Know Your UV Numbers

New research shows effectiveness of sunscreen depends on the thickness of the applied layer

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It’s common knowledge that exposure to the sun can cause skin cancer. And you know you should wear sunscreen when you go out in the sun. But do you also know that the effectiveness of sunscreen depends on the thickness of the applied layer?

Many studies have shown that we typically apply sunscreen at a layer thinner than the recommended two milligrams per square centimeter. This results in a significant reduction of the sun protection factor, or SPF, that a product promises, and increases the likelihood of associated health problems.

A recent study observed 40 subjects applying an SPF 50+ lotion at a thickness of 0.75 milligrams per square centimeter, which is typical of how much consumers use, and found that they were only getting an SPF of about 20.

“You will get protection from DNA damage even if you apply at a lower concentration than the SPF tested by manufacturers. But the thicker the application is always better,” said Antony Young, the study author and experimental photobiologist at King’s College London.

The study also looked at molecular changes in the dermis. The subjects were exposed to simulated solar radiation at two intensity levels that mimic the amount of sunlight that one receives in a normal day, and spending a day at a beach or in a tropical environment.

It was found that when people wore sunscreen at the recommended thickness, they would get a quarter of the damage to their skin under the most extreme condition.

“People tested were Caucasians with skin types which are most sensitive to sunburn. It means they have the greatest risk of skin cancer. If they receive that dose without sunscreen, it will burn their skin. So it’s a very significant reduction of damage,” Young said.

Inadequate protection from the sun can cause health problems not only for Caucasians, but those with a darker complexion. Ultraviolet light from the sun is one of the proven causes of skin cancer. And the incidence rate of the disease has been rising rapidly worldwide.

While the reported cases of skin cancer were lower in Hong Kong than in the West, the numbers are increasing, possibly because of increased participation in outdoor activities and an aging population.

Data of the Hospital Authority’s Hong Kong Cancer Registry show that new patients almost doubled between 1997 and 2007, rising from 430 to 815. Non-melanoma skin cancer, which is linked to sun exposure and artificial tanning, is the most common type of skin cancer.

“The sun causes many effects to the skin – sunburn, tanning, immunosuppression, inflammation, and molecular changes and damage to DNA. They are the reasons that sun exposure has a long-term health effect, such as skin cancer and photoaging,” Young said.

“While some may regard photoaging as a cosmetic problem, a lot of evidence suggests that it plays a role in skin cancer. A factor that causes the majority of skin cancers is damage to DNA, resulting in a particular type of lesion called cyclobutane pyrimidine dimer.”

Most people, except xeroderma pigmentosum patients, have a natural capacity to repair these lesions. But it takes time as it has a half-life of 33 hours, and can accumulate under repeated sun exposure. If our body has to repair 100 such lesions, it means you have to stay shielded from the sun for roughly 13 days.

“People with xeroderma pigmentosum lack the ability to repair DNA. The consequence of this is their incidence of skin cancer is thousands of times greater than normal. They will get skin cancer at about 10, possibly die at about 30, if they do not avoid the sun,” Young said.

“In normal cases, our cells can efficiently repair DNA damage caused by the sun. However, this ability declines with age, and is not 100 percent foolproof. When the fail-safe mechanism fails, you only need one failed repair to cause a problem.”

Skin cancer has a low mortality rate. It can be spotted and treated early but the most important first step is prevention. Sunscreen is a cheap and effective choice.

“Use a high SPF product even if you don’t theoretically need SPF 50 or 70. Do a couple of applications within a space of 20 minutes. That way you are building up the thickness to get the real SPF,” Young said.

“And if you miss a gap, you are more likely to cover that up.”

Be careful when you use certain products that claim to have specific protection against ultraviolet A or UVA or UVB. UVA is more abundant in the atmosphere and goes deeper into the skin, resulting in more cell damage; UVB is much more effective in causing sunburn.

While a product that only blocks UVA might be suitable for inactive people who wear sunscreen to avoid their complexion from getting darker, it is not appropriate for outdoor use. And the product that only blocks UVB can take away the burning effect but you will still get high doses of UVA.

The article first appearedĀ in the Standard on July 11, 2017.

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