Q. I don't understand all the different labelling on sun products or the difference between chemical and physical screens?

A. It’s tricky. So here’s the lowdown from consultant dermatologist Dr. Stefanie Williams of Eudelo, Dermatology and Skin Wellbeing Clinic (eudelo.com).   

SPF stands for Sun Protection Factor but it only tells you about protection against UVB rays, the ones that burn (think B for burn). SPF ratings go from 6-50, but most dermatologists recommend not going lower than 15, 20, or 25, which count as medium protection. SPF 30 is high, SPF50 very high.

The ratings tell you how much longer you can stay in the sun without burning. So if you have a fair skin that burns after ten minutes, SPF15 should allow you to stay in the sun for 150 minutes.

But very importantly, the SPF is only correct if the product is applied in the same way as when it’s tested. That means applying it evenly in a thick layer, not rubbing it in, and reapplying frequently. Most people apply less than half the recommended amount so we hardly ever reach the factor on the packaging, which is why many dermatologists recommend using a medium to high/very high SPF. 

UVA, the ageing ray, is usually represented by a star system, from 1 to 5 stars. However, the stars only indicate a ratio between the levels of UVA protection to UVB protection in a product. The safe option is to go for a medium to high SPF with four or five stars.

There’s another pair of ratings just to confuse things further.  The PA System from Japan (more widely used in Asian countries) measures the level of UVA protection in a product, based on PPD or persistent pigment darkening.  In the same way as SPF measures UVB-induced redness/burning of the skin, so PPD measures UVA-induced darkening/tanning.  But there is a catch. While Dr. Williams feels the PA System is the best approach, it can’t be measured as precisely as SPF. If you do find a product labelled with PA and a plus sign, look for PA+++, she says, with an SPF of at least 30-50.

Another point: no sunscreen is 100% waterproof and 85 per cent of a product can be removed with towel drying so always reapply a sun prep after swimming, and every two hours if you are in the sun.

Now to chemical or physical filters/screens: ones that contain synthetic chemicals work by absorbing the harmful UV rays when they enter the skin. Then they convert them to infrared, which is less harmful, and send them out again. Physical screens use natural agents like zinc oxide or titanium dioxide, which form an actual (physical) protective layer on the skin and reflect them away.

If you have a touchy skin, it’s usually thought best to opt for a physical screen. These have tended to leave a white veil in the past but more sophisticated formulations and technology mean this is seldom a problem nowadays.

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