected. “You are not without risk being indoors either. UVB can’t penetrate glass, but UVA can!” she adds. “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.”