Sun Protection Factor (SPF)

Protecting your skin with a moisturizer, primer or foundation rated SPF 30 or greater—or layering all three for even better protection—is an essential daily step for remarkably healthy, younger-looking skin. The Sun Protection Factor (SPF) rating on your skin care products measures how much UVB protection a product provides when liberally, and evenly, applied to skin.

What most people don't know is that SPF actually has no relation to UVA protection. In fact, UVA protection is measured with a completely different scale called the PA system. In order to get both UVA and UVB protection, you must look for sunscreen products labeled "Broad Spectrum".

But how does sunscreen accomplish protection? The science of how SPF works is fascinating but can also be confusing to understand due to numerous variables. But before we get into that, let's dive into a few more specifics about UVA and UVB rays.

UVA vs UVB: what's the difference? The sun emits invisible ultraviolet radiation that reaches Earth's surface and damages unprotected skin, even on cloudy or rainy days. These rays are known as ultraviolet A (UVA) and ultraviolet B (UVB).

UVB rays affect the skin's surface and cause sunburn, which you can see and feel, but their inflammatory damage also causes skin cells to become abnormal. UVA rays penetrate deeper into skin, destroying everything in their path—including the vital supportive substances skin needs to look young and healthy. Shorter wavelengths of UVA can also contribute to painful sunburn.

Both UVA and UVB rays play a role in causing skin cancers. While UVB rays are responsible for near-instant visible damage, such as the redness of sunburn, UVA rays cause skin to tan, a sign of damage to every layer of skin.

While both UVA and UVB rays are present outside year-round, and in all types of weather, UVA rays are more persistent, penetrating glass, including car and office windows*. That's why it's so important to wear sunscreen even if your outdoor time is minimal, and to choose products labelled "broad-spectrum" since these protect against both types of UV rays.

Some automobile and office windows have a UV-protective coating, but most do not, so unless you know for sure, it's best to act as if all such glass lets UVA rays in.

It's important to know that UVB rays are most intense between the hours of 10:00 AM and 4:00 PM, at high altitudes, and as you get closer to the equator. UVA rays are present all day long at a fairly constant intensity no matter where you are in the world. At any hour, if you can see daylight, UVA rays are present and are damaging unprotected skin!

There's no such thing as a safe amount of UV light exposure.

How is SPF calculated? Before we explain how SPF is calculated, it's important to point out that the testing to determine SPF ratings has limitations because it doesn't always translate to real-world situations. Although the testing is done on people's skin, the conditions are controlled, and the intensity of UV light is fixed.

In the real world (and as explained above) UVB intensity varies while UVA is consistent. The shifting intensity of UVB light impacts how long it will take skin to turn red (burn) with any given sunscreen. And remember, even darker skin can experience sunburn although you won't see the pink to red coloration apparent on lighter skin tones. If you have dark skin, pay attention to how hot and itchy your skin feels when outdoors, as these are signs of potential sunburn.

Remember, the Sun Protection Factor (SPF) rating only measures how much UVB protection a product provides when liberally applied to skin. SPF numbers can also seem misleading because there is less of a difference in protection as the SPF rating increases.

Based on regulated testing: SPF 15 shields skin from 93% of UVB rays SPF 30 blocks nearly 97% SPF 50 blocks 98% SPF 100 blocks 99%

However, looking at it another way: SPF 30 lets about 3% (1/30th) of UVB rays through SPF 50 only allows about 2% (1/50th) through That's 50% more UVB rays penetrating your skin if you use SPF 30 vs SPF 50.

For daily use, most people will do great with SPF 30 to SPF 50. When you know your skin will see intense midday sun, you may want to consider a higher SPF along with other protective measures, such as hats and UV-rated clothing.

But there's a tradeoff: super-high SPF ratings mean increased potential for a sensitized reaction. And if mineral filters (zinc oxide or titanium dioxide) are used to reach an SPF rating over 50, aesthetics will likely be compromised (hello, pasty white cast!).

One way around this is to use a tinted sunscreen with iron oxide to cover the white cast, although it can be tricky to find a tinted option that works for all skin tones.

How long can you stay in the sun with SPF 15 vs SPF 30? We wish there was a quick and easy answer to this question. In the past, it was thought that simple math was enough: if you know your skin turns pink or begins to darken within ten minutes of sun exposure without sunscreen, then SPF 15 would provide 150 minutes of protection (10 x 15 = 150). SPF 30 would net you 300 minutes, and so on.

The above math is how we used to explain SPF ratings to people, but our efforts to simplify turned out to be a shortcoming. SPF ratings aren't just about the amount of time skin is exposed to UV light; rather, they're about time and the amount of exposure meaning the intensity of UVB based on where you are in the world plus other factors such as amount of cloud cover, latitude, season, and proximity to reflective surfaces like water and sand.

Summing it all up, the intensity of solar energy the sun emits impacts the amount of UV radiation your skin receives. To illustrate... 1 hour at 9:00 a.m. 15 minutes at 1:00 p.m. 1 minute at 1:00 p.m. at high altitude with reflecting snow ...are each capable of delivering the same intensity of UV radiation to skin despite dramatically different time stamps.

