In the past, research has warned that although most of us know that we should apply sunscreen for protection before going out in the heat of summer, we don't know how to apply it correctly.
This is a worrying thought; though we may think we have ensured our skin's health and safety, our protective measures are actually ineffective.
And, a new study from King's College London in the United Kingdom — now published in the journal Acta Dermato-Venereologica — has drawn a similar conclusion, suggesting that people typically apply too little sunscreen for it to be at all helpful.
"There is no dispute that sunscreen provides important protection against the cancer-causing impact of the sun's ultraviolet [UV] rays," says study author Prof. Antony Young.
"However, what this research shows is that the way sunscreen is applied plays an important role in determining how effective it is."
Prof. Antony Young
Are you applying enough sunscreen?
The researchers assessed the effectiveness of sunscreen, as people usually apply it, by checking for DNA damage in the skin of study participants.
Typically, sunscreen manufacturers advise applying the cream with a thickness of 2 milligrams per square centimeter (mg/cm2) of skin, and this is the amount that the creators use as a "landmark" when they establish the product's sun protection factor (SPF) rating.
However, when we apply sunscreen, it is hard to estimate exactly how thickly, or how well, we apply it, which impacts its effectiveness.
In order to test exactly what happens when people apply sunscreen in their usual manner, the scientists worked with 16 volunteers — six women and 10 men — with fair skin (who are likely to be more sensitive to UV damage).
The researchers divided the participants into two groups, each numbering three women and five men. One group was exposed to UV radiation (UVR) simulating sunlight and asked to apply high SPF sunscreen in various degrees of thickness — from 0.75–1.3 mg/cm2 to 2 mg/cm2 — on different skin areas.
Participants in the second group received UVR exposure for 5 consecutive days so as to simulate the kind of exposure to summer sun that one would experience while on holiday, for instance.
Moreover, to complete the simulation, the amount of UVR was varied in order to mimic the different conditions that one might encounter in different popular destinations, such as Florida, Brazil, or Tenerife.
Best to use a higher SPF lotion
Prof. Young and team collected skin samples on which they performed biopsies. This revealed that, in the case of participants who had been repeatedly exposed to UVR, the skin areas on which no sunscreen was applied showed significant DNA damage despite the low dose of radiation the scientists had used.
For skin areas on which sunscreen had been applied thinly — at 0.75 mg/cm2 — DNA damaged was reduced. Furthermore, it was significantly reduced in skin on which sunscreen had been applied thickly — at 2 mg/cm2 — even in the case of exposure to higher doses of radiation.
After 5 days of high-dose UVR exposure, skin on which protective lotion had been applied at the recommended thickness of 2 mg/cm2 had significantly less DNA damage than skin exposed to just 1 day's worth of low radiation, but without any sunscreen protection.
In a nutshell, the researchers found that sunscreen with a high SPF (SPF 50), if applied thinly — the way individuals often do — would actually provide no more than 40 percent of the expected protection at most.
For this reason, Prof. Young and colleagues urge people to use sunscreen with a much higher SPF than they normally think they need to ensure that they get the right amount of protection, even if they don't apply the lotion in a thick enough layer on the skin.
"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," he advises.
"This research demonstrates why it's so important to choose an SPF of 30 or more," adds Nina Goad, from the British Association of Dermatologists.
"In theory," she notes, "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."
That is why, Goad urges, it is equally important to protect ourselves from the sun's rays using other means, such as wearing hats and appropriate clothing, as well as seeking refuge from direct sun.