Science of Summer: What Causes Sunburns?

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Editor's Note: In this weekly series, LiveScience looks at scientific aspects of the summer season.

Summer means lots of out-of-doors time. Whether at beaches, barbeques, hanging out in the park or at the pool, most people catch more sun rays this season than other times of the year. In the process, some will get a suntan while others, unfortunately, will experience the painful redness, peeling and blistering that can occur with a bad sunburn.

So what is the skin up to when it starts soaking up sunlight and changing its hue this summer? Essentially, a suntan results from the body's natural defense mechanism kicking in against damaging ultraviolet sun rays. When the defenses are overwhelmed, a toxic reaction occurs, resulting in sunburn

The defense mechanism is a pigment called melanin, which is produced by cells in our skin called melanocytes. Melanin absorbs ultraviolet light and dissipates it as heat.

"Melanin is a natural sunscreen," said Gary Chuang, an assistant professor of dermatology at Tufts University School of Medicine. "When your body senses sun damage, what it does is it starts sending out melanin into surrounding cells and tries to protect them and shield them from getting more damage." [7 Common Summer Health Concerns]

Everyone has about the same number of melanocytes, Chuang said, but people vary in how much and what colors of melanin they produce. Darker skinned people have more natural sunscreen at their disposal. Even when getting a boost from artificial sunblock creams and the like, though, people are all ultimately vulnerable to the sun's ultraviolet wrath. 

"It doesn't matter how much sunscreen you have on — if you are lying there forever and ever, some of the radiation will definitely penetrate through," said Chuang. "Even if you have a tan you can get a sunburn, and people with dark skin types can get a sunburn if out long enough."

DNA buster

Invisible ultraviolet light carries more energy than the light visible to humans, and this energy packs a tiny punch.

When a UV photon strikes the skin, it can damage the DNA in the body's cells. It does this by breaking the orderly bonds between the four nucleotides, adenosine, thymine and guanine. So-called thymine dimers form, when two thymine nucleotides bind together, throwing the whole shape of the DNA molecule out of whack.

The cell with the messed-up DNA usually then commits suicide, a process called apoptosis. "The cells receive so much radiation that they undergo apoptosis," said Chuang.

Crimson carnage

The body senses this destruction and over the course of several hours starts flooding the area with blood to help with the healing process. Painful inflammation occurs as well. Usually within half a day of overindulging in the sun, the characteristic steamed-lobster look of a sunburn begins to make itself known, and felt.   

With very bad sunburns, thermal damage in the manner of second-degree burns not unlike that caused from being too close to a fire can set in. The skin blisters as a result, with liquid-filled, protective bubbles forming over areas of tissue damage.

Several days after the initial sun-wrought carnage, dead skin cells in the blasted region will start to peel off. "The cell signals the body that it has received enough radiation and has a chance of becoming mutated, so [the cell says] 'Now you need to die off before it becomes problematic,' and you get that sloughing of the skin," said Chuang.

Sometimes the cells with sun-caused mutated DNA do turn into problem cells, however, that do not call it quits and keep proliferating as cancers. "The UV light causes random damages in the DNA and DNA repair process such that cells acquire the ability to avoid dying," said Chuang.

Skin cancer is the most common form of cancer in the United States. People who allow themselves to get sunburnt repeatedly are at much higher risk. The risk for the deadliest form of skin cancer, called melanoma, doubles for someone who has received five or more sunburns, according to the Skin Cancer Foundation.

Beating back the sun

To avoid skin cancer, as well as the painful nuisance of a sunburn, Chuang advises people to cover up and apply sunscreen liberally.

"The sunscreens I like are physical blockers," Chuang said. In sunblock formulas, look for the ingredients of titanium dioxide or zinc oxide, because they "reflect off UV radiation," he said. [Infographic: How to Read Sunscreen Labels]

Chuang is also is a big fan of hats. "Wear hats," he said. "People think hats are going out of fashion, but they are a very basic physical blocker of the sun."

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