Protecting your skin with a moisturiser, primer or foundation rated SPF 30 or higher (or layering all three for even better protection) is an essential daily step for remarkably healthier and younger-looking skin.
The Sun Protection Factor (SPF) rating measures how much UVB protection a product provides when liberally and evenly applied to the 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 sun cream with the “Broad Spectrum” label.
But how does sun cream accomplish this protection? The science of how SPF works is fascinating but can also be confusing to understand due to numerous variables. Before we break it down, it helps to know a few more specifics about UVA and UVB rays.
The sun emits invisible ultraviolet radiation that reaches the Earth’s surface and damages the skin when left unprotected, even on cloudy or rainy days. These rays that impact the 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 the skin, damaging everything in their path – including the vital supportive substances that the 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 or deeper skin tones that come with sunburn, UVA rays cause the skin to tan which is a sign of damage to every layer of the skin. While both UVA and UVB rays are present outside year round and in all types of weather, UVA rays can also penetrate through glass, including car and office windows.* That is why it’s so important to wear sun cream even if your outdoor time is minimal and to choose products labelled "Broad Spectrum” since these products will protect your skin from both types of UV rays.
*Some car and office windows have a UV-protective coating but most do not. So unless you're sure, it’s best to act as if all 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. In comparison, 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.
Before we explain how SPF is graded, it’s important to point out that the testing used to determine SPF ratings has limitations because it doesn’t always translate to real world situations. Although the testing is performed on people, the conditions are controlled and the intensity of UV light is fixed. In the real world, UVB intensity varies while UVA is consistent. The shifting intensity of UVB light impacts how long it will take the skin to burn (turn red or a deeper brown/black) with any given sun cream.
Remember, the Sun Protection Factor (SPF) rating only measures how much UVB protection a product provides when liberally applied to the skin: SPF numbers can also seem misleading because there is less of a difference in protection as the SPF rating increases. So what does SPF 30 mean? Or SPF 50? Based on regulated testing:
However, looking at it another way:
That means 50% more UVB rays can penetrate your skin if you use SPF 30 compared to SPF 50. When you know you won't be able to find shade in the 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 trade-off: super high SPF ratings mean increased potential for a sensitised reaction. And if mineral filters are used to reach an SPF rating over 50, aesthetics will likely be compromised (hello, white streaks!) We recommend you experiment with our Advanced Sun Protection Daily Moisturiser SPF 50 which leaves zero white cast and contains a blend of five next-generation synthetic UVA & UVB filters tolerable even for sensitive skin – as you might just find it is the best sunscreen for your face.
We wish there was a quick and easy answer to this question. In the past, it was thought that simple maths was enough: if you know your skin turns pink or begins to darken within ten minutes of sun exposure without protection, then Factor 30 sun cream would provide 300 minutes of protection (10 x 30 = 300). How long does Factor 50 last? 500 minutes. Factor 15 sun cream? 150, and so on.
But it just isn't that simple. SPF ratings aren’t only about the amount of time that the skin is exposed to UV light; rather, the ratings are about the time and the amount of exposure combined, 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, sand and snow.
Summing it all up, the intensity of solar energy that the sun emits impacts the amount that your skin receives. To illustrate …
... are each capable of delivering the same intensity of UV radiation to the skin despite dramatically different time stamps.
Finding the SPF rating that's best for your skin really depends on how long it takes for your skin to change colour (to a red, tanned or deeper skin tone) when exposed to UV light without sun protection. Once you have that timing down – and you want to estimate rather than test – multiply that number by the SPF rating to get a baseline of the amount of time your chosen sun cream will protect the skin under normal conditions.
“Normal conditions” means the amount and intensity of UV light that usually impacts your skin. When you know your skin will be getting more intense and/or prolonged UV exposure, you will need to choose sun cream 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 back garden in June, and you use a sun cream rated SPF 30, you will get five hours of sun protection (10 minutes x 30 = 300 minutes, or 5 hours). If your skin would normally change colour after 20 minutes of sun exposure at 9:00 am in June in your back garden, SPF 30 would grant you 10 hours of protection. But this is only true if you’ve applied your sun cream liberally and studies show that most people tend not to. In reality, people apply much less sun cream than how they apply it when conducting tests to determine the SPF rating in labs, so you are likely not getting the SPF level of protection indicated on the label.
Another great way of enhancing whichever SPF you choose to use is by adding an antioxidant-rich product into your routine. One ingredient to look out for is coffee-fruit extract. This ingredient contains antioxidant compounds known as chlorogenic acids that can reduce the visible effects of sun exposure by stopping the cascade of the redness-triggering damage that UV light causes. Find coffee fruit extract in our skin-refining serum, the DEFENSE Antioxidant Pore Purifier.
The bottom line is when you know you’ll be outside for longer than usual or when the sun is more intense in your neighbourhood, opt for higher SPF ratings of at least 30 and reapply every 2 hours. Now that you know how the SPF rating in sunscreen works, check out our top tips on sun protection.
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
Sign up for our newsletter and become a Paula's Choice Member. Your benefits:
+ Welcome gift*
+ Exclusive deals*
+ Skincare advice
+ Birthday gift
Need help with finding the right product for your skin? Our customer care team are skincare and product experts and can guide you to the best skin of your life.
Sign up for our newsletter to receive exclusive offers and expert skincare advice, plus get £5 off your next order.