What is SPF & how does SPF work?

Protecting your skin with a moisturiser, 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.

What does SPF mean?

The Sun Protection Factor (SPF) rating measures how much UVB protection a product provides when liberally and evenly applied to skin. SPF has no relation to UVA protection; UVA protection is measured with a different scale called the PA system. In order to get both UVA and UVB protection, look for sunscreen products labelled “Broad Spectrum”.

But how does sunscreen accomplish this 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, you need to know a few more specifics about UVA and UVB rays.

UVA vs UVB: What’s the difference?

The sun emits invisible ultraviolet radiation that reaches the Earth’s surface and damages skin, unprotected skin, even on cloudy or rainy days. These rays that impact skin 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 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 cause painful sunburns. 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 can also penetrate 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.

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 a difference in protection as the SPF rating increases. Want to know what suncream factors mean? 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 sensitised reaction. And if mineral filters are used to reach an SPF rating much 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.

How long does SPF 50, 30 and others last?

Want to know how long SPF 15, 30 and 50 last? 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 get 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 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 first knowing how long it takes for your skin to change 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 to get a baseline of the amount of time your chosen sunscreen will protect under normal conditions for you.

“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 sunscreen with a higher SPF rating and reapply often. All UV filters break down and become less effective with ongoing exposure to sunlight. Reapplying at regular intervals ensures that sun protection is maintained.

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 9am in June in your own backyard, SPF 30 would give you 10 hours of protection. But this is only true if you’ve applied your sunscreen liberally, which studies have shown that most people do not. In reality, people apply much less sunscreen than what is tested in the labs to determine SPF rating, so you are likely not getting the SPF level of protection indicated on the label.

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. Want to know more about how often you need to reapply sunscreen?

Now that you know how the SPF rating on sunscreen works, check out the must-know tips in our sun protection articles.

References for this information:

Photodermatology, Photoimmunology, and Photomedicine, September 2020, pages 351–356; and May 2020, pages 192–199
Clinics in Plastic Surgery, volume 43, 2016, pages 605–610
Journal of the American Academy of Dermatology, December 2013, pages 867.e1–867.e14
Dermatologic Clinics, July 2014, pages 427–438
The Journal of Clinical and Aesthetic Dermatology, January 2013, pages 16–24; and September 2012, pages 18–23
Indian Journal of Dermatology, September-October 2012, pages 335–342
Journal of the American Academy of Dermatology, May 2008, pages S149–S154

Related articles

• Sun Damage Explained
• Sunburned skin?
• Be smart in the sun: things you definitely need to know
• How should you use a sunscreen product?