Thursday, August 30, 2012

Sunscreen: Let Me Tell You About It

While reading this article today ("Sunscreen Myths And Misconceptions",) it occurred to me that writing my own post on the topic is long overdue. As a sunscreen disciple who has done a lot of research into ingredients and effectiveness, I've become very confident in my own sun protection, and dispense advise (both solicited and not) frequently to those around me.

I don't feel that I need to begin by justifying why sunscreen is good for you. Everything I've read that suggests otherwise is pseudoscientific bunk. Similarly, I don't want to spend a lot of time talking about cancer risk, since, again, the only people that deny that sunscreen decreases your risk of skin cancer are woo practitioners. I'd much rather delve into trying to break down some of the technical aspects of sunscreen itself and try to make it easier for neophytes and non-chemists to understand (yes there may have just been a GPB reference there.) If you're interested in the pesudoscience/cancer discussion, the link above does a good job debunking some of the myths behind anti-sunscreen proclamations.

First up, I want to talk about the difference between UVB and UVA, and what they mean for your skin.


In the most simplified terms, UVB gives you a visible, red, sunburn. UVB penetrates the outer-most epidermis only, and produce more severe immediate damage to those layers. UVA penetrates more deeply, and it will very rarely produce a red burn, but it primarily contributes to tanning and longer-term indicators of damage, like wrinkles and spots. Tans, though perceived as attractive, are actually evidence of sun damage, but since they aren't painful like sunburns, we tend not to think of them this way. (I should note that when I say "tans are sun damage" I am of course NOT referring to skin that is of a naturally darker complexion; I'm talking about the changing of one's complexion.) Both types of rays will contribute to skin cancer risk, especially over time; though, generally speaking, burns caused by UVB increase risk more immediately. This is a nice segue way to:

Probably everyone has heard of Sun Protection Factor (SPF); it's the nice number on your sunscreen that tells you how good the protection is. The SPF number informs you how strong the protection is with the following formula:

SPF X absorbs 1 - 1/X of UVB rays.
So for example, SPF of 30 absorbs (1-1/30), or about 96.7% of the sun's UVB rays. SPF 55 absorbs 98.2% of the sun's UVB rays. SPF 100 absorbs 99% of the sun's UVB rays. This is all why people say there isn't much of a point in spending more money on some of the higher SPF sunscreens beyond 45 or 50 -- because there isn't much of a difference between 98.2% and 99.0%, even though the 100 SPF is almost twice as high as SPF 55.

Astute readers who didn't fall asleep during the math section may notice that the SPF formula only covers UVB. That's correct! Persistent Pigment Darkening (PPD), the measure of UVA protection, has never been required information for United States sunscreens, so it's harder to judge how well a sunscreen will protect you against these longer waves. The best you can do is try to know your filters -- the exact chemical compounds in sunscreen that block or absorb UV radiation. This isn't always easy, because there are a lot of them, so I'll try to stick to the most important ones.

Physical Filters
The two main physical filters used in sunscreen are Zinc Oxide (ZnO) and Titanium Dioxide (TiO2). A "physical" filter means that the particles sit on top of your skin and act as a physical barrier to scatter, reflect, and absorb UV rays. Both of these two filters protect against UVB rays, but TiO2 has an absorption spectrum that only covers the shorter range of UVA: the "half" of the UVA spectrum that is closer to UVB, also known as UVA II. ZnO covers the full range of the UV spectrum, so sunscreens that use only physical filters offer much better protection if they include ZnO.

Chemical/Organic Filters
There are tons of these! So like I said, I'm going to try to keep it as simple as possible. Chemical/organic filters penetrate the epidermis and create an in-skin barrier that absorbs UV rays and metabolizes them into heat. It doesn't make you feel hot, not hotter than sitting in the sun anyway. It's just an energy-releasing reaction. And why the name "organic"? Well, organic compounds are literally those that have carbon in them, which these filters do, as opposed to the molecules that comprise physical filters. The two main things you need to be concerned about with when selecting sunscreens with organic filters are:
1. Coverage of the UV spectrum (like with physical filters,) and
2. Photostability.

