A new study from King's College London assessed just how much sun protection people actually receive, based on typical use.
Most people don't receive the full UV radiation blocking benefit of sunscreen, because they apply it more thinly than manufacturers recommend.
Results showed that sunscreen with a sun protection factor (SPF) of 50, applied in a typical way, would, at most, provide 40% of the expected protection. The researchers suggest we use a much higher SPF sunscreen than we consider necessary and be careful about how we apply it, to ensure we're protected from sun damage. It’s suggested an SPF of 30 or is better (ideally SPF 50) to deal with “real-world” situations and not to rely soley upon sunscreen for protection. We should also use clothing, hats and shade and remember to be careful outside, especially between 11am and 3pm.
When using lotions, as the bare minimum, the British Association of Dermatologists recommends you should to apply at least six full teaspoons (approximately 36 grams) to cover the body of an average adult, which is more than half a teaspoon of sunscreen to each arm and the face/neck (including ears), and just over one teaspoon to each leg, front of body and back of body.
The overall message in terms of sunscreen use is “more is better.” It's also easy to forget to reapply sunscreen as often as necessary. Apply sunscreen 15 to 30 minutes before going out in the sun to allow it to dry, then again shortly after heading outdoors to cover any missed patches and to make sure you’re wearing a sufficient layer. Reapply it at least every two hours, and immediately after swimming, perspiring and towel drying or if it has rubbed off.
"Water resistance" is tested by the ability of a sunscreen to retain its sun protection properties following two 20 minute intervals (40 minutes total) of moderate activity in water. However, up to 85% of a product can be removed by towel drying, so you should reapply after swimming, sweating, or any other vigorous or abrasive activity. Another important factor is the reflection of the sun’s rays, which can greatly increase the power of the radiation, by the following percentages: snow up to 85% increase, sand up to 17% increase, water up to 5% increase.