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Sunscreen that is spread on the skin too thinly may provide less than half the expected degree of protection, a study has shown.
Scientists analysed DNA damage from ultraviolet (UV) rays in the skin of volunteers at various levels of sunscreen thickness.
They found that sunscreen lost much of its effectiveness if applied below the recommended thickness of two milligrams per square centimetre of skin (2mg/cm2).
That is the thickness level on which manufacturers’ sun protection factor (SPF) ratings are based.
But people are known to be far more conservative in their use of the creams and lotions.
Previous studies suggest that sunscreen is typically spread as thinly as 0.8mg/cm2.
The new study showed that at this level of usage, a sunscreen with a high SPF of 50 would at best provide just 40% of expected protection.
Lead researcher Professor Antony Young, from King’s College London, said: “There is no dispute that sunscreen provides important protection against the cancer-causing impact of the sun’s ultraviolet rays.
“However, what this research shows is that the way sunscreen is applied plays an important role in determining how effective it is.
“Given that most people don’t use sunscreens as tested by manufacturers, it’s better for people to use a much higher SPF than they think is necessary.”
Part of the research involved exposing eight fair-skinned volunteers to a blast of UV radiation, simulating sunlight.
Target areas of their skin were treated with high SPF sunscreen of varying thickness, ranging from 0.75mg/cm2 to 2mg/cm2.
Another group of eight received exposures on five consecutive days, to mimic a sunshine holiday experience.
The amount of UV radiation was varied to replicate conditions in different holiday destinations, such as Tenerife, Florida and Brazil.
Biopsy tissue samples showed that repeated UV exposure caused “considerable” DNA damage to unprotected skin areas, even when the radiation dose was very low.
Damage was progressively reduced as the thickness of applied sunscreen increased.
Five days of high dose UV with 2mg/cm2 of sunscreen produced significantly less damage than just one day of low-dose exposure when skin was unprotected.
The findings are reported in the journal Acta Dermato-Venereologica.
Nina Goad, from the British Association of Dermatologists, said: “This research demonstrates why it’s so important to choose an SPF of 30 or more.
“In theory an SPF of 15 should be sufficient, but we know that in real-world situations we need the additional protection offered by a higher SPF.
“It also shows why we shouldn’t rely on sunscreen alone for sun protection, but we should also use clothing and shade.
“An extra consideration is that when we apply sunscreen, we are prone to missing patches of skin, as well as applying it too thinly.”