The most common organic filters you'll see these days in United States sunscreens are Avobenzone, Octisalate, Octocrylene, Homosalate, Oxybenzone, and Octinoxate. Most sunscreens avoid using PABA these days, as it has been demonstrated to be potentially damaging -- so you should avoid sunscreens with PABA if you happen to see them. Avobenzone covers UVAII, and is the only one of these that protects against the longest-wave UVA rays up to 400nm, also known as UVA I. (Feel free to keep checking on the graphics above when I'm talking about short and long waves for reference!) The rest are all UVB filters, except for oxybenzone, which covers both UVB and UVA II. Oxybenzone is a good filter, but it does tend to be absorbed through the skin intact (rather than degraded into different, inert compounds) into the bloodstream more than the others. So far, no research indicates that there is any adverse effect from this, but if it makes you wary, you can avoid this filter. Due to the overlapping and complementary nature of these filters, they are often used together to boost UVB protection and cover the full UV spectrum.

So, what is photostability? Well, organic filters, as they absorb and metabolize UV light, are themselves degraded. Some degrade quickly enough in sunlight that they are considered photo-unstable. Avobenzone is one of these, but we like to see it in sunscreens because it protects against UVA I, while the other filters don't. Fortunately, sunscreen makers have found a way to stabilize it in solution by adding octocrylene. So that's a good green flag: if you see avobenzone and octocrylene in a sunscreen, chances are it's in effort to stabilize avobenzone so that it lasts longer. Avobenzone also has an enemy, though: it's already unstable on its own, but if you add octinoxate to the mix, it helps degrade avobenzone even faster! If you need help remembering this, a mnemonic that has helped me remember is "OctiNOxate," as in "octiNOxate is NO GOOD with avobenzone."

Why is photostability important? Well, first know that it is better to wear an unstable sunscreen than no sunscreen at all, since you're at least getting some protection. Beyond that, a photostable sunscreen is important because most people don't reapply sunscreen throughout the day when they are in the sun, and a stable sunscreen will simply last longer and offer better protection for longer. If you are the type that doesn't mind conscientiously re-applying every 2 hours, then maybe stability isn't as much of a concern for you. But I'd say that type of person is about 5% or less in the population, so most of us are better off seeking out photostable sunscreens.

I also want to talk about two other filters that have been prevalent in European sunscreens for at least 10 years, but are either not approved or very recently recently approved by the FDA here in the US. Tinosorb S and Mexoryl are both broad-spectrum filters that absorb the majority of the UVB and UVA spectrum, with drop-offs in absorption toward the extreme ends of the spectrum (as with all filters.) They are also both photostable, and Tinosorb actually can also stabilize avobenzone in solution. These are awesome, multipurpose beasts of UV filters, and the FDA is just taking forever to approve them. Come on, FDA! Fortunately, sunscreens using these filters are not too difficult to get online.

That should about wrap it up. In summary, here is what you should consider when selecting a sunscreen:

1. Physical or organic filters? Generally, the recommendation seems to be to start with organic filters and see how your skin takes to them. If you have sensitive skin or find that you react to most sunscreens that use organic filters, it may be time to get physical. If you're doing so, make sure you get one with zinc oxide, since that provides the best protection.
2. If organic works for you, make sure it has stable, broad-spectrum filters. Look for 30+ SPF sunscreens with avobenzone and octocrylene, first and foremost -- not one or the other. If they also have homosaliate and octisalate, that's fine; they're just contributing further UVB protection. Same goes for oxybenzone, unless you'd rather avoid it. If you can find sunscreens with Mexoryl and Tinosorb, try those out too! (They're personally my favorites, but that's not a scientific bias so much as what formulations I like best from experience.)

I hope this all helped! If there is interest, I can do a separate post for questions and/or my personal sunscreen picks!

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