Sunscreen Safety
Last week's recall of five sunscreen spray products (story here) got me thinking about how we use sunscreens, a topic that ultimately illustrates how statistics both saves lives and endangers them. (Is that a melodramatic choice of words? I'll let you decide.)
My focus here is on Sun Protection Factor, or SPF. Most of us are familiar with this acronym, and we know that higher numbers are "better". But what is SPF exactly?
The sunlight that reaches the earth contains two types of ultraviolet radiation that can damage your skin, UVA and UVB. The SPF tells you how much UVB radiation will penetrate sunscreen that has been suitably applied. ("Suitably" means 2 milligrams per square centimeter of skin, as per FDA testing requirements.) A 30 SPF sunscreen allows 1/30 of the UVB radiation to penetrate, a 50 SPF sunscreen allows 1/50 of the UVB radiation to penetrate, and so on.
Unfortunately, many people equate SPF with how much time you can spend in the sun without getting burned. SPF actually only tells you how much UVB exposure you would get, under laboratory conditions, over a certain period of time. That particular amount of time is influenced by a gazillion variables (I counted), including the weather, your skin type, the extent of your exposure to water, how much you're sweating, how well you've applied (and reapplied) the sunscreen, and so on. A thick layer of lower-SPF product can actually offer greater protection than a thin layer of higher-SPF product.
So, my first point about SPF numbers is that they illustrate the need for caution about safety-related statistics. Specifically, about what the statistics tell us. Using your sunscreen's SPF to estimate how long you can spend in the sun is risky at best.
(Stats people may interrupt here to say: An SPF value is just a number, not a statistic. Actually, it's a statistical result – a measure of central tendency derived from groups of people who participated in FDA-mandated testing studies. Thanks by the way to those people for going to labs, allowing scientists to shine UV light on parts of their body that don't usually experience it – i.e., their buttocks – and hanging in there until sunburn is detected. I hope they had soft chairs waiting at home...)
On to more serious matters. We wear sunscreen not just to prevent sunburn, but also because we know that prolonged exposure to sunlight causes aging skin and even skin cancer. Fortunately, sunscreens do provide some protection against premature aging (e.g., wrinkling), as well as skin cancers (particularly melanoma and squamous cell carcinoma, according to a small number of studies). We should thus be using high-SPF sunscreen, right? Well, yes. Sort of...
1. Broad-spectrum sunscreen gives you protection against the two kinds of UV radiation that cause skin aging and cancer. However, as per FDA requirements, SPF only tells you how well a product screens out UVB rays. Longer-wave UVA rays penetrate skin more deeply and also promote skin aging and cancer. (Unfortunately, UVA radiation doesn't cause sunburn the way UVB radiation does, so a product that prevents sunburn doesn't necessarily prevent UVA damage.) In short, higher SPF products shield you more from UVB radiation, but they don't necessarily give you more protection against UVA. They just give you some. How much is not known.
2. SPF values aren't reliable even under laboratory conditions. The Environmental Working Group provides a succinct description of some of the challenges that sunscreen manufacturers face when they apply FDA rules for measuring SPF:
The test methods require someone to determine a change in the skin redness of a small handful of human participants exposed to UV light in a lab. These results may differ based on the evaluator, testing instrumentation or participant skin type....
One study highlighted the potential for variability in SPF. When Procter & Gamble tested a competitor’s SPF 100 product at five different labs, the results varied from SPF 37 to SPF 75. A 1.7 percent difference in light transmission yielded an SPF measurement of 37 instead of 100. The company concluded that a very small difference in test conditions could dramatically affect the calculated SPF. Small differences in application thickness could have a similar effect.
3. Even if SPF values were reliable under laboratory conditions, the real world factors I mentioned earlier (weather, sweating, reapplication time, etc.) would cause fluctuations in the actual level of protection.
So, what should you do? And, what does all this tell us about the impact of statistics? (I mean, what about my seemingly wild claims about lives endangered and saved?)
First, here's what you should do:
1. Slather it on. That is, wear a lot of sunscreen if you're spending time in the sun. How much is "a lot"? It's hard to say exactly. SPF ratings assume 2 milligrams of sunscreen applied to each square centimeter of bare skin. Experts describe that as about an ounce of sunscreen in total, but the actual amount depends on how much skin you have and and much of it you're exposing. In any case, the research seems pretty clear that some sunscreen is better than none at all. And, all else equal, higher SPF is preferable to lower SPF, albeit for only one kind of UV radiation.
