How Sun Protection Factor (SPF) Works

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 step to stunningly healthier, younger-looking skin. But how does sunscreen accomplish all this? Great question!

The science of how sunscreen and sun protection factor (SPF) ratings work is fascinating, but complicated. We’ll shed some light on why sunscreens are nothing short of a miracle.

UVA and UVB Rays

The sun emits invisible radiation that damages skin, even on cloudy or rainy days. The 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 to become abnormal. UVA rays, on the other hand, which you never feel, penetrate deep into skin, destroying everything in their path – including the vital supportive substances skin needs to look young and healthy. Both UVA and UVB rays play a role in causing skin cancers.

It’s important to know that UVB rays are most intense between the hours of 10:00 AM and 4:00 PM, while UVA rays are present all day long at a fairly constant intensity. At any hour, if you can see daylight, UVA rays are present and silently damaging unprotected skin! There’s no such thing as a safe amount of UV light exposure.

Sun Protection Factor

The Sun Protection Factor (SPF) rating measures how much UVB protection a product provides when liberally applied to skin. A product with a higher SPF shields the skin from more of the sun’s rays, but the protection does not increase exponentially as the SPF rating goes up. For example, a product rated SPF 30 will protect skin from almost 97% of the sun's UVB rays, while an SPF 50 product will shield you from 98%.

The SPF rating system is a guide to how long you can stay in direct sunlight before your skin starts to burn (remember, it’s the UVB rays that cause sunburn).

Here’s where it gets complicated and a bit mathematical, but hang in there, this is VERY important. To work this out, you need to know about how many minutes it takes your skin to turn pink without sun protection. If you normally start to burn after 10 minutes of unprotected sun exposure, multiply the number of minutes by the SPF rating of the sunscreen you’re considering – that’s how long your sun protection will last.

For example, if your skin normally changes colour after 10 minutes of unprotected sun exposure 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, SPF 30 would give you 10 hours of protection. But this is only true if you’ve applied your sunscreen liberally. If you don’t, you won’t be getting the SPF level of protection indicated on the label.

While UVB rays are responsible for fairly 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 whether you go outside or not and to choose products labelled “broad-spectrum,” as these shield skin from both types of UV rays.


When to Reapply Sunscreen

Sunscreen gets broken down by direct exposure to daylight, so how often you need to re-apply sunscreen depends on the amount of time you spend outside. If you are in the office all day, then the sunscreen that you applied in the morning will still be effective when you travel home at the end of the day.

If you’re spending a longer amount of time outside in direct sunlight you need to reapply it regularly, at least every 2 hours, to make sure you are getting full sun protection. If you are swimming or sweating, your sunscreen will wear off more quickly (even if it is labelled water-resistant) and you will need to reapply it every 40 to 80 minutes.

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:

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
Indian Journal of Dermatology, September-October 2012, pages 335–342
Journal of the American Academy of Dermatology, May 2008, pages S149–S154


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