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Sun Protection Tips: What Is SPF (Sun Protection Factor)?

Written by: Paula's Choice Research Team

Protecting your skin from sunburn with a moisturizer, primer or foundation rated SPF 30 or greater—or layering all three for even better sun protection—is an essential daily step for remarkably healthy, younger-looking skin.

The Sun Protection Factor (SPF) rating on your skincare 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 labelled “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 those tips, let’s dive into a few more specifics about UVA and UVB rays.

UVA vs UVB: what’s the difference in sun protection?

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 the 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 on the face and body.

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 and help to prevent sunburn.

*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 colouration 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. However, SPF numbers can be misleading because the level of protection doesn't increase proportionally with the SPF rating. 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, a tip is to look 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 sensitised 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 without getting a sunburn?

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 am
  • 15 minutes at 1:00 pm
  • 1 minute at 1:00 pm 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 to prevent sunburn?

Which SPF rating to choose comes down to knowing how long it takes for your skin to turn colour (either pink to red or to show signs of a tan) when exposed to UV light without sun protection. Once you have that timing down—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.

As you now know, conditions vary greatly. “Normal conditions” means the amount and intensity of UV light that normally impacts your skin. When you know your skin will be getting more intense and/or prolonged UV exposure, you will need to choose a sunscreen with a higher SPF rating and reapply often. Reapplying at regular intervals ensures that sun protection is maintained because all UV filters break down and become less effective with ongoing exposure to sunlight.

For example, if your skin normally changes colour 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). If your skin would normally change colour after 20 minutes of sun exposure at 9 am 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.

Bottom line: when you know you’ll be outdoors longer than usual or the sun in your area is more intense, opt for higher SPF ratings of at least 30+ and reapply every 2 hours to prevent sunburn.

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 sun 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 labelled 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 or sunburn on the face or skin when using SPF and broad-spectrum sunscreen. Of course, the end goal should not be to get a tan or sunburn—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 care 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 protection is rated SPF 30 or higher, like the RESIST Anti-Aging Lip Gloss SPF 40 Sheer Pink.

Now that you know how the SPF rating on sunscreen works, check out the must-know tips in our articles: How to Apply Sunscreen, How to Apply Water-Resistant Sunscreen, and Mineral vs. Synthetic Sunscreen Ingredients.

Learn more about the age-defying effect of protecting your skin with sunscreen here.

Shop for skincare products online such as face products and body products to protect yourself against sunburns today.

References for this information

Back to Sun Care

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