How Sunscreen Works (And Why You're Wrong About It)

Your skin shouldn't look like a package of pork cracklins after spending the day outdoors; that's why we invented sunscreen. However, there's a right way and a wrong way to slather on your protection—screw it up and you could get burned.

It's not the visible light from the sun that causes your skin to crisp, but rather the invisible ultraviolet waves. As assistant professor of dermatology and director of photomedicine at Tufts University School of Medicine, Jeffrey M. Sobell, explains:

A sunburn—manifested by cutaneous redness, swelling and pain—is an acute toxic reaction caused by exposure to the sun's ultraviolet radiation. Although the precise mechanism by which a sunburn occurs has not been clearly identified, complex chemical reactions and pathways take place that most likely result in the clinical symptoms.

The energy from ultraviolet radiation can damage molecules in the skin, most importantly DNA. One consequence of this is the synthesis of different proteins and enzymes. The effects of these proteins, notably prostaglandins and cytokines, lead to dilation of the cutaneous blood vessels and recruitment of inflammatory cells. This, in turn, produces a sunburn's characteristic redness, swelling and pain. Once the signal of excessive radiation exposure is initiated, it generally takes four to six hours for these proteins to generate. Sunburn symptoms thus don't appear until well after exposure. (DNA damage can also result in the destruction of the involved skin cell. This is one of the reasons why skin peels after a bad sunburn.)

Ultraviolet radiation arrives most often in three distinct wavelengths, designated UV-A, -B, and -C. UV-A has the longest wavelength at 400 nm - 320 nm and therefore penetrates both the ozone layer and glass as well as deeper into your skin than the others, causing age spots and wrinkling. UV-B, with a wavelength of 320 nm - 290 nm, is partially blocked by the ozone layer and cannot travel through glass. This wavelength is the primary cause of sunburns and the targeted range for protection, though Broad-spectrum sunscreens will inhibit both UV-A and UV-B. UV-C has a wavelength of 290 nm - 100 nm, though it is completely blocked by the ozone layer and only an issue if you use tanning beds.

To block these harmful solar emissions, sunscreens employ a mix of organic and inorganic ingredients that either reflect, scatter or absorb the radiation and dissipate it as heat. Titanium dioxide and zinc oxide are two of the most commonly used inorganic materials used to physically block UV rays from reaching the skin. Previously, these ingredients would appear white (see below), however modern micronizing techniques break down the particles so as to be invisible without diminishing their functionality.

How Sunscreen Works (And Why You're Wrong About It)

These inorganic compounds are supplemented with UV-B absorbing organic chemicals such as cinnamates, octyl methoxycinnamate, para-aminobenzoic acid, and Benzophenones, that convert the incoming energy into harmless waste heat.

Together, these compounds provide a level of protection measured in factors. That is, if your baseline, unprotected time limit for sun exposure is say 30 minutes before burning, an SPF (sun protection factor) 30 sunscreen would allow you to remain outside for 30 times that duration, or 15 hours. If your baseline is 15 minutes, an SPF 30 would only hypothetically provide 7.5 hours of protection.

Of course, these products only work if you actually put it on and keep it on. Unfortunately, many folks fail to realize that fact and are putting themselves at long term risk for skin cancer and freckles by not doing so. These are a few of the most common sunscreen misconceptions:

1) I'm naturally tan, so I don't need sunscreen

Hahaha, no. The American Cancer society reports more than two million people are diagnosed annually with skin cancer. Roughly 76,000 of them will be cases of invasive melanoma, skin's deadliest cancer, and it will kill an estimated 9,200 people. It's not just the pastiest of us that are at risk, all skin burns eventually. Naturally darker skin simply offers a higher baseline protection factor, not an unstoppable UV barrier.

Instead, slather on a broad-spectrum sunscreen with an SPF of at least 30, water resistant if you've got it. The SPF 30 will block 96 percent of the harmful incoming rays and the water resistant properties will prevent it from washing away as you sweat.

"To cover your whole body, you would have to fill a shot glass," dermatologist Dr. Jennifer Stein of New York University Langone Medical Center said. It's not as though you need to apply it to every crack and crevice, but be sure to get the tips of the ears and use lip balm with a sufficient SPF.

2) I got my face, neck, legs, and forearms this morning, I'm good for the day

Sunscreen does not last all day. "The general principle is to reapply every two to four hours," dermatologist James Spencer, MD, of St. Petersburg, Fl, told WebMD. "Sunscreen does go away with time."

In addition to touching up your exposed bits every couple of hours, you should try to cover up as much as possible with long sleeves and broad-brim hats and avoid being under the sun when it's at its midday peak between 10am and 2pm.

3) It's cloudy out, I'm fine

The weather makes little difference with overcast conditions blocking as little as 20 percent of the inbound radiation. What's more, geographic features like snow, sand, and water can all reflect as much as 70 percent of the sunlight that hits them and higher altitudes actually increase UV exposure (yes, because you're closer to the Sun). As such, you shouldn't rely on the local forecast to protect you.

4) Anything higher than 30 is a waste

That's partially true, but don't count out the higher stuff entirely. SPF 90 only provides a three percent boost in protection over SPF 30. "You get 99 percent sunburn protection with SPF 90, versus 96 percent with SPF 30," says Howard Sobel, M.D., of Lenox Hill Hospital in New York City.

For most people, that extra bit of protection may not be all that necesssary. However for exceptionally fair skinned folk that extra three percent over a lifetime really adds up, and could determine whether or not you come home looking like Zoidberg.

How Sunscreen Works (And Why You're Wrong About It)

[WebMD - Women's Health - CNN - Library of Congress - American Assoc. of Dermatologists - EPA - About - hat tip: thanks Travis B - Images: haveseen / Shutterstock]