The Grapevine

How You Apply Sunscreen Decides How Much Protection It Provides You

The level of protection you have from sun exposure depends on how you spread sunscreen across your skin. Researchers from King's College London, England conducted an experiment to find out the thickness of product people should aim to use. 

The study titled "Sub-optimal Application of a High SPF Sunscreen Prevents Epider­mal DNA Damage in Vivo" was published in the journal Acta Dermato-Venereology on July 24.

"There is no dispute that sunscreen provides important protection against the cancer-causing impact of the sun's ultraviolet rays," said report author Antony Young, a professor at King's College London. "However, what this research shows is that the way sunscreen is applied plays an important role in determining how effective it is."

As part of the study, 16 fair-skinned people were recruited and split into two groups of three women and five men each. Varying levels of sunscreen and UV radiation (UV ray) exposure were tested on each group over the course of the experiment. In the end, skin samples were tested to measure DNA damage.

UV ray exposure spread across five days could still cause considerable DNA damage on parts of skin without sunscreen. The damage was reduced in the areas of skin where sunscreen was applied at a thickness of 0.75mg per cm2, while adequate protection was found in areas using 2mg per cm2 of sunscreen.

Sun protection factor (SPF) of 50, when applied in a typical way, was found to provide 40 percent of the expected protection. 

"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," Young said.

The findings showed applying sunscreen at a thickness of 2mg/cm2 could lead to significantly reduced damage across five days compared to just one day of UV ray exposure without sunscreen.

As the United Kingdom and several other regions are in the midst of a heatwave, the study offered a reminder on why sunscreen alone may not be enough for sun protection. 

recent study from the United States also supported this recommendation after finding fair-skinned people were particularly vulnerable to UV damage if they only used one method of sun protection. Apart from sunscreen, one can consider wearing a hat and protective clothing or spending as much as possible time under a source of shade.

Nina Goad of the British Association of Dermatologists recommended choosing an SPF of 30 or more when purchasing sunscreen. 

"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." 

The important takeaway from the findings, Goad stated, was to practice applying the product evenly and in the correct quantity.

"An extra consideration is that when we apply sunscreen, we are prone to missing patches of skin, as well as applying it too thinly," she said.