Sunscreen: how to use it so it actually works

The SPF number is measured with a thick layer

Laboratory testing uses about 2 mg of product per square centimetre of skin. In real life most people apply roughly a third of that, so an SPF 50 behaves more like an SPF 15. Applying more matters more than buying a higher number.

How much to use

About a teaspoon for the face and neck, and roughly a shot-glass amount for the whole body in swimwear. Reapply every two hours outdoors, and after swimming, heavy sweating or towelling.

What sunscreen does and does not do

It reduces sunburn, photoageing and the risk of skin cancers. It does not make unlimited sun exposure safe, and it should be combined with shade in the middle of the day, a hat, and clothing, which are more reliable than any cream.

Vitamin D concerns

Regular sunscreen use has not been shown to cause vitamin D deficiency in real-world studies. If levels are low, supplement rather than choosing to burn.

Check your skin

Once a month, look for a mole that changes shape, colour, size, or that itches or bleeds. Anything new and evolving after age 40 deserves a dermatology opinion.