How Sun Protection Factor (SPF) Works

We all recognise the importance of protecting skin with SPF products. But understanding how they actually work is a little more complicated. In this article, we’ll explain what SPF is, how long it lasts and the different types of sunscreen available.

UVA and UVB Rays

The sun's radiation consists of UVA and UVB rays, which damage the skin in different ways. UVB rays only reach the skin’s surface and cause sunburn, while UVA rays can penetrate deeper into the skin and cause DNA damage.

Sun Protection Factor

Sun Protection Factor (SPF) measures how much UVB protection a product provides skin. A higher SPF blocks more of the sun’s rays. A product containing SPF 30 will protect skin from almost 97% of the sun's UVB rays (when applied liberally). The SPF is a guide to how long you can stay in direct sunlight before your skin starts to burn.

To work this out, you’ll need an idea of how many minutes it takes your skin to turn pink without sun protection. If you normally burn after 10 minutes of unprotected sun exposure , you multiply that number by the SPF rating that you are using. That is how long your sun protection will last. For example, if you burn after 10 minutes of unprotected sun exposure and you are wearing SPF 30 you will get 5 hours of sun protection (10 minutes x 30 = 5 hours).

While UVB rays are responsible for visible sun damage, you also need to protect skin from UVA rays. UVA rays are present all year-round, whatever the weather, and can penetrate windows and glass. This makes it really important to choose products with a broad-spectrum sunscreen that shields skin from both UVA and UVB rays.

Mineral and Synthetic Sunscreens

Synthetic sunscreens, like homosalate or avobenzone, absorb UV radiation in your skin. They need to be fully absorbed into your skin to work so make sure you apply them at least 20 minutes before you go outside. Synthentic sunscreens tend to have a lightweight texture, making them a good choice for combination to oily skin or skin that is prone to breakouts.

Paula’s Choice products with synthetic sunscreen:


Mineral sunscreens, like titanium dioxide or zinc oxide, work by reflecting UV radiation as it hits your skin, a bit like a mirror. They work immediately after application, so you don’t need to wait before going outside. If you have sensitive skin, mineral sunscreens are the best choice as they are less likely to cause irritation.

Paula’s Choice products with a mineral sunscreen:


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 re-apply 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.

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