What SPF do I need? Which SPF rating to choose comes down to a combination of factors: knowing (or at least having a good idea of) how long it takes for your skin to turn color (either pink to red or to show signs of a tan) when exposed to UV light without sun protection, whether you'll be outside or not, the intensity of that day's UV rays in your area and your overall health. This might sound complicated, but we're breaking it down for you.

The baseline rule: Apply broad spectrum sunscreen rated SPF 30 or greater liberally and evenly and reapply as needed. Although not an exact science, once you have the timing of how long it takes your skin to redden or darken—and you may need to estimate—multiply that number by the SPF rating you're using to get a baseline of the amount of time your chosen sunscreen will protect under normal conditions for you, based on your location, skin color, and how long it takes for a color change to occur.

For example, if your skin normally changes color after 10 minutes of unprotected sun exposure at noon in your own backyard in June, and you use a sunscreen rated SPF 30, you will get five hours of sun protection (10 minutes x 30 = 300 minutes, which is 5 hours of protection) at this location. If your skin would normally change color after 20 minutes of sun exposure at 9am in June in your own backyard, SPF 30 would give you 10 hours of protection. This is only true if you've applied your sunscreen liberally, which studies have shown that most people do not.

However, if you're going to be outside continuously during hours of peak UV ray intensity (10AM-2PM), when the UV index is 8 or higher, or when visiting places that have more intense sun than where you live, opt for SPF 50+. If you have health risk factors, always follow the recommendation of your doctor and dermatologist when it comes to proper sun protection.

Bottom line: when you know you'll be outdoors longer than usual or the sun in your area is more intense (which is a given as elevation above sea level increases), opt for higher SPF ratings of at least 50+ and reapply every 2 hours, plus wear a hat, sunglasses, and UV-protective clothing. If you'll be inside, or outside for shorter amounts of time in less intense sun, choose SPF 30+.

Does higher SPF mean less tan? While in theory a higher SPF seems less likely to lead to a tanning response in skin, it's not a clear-cut correlation. It's important to remember that the SPF number only correlates to the level of protection from UVB rays. As you now know, UVB rays aren't chiefly responsible for skin tanning—that reaction comes more so from UVA rays.

Remember, to ensure protection from UVA rays, your sunscreen must be labeled as broad spectrum. Also keep in mind that no sunscreen can prevent 100% of sun exposure. Even if you think you're pretty good about sun protection, data shows most of us still aren't applying sunscreen as liberally as we need to be.

Long story short, there's a chance you could still see some level of tanning on skin when using SPF and broad-spectrum sunscreen. Of course, the end goal should not be to get a tan—that's a red flag for the skin-damaging inflammation taking place below the surface—but you already knew that.

Do my lips need SPF? Lips need SPF just like the rest of your skin. In fact, protecting your lips from sun exposure is one of the most significant things you can do to preserve their youthful appearance and decrease the risk of lip cancer. Finding a lip product with SPF that you like the look and feel of and will be willing to reapply throughout the day is key (remember, eating/drinking will cause your lip product to rub off, taking the protection with it). Just like with face and body sunscreen, it's important that your broad-spectrum lip sunscreen is rated SPF 30 or higher.

An SPF rating is produced by UV filters — a small group of permitted ingredients dispersed through the formula that sit as a thin film on the skin's surface and intercept ultraviolet photons before they reach the skin beneath.

Organic (often called chemical) filters such as butyl methoxydibenzoylmethane (avobenzone), octocrylene and bis-ethylhexyloxyphenol methoxyphenyl triazine have molecular structures that absorb UV energy and release it again harmlessly, largely as a very small amount of heat. Each one absorbs a particular slice of the UV spectrum, which is why formulators blend several together to cover UVB and UVA at the same time. Some filters, avobenzone especially, break down under sunlight and need stabilising partners to keep performing through the day.

Mineral (inorganic) filters — zinc oxide and titanium dioxide — were long described as bouncing UV away like tiny mirrors. Laboratory work suggests the picture is more nuanced: they also absorb most of the UV that reaches them, with reflection and scattering playing a smaller part.

Because the whole thing depends on that film, the rating is only realised when the film is complete and even. Protection scales with how much you apply, so a thin layer delivers considerably less than the number on the pack. Rubbing, sweating, towelling and water all thin it out.

SPF only performs if you use enough of it — and then use it again.

• Use enough. Around half a teaspoon for face and neck, or the "two-finger" rule: a line squeezed along the full length of your index and middle fingers. Six to eight teaspoons covers the whole body.
• Apply it last in your morning routine, after moisturiser and before make-up, and give it a few minutes to settle before you dress or add foundation.
• Reapply every two hours when you are outdoors, and immediately after swimming, heavy sweating or towelling — water-resistant formulas included.
• Cover the bits people miss: ears, hairline and parting, eyelids, neck and chest, backs of hands, and lips with an SPF 30+ balm.
• Read the UVA marking as well as the number. NHS guidance in the UK is at least SPF 30 with a minimum four-star UVA rating.
• Treat SPF in moisturiser or foundation as a bonus, not a replacement — almost nobody applies these thickly enough to reach the stated rating.
• Wear it year-round. UVA is present whenever there is daylight, including through glass.