2. Don't expect highly specific guidance. Let's say you're going on an errand, or taking the dog for a walk, and it's a sunny day. How many minutes can you safely spend in the sun? Again, that depends on the intensity of the sun, your skin type, how much you're sweating, and a lot of other variables you won't be measuring. (Even if you could measure them, the science doesn't tell us about the long-term consequences of any one instance of sun exposure.)
3. Don't be cocky. Even high SPF products allow some UV radiation to penetrate. The best you can do is to apply sunscreen liberally – and make sure you've purchased a "broad spectrum" product, so that you know you have at least some protection against both UVA and UVB rays.
4. Follow the instructions. (I'm really bad at doing this, but I'm recommending it anyway.) For example, most products tell you to reapply after 80 minutes and, indeed, studies show that you do need to reapply within about 2 hours to maintain full protection. Also, "water-resistant" doesn’t mean "water-proof," so if you're staying in the sun after you swim, reapply. Et cetera.
5. Check out the guide to sunscreens that the Environmental Working Group has published each year for the past 15 years. Although EWG has its biases, this guide includes useful recommendations, warnings, tips, and an admirably thorough description of the methodology used to evaluate product safety.
So, back to my original claim... Surely SPF ratings are saving lives, by persuading some consumers to use higher SPF products, while persuading others (through the legitimizing power of statistics) that if the product has a numerical rating, it must be effective. Some people who have switched from lower to higher SPF sunscreens, and some people who have begun to use sunscreens, must have warded off skin cancer by now. I can't prove this, but it strikes me as a reasonable inference, all things considered, and a good illustration of the value of statistics.
At the same time, it's well-established that the prevalence of skin cancers has continued to increase, even after (a) the first wide-scale production of sunscreens (and FDA regulation beginning in 1974), (b) the introduction of the SPF system in the late 1970's, (c) the creation of the first broad-spectrum and water-resistant products in the 1980's, and (d) growing public awareness of the dangers of sun exposure in recent decades, resulting in increasing use of sunscreens.
So, how is it that during the decades that sunscreens were marketed, improved, and used more widely, the rates of skin cancer continued to increase?
Part of what's happening is that higher SPFs are creating a false sense of security. Several studies have shown that people spend much more time in the sun when using higher SPF sunscreen. But, the actual increase in protection is not as much as the numbers seem to imply...
Look back at the definition of SPF at the beginning of this newsletter. An SPF of 50 would mean that 1/50, or 2% of UVB radiation is blocked. An SPF of 1/100 would mean that 1% of UVB radiation is blocked. In other words, the SPF 100 sunscreen only blocks 1% more UVB radiation than the SPF 50 product. Meanwhile, some degree of UVA exposure is still occurring. (We're assuming too that the sunscreen was appropriately applied and reapplied in the first place.)
Mathematically speaking, what's important here is that SPF represents a logarithmic rather than linear scale. In practical terms, this means that at lower SPF values, a little more sunscreen translates into a big jump in effectiveness, while at higher values, a little more barely influences effectiveness. Here's an example:
—An SPF 80 sunscreen allows 1.25% of UVB radiation to penetrate the skin. Increase the SPF by 20, and you find that an SPF 100 sunscreen allows 1% of UVB radiation to penetrate. Not much of a difference.
—An SPF 5 sunscreen allows 20% of UVB radiation to penetrate the skin. Increase the SPF by 20, and an SPF 25 sunscreen allows 4% of UV radiation to penetrate. A much bigger difference.
In light of these numbers, experts often recommend that SPF 15 or SPF 30 is sufficient. In the EU, Japan, and Canada, the SPF values that can be listed on sunscreens are capped at 50 – other countries use even lower caps – because the fractionally tiny advantages of higher SPF values are far outweighed by the public health risks of overconfidence. Our FDA shares this perspective, and in 2019 it proposed a legislative cap of "60+" on sunscreen products. We'll see soon whether this cap is implemented.
My point here is that simply presenting SPF statistics, without context, promotes unsafe behavior. Here again, I can't prove that a false sense of security has caused some users of high-SPF products to overexpose themselves to sunlight to the point of developing skin cancer, but that seems like a plausible inference from the research. Stats taken out of context can be dangerous. People may find it tedious to figure out what SPF means, but doing so seems worth the effort.
So, if you're going to be in the sun for a while, slather it on. Just don't ask me to define "slather". Or participate in a sunscreen testing study...
Thanks for reading!