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Ask A Dermatologist: What Is SPF?

SPF labelling has plenty of myths around it. Skin + Me’s resident expert, dermatologist Dr Malvina Cunningham is on hand to explain what SPF actually means.

What does SPF really mean?

Dr Malvina explains, “SPF stands for sun protection factor, and is the measure by which sunscreens are rated for their protection against sunburn. SPF is measured in a lab by comparing the amount of time it takes UV radiation to cause mild sunburn versus the time it takes with sunscreen applied. When it takes 10 minutes to burn without sunscreen and 150 minutes to burn with sunscreen then the SPF of that sunscreen is 15 (150/10).” 

What does “broad spectrum” SPF mean?

If you’ve come across the term broad-spectrum, you probably know that it’s what we’re supposed to be wearing, but possibly not what it actually means. Once again, Dr Malvina explains…

“SPF is mostly a measure of UVB radiation but it does not tell you about UVA protection. To ensure adequate protection for both, choose a broad-spectrum sunscreen and look out for the star rating (choose at least 4 out of 5 stars) or the UVA sign in a circle, this indicates it has a European seal of approval and protection is at least a third of that of UVB.”

Is there anything else we should know about SPF labelling?

Unfortunately, yes. Dr Malvina explains the big problem with how SPF is measured: “SPF labelling is based on applying an arbitrary number of 2mg/cm2 of sunscreen under laboratory conditions. 

2mg/cm2 of application is equivalent to applying a quarter of a teaspoon of sunscreen to the face. This means a 30g gram tube should last you about 3 weeks if applied at that quantity to the face only. One teaspoon is adequate for the face and neck, two teaspoons for the front and back torso, 1 teaspoon for each arm and 2 teaspoons for each lower leg. Sunscreen should be applied 15 to 30 minutes before you go outside and reapplied every two hours.”

How can I best protect my skin from the sun?

As Dr Malvina has explained, while SPF is essential, no amount of sunscreen will fully protect you. As such, good sun behaviour is something we should all be practising. This means wearing sunscreen daily, staying out of the sun during the hottest part of the day and wearing protective clothing and hats. Skin + Me has plenty of recommendations for sunscreens that will work for all skin types – and if you’re oily you might even be able to save by skipping moisturiser in the morning.