Applying some chemistry

The last few days have been an early taste of summer here in the Northeast, with plenty of sunlight, and temperatures as high as ninety degrees fahrenheit. This has (of course) gotten me thinking about the science of summer, and reminded me of a special topic request (hi, Abby!). Yes, I know its a Wednesday, which means chemistry. However, there is plenty of chemistry out there, and I suppose we could use a summer break from the periodic table. Instead, we’ll take a look at the chemistry of sunscreen. Next week, it’s back to the periodic table and our meeting with the metalloids.

Sunscreen is one of those things we just don’t think too hard about. Oh, sure, plenty of people spend a bit of time pondering just how high an “SPF” to go with (a devil-may-care 15? a moderate 45? a geek-declaring 90?), but how much time do we spend thinking about just what SPF means? In fact, does anyone even know what SPF stands for?

SPF stands for “sun protection factor”. It would be nice if this was some kind of simple, straightforward measurement, but instead, it refers to what percent of incoming UVB radiation (we’ll get there in a minute) the sunscreen in question protects the skin from. Even the “protects the skin from” part is a little bit complicated - all human skin burns in sunlight, but the pigment melanin provides skin with some protection. Skin with higher concentrations of melanin takes somewhat longer to burn, so the FDA divides human skin into six categories, based on melanin content. The SPF scale refers to percent protection offered to the three categories with the lowest melanin content.

 As you can see, the SPF scale also isn’t a straight line. So a sunscreen with an SPF 30 does not protect from twice as much UVB radiation as an SPF 15, and an SPF 60 does not protect from twice as much UVB radiation as an SPF 30. There’s also the time component (or not) - SPF measures how much UVB radiation the sunscreen protects from at a given moment. It does not measure how long the effect lasts. Industry standards  recommend reapplying ever 2 hours, regardless of SPF.

Ok, so that’s how we measure the effectiveness of sunscreen at blocking UVB radiation. What is UVB radiation? And what about that UVA radiation you sometimes see referred to on bottles? Time to revist our old friend, the EM spectrum.

 UV, or ultraviolet radiation, is the wavelength band right before visible light. This means that UV radiation has a wavelength just slightly below that of visible light (in fact, the wavelength of visible light increases as you move from violet to red, hence, ultraviolet and infrared). The sun emits UV radiation, along with visible light (and pretty much everything else on the EM spectrum). The ultraviolet region of the wavelength spectrum is subdivided into four regions - UV vacuum (sometimes further divided into UV vacuum and far UV), UVC, UVB and UVA, in order of increasing wavelength. UV vacuum and UVC are absorbed by the atmosphere, while UVB and UVA make it down to the surface of the Earth, and necessitate the wearing of sunscreen.

 Neither UVB nor UVA is great for human skin. UVB is what actually causes sunburns. Even after the burn heals, that area of skin is at a greater risk for developing cancerous tumors. UVA radiation does not burn, but it does penetrate deeply into the skin, and can have an immune-suppressing effect. UVA radiation contributes to wrinkling and spotting in human skin with age, and can also increase the risk of developing cancer. Clearly, you want to minimize both burns and UVA penetration, hence the sunscreen application. Now, how exactly does sunscreen protect?

Actually, that’s a question with two answers. Sunscreen can either protect from UVB and UVA radiation through physical or through chemical means. Physical sunscreens reflect incoming UV radiation away from the skin, while chemical sunscreens harmlessly absorb incoming UV radiation before it reaches the skin. Titanium dioxide and zinc oxide are the only physical sunscreens out there, but have the effect of providing full or considerable protection from both forms of UV radiation. There are a number of chemical sunscreens out there, each of which absorbs a very small portion of the UV spectrum. 

As the above graph would indicate, an effective entirely chemical sunscreen contains a blend of different chemicals. Chemical sunscreens have a small risk of skin irritation, but are still more popular than physical sunscreens, if only because chemical sunscreens are also transparent. 

 Not the most popular beach look.

Whichever option you pick, as long as its FDA-approved and you follow all of the labels, sunscreen harnesses the power of chemistry (and some physics) to protect your skin. So, slather it on, and celebrate the science of summer!