Applying sunscreen while out in the sun is critical in order to avoid the harmful, DNA-damaging effects of ultraviolet radiation (UVR). However, most of us are applying sunscreen wrong, according to a new study. On average, people are only getting 40% of the SPF protection offered by a correct dosing of the product.

Credit: Pixabay.

There are three primary types of ultraviolet radiation: UVA, UVB, and UVC. UVA stands for Aging because it penetrates deep into the skin and is responsible for premature aging of the skin and skin cancer. Tanning beds can emit two to five times more UVA radiation than the sun. UVB stands for Burning. It mainly affects the outer layers of the skin, causing sunburns, premature aging of the skin, and skin cancer. UVC radiation is the strongest, most dangerous form of UV light. But you don’t have to worry about this latter class of ultraviolet radiation: they’re all blocked by the planet’s atmosphere and never reach your skin.

According to the National Cancer Institute, the rate of new melanoma cases among American adults has tripled since the 1970s, from 7.89 per 100,000 population in 1975 to 22.7 in 2010 (NCI 2015). Of course, some of the increased incidence rates can be attributed to better diagnosis, but overexposure to the sun and indoor tanning also definitely play a major role. This is why most doctors stress that you should always wear sunscreen when you know you’ll be spending your day on the beach. The most vulnerable hours are those between 11AM and 4PM.

The SPF was introduced in the 1960s and for decades, SPF 30 protection seemed like more than enough to prevent surface sunburns. Today, there are companies that sell sunscreen labeled SPF 50, 70 and even 100.

If you’ve always wondered what the heck SPF is in the first place, today’s your lucky day. Basically, the SPF rating indicates how many times a person’s UVB exposure will be reduced once the sunscreen is applied. For instance, if it would take 15 minutes in the sun for your skin to get burned, wearing SPF 15 will extend the window 15 times, meaning it now takes three hours and 45 minutes for your skin to feel the same effect. A sunscreen with an SPF 100 index means the same person would be protected for more than 24 hours.

The formula below explains how the SPF index works.

SPF INDEX x the time it takes to burn = time needed to receive the same dose of UV you would have gotten otherwise 

The effectiveness of sunscreen, however, is highly dependent on applying the product correctly — and most of us are using too little of it.

Generally, you should apply around 2 mg of sunscreen per centimeter squared to reap all the protective benefits. That’s about twice as much people typically apply on their skin, according to Antony Young, a professor of experimental photobiology at King’s College London. In fact, a previous study found that the large majority of people apply about a third of the recommended sunscreen dosage.

To get an idea of how important the sunscreen layer’s thickness is for effective SPF protection, imagine that applying a 0.75 mg/cm^2 layer of SPF 20 sunscreen is equivalent to the protection of an SPF 4 product.

Young and colleagues performed a series of experiments that measured the real-world effects of sunscreen during conditions typically seen during a holiday in popular vacation destinations, such as Tenerife or Florida.

There were two groups of volunteers, each comprised of three women and five men. One group received a single UVR exposure to areas of the skin where sunscreen of varying thickness was applied. The other group received five consecutive days of UVR exposure; each day, the volunteers received a different level of exposure and used a different amount of sunscreen.

When sunscreen wasn’t applied at all, biopsies of regions of the skin exposed to UVR showed considerable DNA damage. That was far from surprising, but on the other hand, what was unexpected was just how much of a difference the amount of sunscreen can make.

DNA damage was only slightly reduced when sunscreen was applied with a thickness of 0.75 mg per centimeter squared, but was considerably reduced when 2 mg per centimeter square was used. The findings applied to the group that received the most UVR exposure, as well.

To get an idea just how important sunscreen is, the study found that five days of exposure to high-intensity UVR rays when sunscreen was applied were less damaging that one day of low UVR exposure without sunscreen on, the researchers reported in the journal Acta Dermato-Venereologica.

The study only involved 16 people, which is a small sample size, but the findings have a high level of significance.

But let’s talk a bit more about SPF. For each minute wearing an SPF 30 sunscreen lotion, for instance, you receive 1/30th, or 3.33%, of the UV exposure that you would have gotten without the lotion. This means SPF 30 protects you from 97% of UVB rays. SPF 80 blocks another 1.75 percentage points of UVB radiation, while SPF 100 blocks 99% of UVB rays. So the difference between SPF 30 and SPF 100 is only 2.3% — a marginal improvement, but confusingly enough, this isn’t what you’d think when choosing lotions based on the SPF index.

However, bearing the new findings in mind, people who’ve bought a high-SPF sunscreen have done themselves good (although I still believe SPF 100 is overkill).

“This research demonstrates why it’s so important to choose an SPF of 30 or more,” said Nina Goad from the British Association of Dermatologists, in a statement.

“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 takeaway is simple: never forget to use sunscreen when you’re at the beach, and make sure to apply the right amount while you’re at it. When in doubt, put on much more than you usually do.