With summer weather and abundant sunshine looming, I thought I’d delve into the mysteries of the sun protection factor (SPF) which appears on all bottles of sunscreen.  Before I get into the details, I’d like to share my favorite sunscreen story.
Some years ago I was shopping with my wife for sunscreen at the Clinique counter at the mall.  My wife described what she wanted and the earnest young sales woman in her white lab-coat style uniform brought out a tube of sunscreen which she extolled as “chemical free.”  These are just the sort of “scientific illiteracy” moments which drive me crazy. In what was perhaps not my best moment I let out a derisive chortle and remarked “What is it, an empty tube?”  I got a blank stare from the sales clerk and a rather disapproving glare from my wife.  Nevertheless I pressed on to inform her that each and every ingredient in the sunscreen was, in fact, a chemical.  After a brief and uncomfortable silence my wife purchased the sunscreen and we left.  Needless to say I am no longer invited on sunscreen shopping ventures.
OK, back to the matter at hand.  The energy from the Sun which reaches the earth is 52% infrared radiation (heat), 43% visible light, and 5% ultraviolet (UV) radiation.  Despite the fact that UV rays account for only 5% of the solar energy, they are responsible for skin damage and cancers.
Sunscreens function by including chemicals which, due to their molecular structures, can adsorb, reflect, or scatter UV radiation.  The active ingredients in sunscreens come in two types, organic molecules which are able to absorb UV radiation and inorganic molecules which can absorb, reflect or scatter UV radiation.  When your skin is exposed to the sun, it responds by manufacturing its own UV absorber, melanin.  Melanin is the pigment that darkens your skin providing a sun tan.  So it is true that tanning does provide some protection from further sun damage, but this protection is not sufficient on its own.
The UV energy from sun comes in two types, UVA and UVB, defined by their wavelength ranges.  UVB rays are what cause sun burns and they can also cause skin cancers.  Since UVB radiation causes sunburn, it was the initial focus for sunscreen products.
The first commercial sunscreen, Gletcher Crème, was developed in 1938 by Franz Greitzer.  It only addressed UVB rays and had an SPF of 2.  Greitzer’s formulas were improved over time and provided the basis for Coppertone sunscreens which became popular in the 1950’s.  Greitzer is also credited with the development of the SPF measurement in 1962.
SPF has traditionally only measured protection against UVB rays and is often mis-described.  SPF is the multiple of the cumulative amount of UVB radiation required to cause sunburn after applying 2 mg of sunscreen per square centimeter of skin area compared to the amount of UVV radiation required to cause sunburn without any sunscreen.  Since discussing cumulative amounts of UVB exposure can be cumbersome, particularly since the amount of UVB radiation reaching the earth surface varies strongly with the angle of the Sun, SPF is often explained in terms of time.  For example, you might read that wearing a sunscreen with an SPF of 15 would allow you to avoid a sun burn for 15 hours versus 1 hour for no sunscreen.  Using time as a proxy for cumulative exposure is not correct.  In an earlier draft of this column I included a paragraph describing the mathematics behind it. When even I found that to be unengaging, I deleted it.  Suffice it to say the use of time as a parameter is incorrect but close enough.
The trend has been for companies to market sunscreens with higher and higher SPFs.  As you can see in the graph at the top of the page, after you reach an SPF of about 30, there is a point of diminishing returns.  In Europe, regulations are being adopted to limit the SPF labels to no more than 50, since SPFs of 50-100 are not demonstrably any better than 50.
Far more important than the selection of SPF, as long as it is >30, is the amount used.  The FDA recommends a usage of 2.2 mg per square centimeter.  This corresponds to approximately one ounce of sunscreen, about enough to fill a shot glass, for an adult in a bathing suit.  Several studies have shown that people tend to use only 25-50% of the recommended amount.  Using an overly thin or incompletely applied layer of sunscreen dramatically reduces effectiveness.  I think people use less than the recommended amount because sunscreen comes in maddeningly small containers which are absurdly expensive.  There is a great business opportunity out there for someone who markets a much larger container of sun screen, with a pump dispenser, at a reasonable price. 
I left the discussion of UVA rays to the end, as they are the most interesting part of the story.  They can cause skin cancer, wrinkles, and other skin damage, but do not give you a sun burn.  I am 46 years old, just about the first generation to start using sunscreen somewhat regularly.  The sunscreen that my generation wore generally only contained UVB protection.  Therefore, the UVA rays were unperturbed on their way into our epidermi and now we all dutifully line up at the dermatologist to have pre-cancerous growths frozen, scraped, and sliced off.
Starting in the summer of 2012, the FDA has defined the term “broad spectrum UVA/UVB protection” to mean that the SPF for UVA is greater than or equal to that for UVB. UVB protection is provided by the organic chemical portion of the sunscreen, while the UVB protection generally comes from an inorganic chemical compounds like zinc or titanium dioxide.
To close let me weigh in on two “Common Science” questions about sunscreen. 

  1. Since your body needs sun exposure in order to produce vitamin D, you often see concerns expressed that overuse of sunscreen will result in a deficiency.  In fact, the amount of sun exposure required to maintain healthy levels of vitamin D is limited.  The AMA recommends 10-15 minutes of non-sunscreen exposure on the arms and/or legs twice a week.  The daily logistics of life should provide more than sufficient sunshine to meet this goal.
  2. The UVB protectants in most sunscreens to degrade and lose effectiveness over time.  So, despite the high cost per ounce, your old sunscreen should be discarded and replaced no less than once a year.

Summer is just around the bend, so go buy (locally!) some broad spectrum UVA/UVB sunscreen, apply 2.2 mg per square centimeter, grab your favorite summer beverage, head outside and enjoy the Carolina summer.
Have a comment or question?  Log in below or send me an e-mail go commonscience@chapelboro.com.