The Application of Sunscreen Advice

In spite of being a part of the high school graduating class of 1997, I occasionally forget the proverbial commencement advice of Mary Schmich, which was made popular by Baz Luhrman two years later in his song ‘Everybody’s Free (To WearSunscreen)’.  But since moving to Arizona 7 years ago, I have taken the part about sunscreen to heart:

“The long-term benefits of sunscreen have been proved by scientists, whereas the rest of my advice has no basis more reliable than my own meandering experience”. 

True indeed.  But perhaps not as simple as it used to be.  As I reached the bottom of yet another tube of SPF 30 this summer, I did some shopping around to find a replacement.  Options abound these days.  There is a whole gamut of SPF values to choose from.  How much UV-A vs. UV-B does one need?  And what about active ingredients?  Much of the marketing is lost on the average consumer.  So here is my attempt to demystify some of the science behind the labels.

UV-A and UV-B

Once upon a time in 1665 England, a 23 year old Isaac Newton proved with a prism that the colors of the rainbow are components of sunlight (and NOT artifacts of the prism).  What we have learned since then is that light exists as a wave (while also being a particle, but we won’t get into that for now), and depending on how big the wave of light is (wavelengths), we get different colors.  And there are wavelengths of light that go way beyond and before the small rainbow of ‘colors’ that we can see.  This is where UV light fits in – it is how we classify a range of invisible wavelengths of light that come just before the wavelengths for ‘violet’.  First UV-A, then UV-B.  If this sounds confusing, then hopefully Figure 1 will be worth another 1000 words of explaining.  Anyway, there is a tradeoff between UV-A and UV-B in terms of sun protection: UV-B is more damaging, but UV-A is about 20 times more abundant.  This is because our friend the ozone layer filters out most of the UV-B (and thankfully all of the UV-C, so we don’t need to discuss that as long as we stay on good terms with the ozone layer).  And so in a nutshell: since both UV-A and UV-B can cause skin damage and cancer in their own ways, a good sunscreen should protect against both (i.e., ‘broad-spectrum’).

Figure 1. A part of the wavelength spectrum of light. UV-A is designated as the range between 315-400 nm, and UV-B is 280-315 nm.  Visible wavelengths manifested as colors are between about 400-700 nm.

Active Ingredients

There are basically two types of sunscreen: those with organic active ingredients, and those with mineral (or inorganic) active ingredients.  Sometimes the former is referred to as ‘sunscreen’ while the latter is dubbed ‘sunblock’.

Unlike when applied to foods, the term ‘organic’ in the context of sunscreen simply means that it is carbon based.  The active ingredients in this class of sunscreen will contain molecules with seemingly abstract names like octocrylene, octyl methoxycinnamate, octyl salicylate, and oxybenzone to name a few.  Octocrylene tends to be the most common, and it is represented here in Figure 2.  The others are similar.  See all the carbon atoms?  The ones with double bonds are important here.  These act like a net to filter or ‘screen’ out UV sunlight by absorbing it.

Figure 2.  The octocrylene molecule, also known as 2-ethylhexyl 2-cyano-3,3-diphenyl-2-propenoate.  Hydrogen atoms are not shown.  UV is absorbed by double bonds between carbons, particularly in the rings.

Mineral sunblocks work in a more intuitive fashion.  These are creams that contain microscopic metallic particles that are either titanium dioxide (TiO₂) or zinc oxide (ZnO).   In principle, the metallic particles in sunblock creams work together to reflect, or ‘block’, UV sunlight the same way as those large reflective car sunshades do.  Because they reflect the light, titanium and ‘zinc creams’  appear white rather than clear when applied.  But this is changing.  The metallic particles are getting smaller  – small enough to be considered sub-microscopic nanoparticles (less than 100 nanometers in diameter).  Nanoparticles are so small that they tend to absorb light like the molecules in organic sunscreens do, rather than reflecting it.  Consequently, nanoparticle creams now can also appear clear when applied.  Because nanoparticles are new and quickly becoming more common in sunblocks (and other cosmetics), some concerns about their safety have been raised and are worth noting.  But bear in mind that other concerns exist regarding some active ingredients in organic sunscreens as well.  For now it has been established that the general benefits of using any sunscreen far exceed any specific risks.

Sun Protection Factor (SPF)

Ever since sunscreen manufacturers began placing a cryptic value on their products in the 1960’s as a means to quantify how much sunburn they prevent, the SPF values of various products have been perpetually escalating.  Neutrogena finally broke triple digits in 2009 by offering the first SPF 100 sunscreen in the U.S.  At this point, the FDA decided that things were getting ridiculous and stepped in to propose that, among other things, we stop counting SPF beyond 50.  Why?  Unfortunately, in order to understand how SPF values are quantified you practically need two semesters of calculus and a philosophical understanding of Zeno’s Paradox.  But check out Figures 3 and 4 instead.  You’ll notice that the amount of UV that is unblocked by sunscreen becomes increasingly insignificant anywhere beyond about 30.  So when you are at the beach and you notice that your friend’s sunscreen SPF is twice as high as yours, relax.  As long as you are using at least SPF 30, you’ll be just fine.  Although a higher SPF would in theory last longer, this is not always the case in practice due to variables such as application, water, sweating, and variations in quality from product to product.  So in conclusion, just remember to apply and reapply – or cover up!

Figure 3.  A plot representing SPF values vs. the percentage of UV rays blocked.  Because the increase is exponential and not a linear relationship, the difference between SPF values becomes increasingly insignificant.

Figure 4.  The significance of SPF can be viewed in this manner as well.  While it is true that SPF 30 permits twice as much UV to pass through as SPF 60, this perspective can be deceiving if percentages are not taken into account.