Explanation of sunscreen photostability |
You would think that by now, skin care companies would be able to make a photostable sunscreen that protects against all UV rays and feels great on the skin without leaving a white residue. But a sunscreen like this is actually harder to make than you think!
Sunscreen UV filters that can theoretically protect against UVB and UVA rays may degrade or destabilize over time or when exposed to sunlight. Many sunscreen actives also don't protect against the whole UV spectrum and must be combined with other sunscreen actives to get full protection. This can sometimes cause problems when the actives interact poorly with one another. Additionally, the amount of UV filters you need put in a sunscreen to make it protect at a certain SPF or PPD can make the sunscreen feel unpleasant to wear on your skin.
Hence the current dilemma in sunscreens: You want the sunscreen to offer broad UV protection, but many sunscreen agents that offer high UVB and/or UVA protection do so at the expense of their stability, their cosmetic finish, or both. That's why there is a whole lot of discussion about photostability and why it's so difficult to find an agreeable sunscreen.
What is photostability?
Photostability means that your sunscreen's UV filters do not degrade or break down when exposed to sunlight. You need to use a stable sunscreen because an unstable sunscreen is not as effective in protecting you against the sun. If your sunscreen breaks down when you walk into daylight, how good will it be at protecting your skin from UV rays when you're out and about?
Physical sunscreen actives like titanium dioxide and zinc oxide are all photostable, so it's really the photostability of chemical UV filters you have to worry about (ex. avobenzone and octinoxate are two of the more unstable chemical sunscreen agents). If a sunscreen active is not photostable, sometimes other UV filters or inactive photostabilizers can help make them more stable. How much more stable it can get will depend on the particular sunscreen's formulation.
The problem with photostability
Physical sunscreen actives like titanium dioxide and zinc oxide offer broad spectrum (UVB and UVA) protection and are highly stable. So, if physical UV filters are all stable and offer good protection, why bother with chemical UV filters and risk photostability issues?
The reason is, sunscreens with physical UV filters usually don't have an elegant cosmetic finish. They can be drying on the skin, feel "heavy", and leave a white cast that makes your face and neck two different colors. So even if a sunscreen does offer great protection and is photostable, it may not be the best option if you don't even like to use it. And getting someone to like using sunscreen is how you get them to wear it every day. And wearing it everyday is how you guarantee your skin is protected from the sun.
You might think that sunscreen manufacturers can just use less physical UV actives to make the sunscreen look and feel better. Unfortunately, if you don't use enough of the active in the product formulation (or apply enough to your skin for that matter), the sunscreen won't protect at the labeled level of SPF or PPD. This is why lower SPF sunscreens tend to look and feel better on your skin and why it's possible for many makeups and moisturizers to offer SPF protection without compromising function. However, the downside of these sunscreens is that they don't offer as wide a range of sun protection (if that's what you're looking for). Of course high SPF and PPD sunscreens that feel good do exist. It just depends on the process of weeding through bad sunscreens to find the one that actually works for you and how your skin reacts to the ingredients.
To get back on point, even though physical UV filters meet all the requirements in terms of UV protection, they fall short when it comes to how the sunscreen feels. So to make the sunscreen more cosmetically elegant, sunscreen manufacturers have turned to using chemical UV filters. Because chemical UV filters tend to be less thick and opaque, they often look and feel better on the skin. Some chemical UV filters can even double as a makeup primer! However, chemical sunscreen actives aren't perfect and come with their share of issues.
Chemical UV filters can cause skin irritations for some people. If you feel like your sunscreen burns when you put it on, the chemical actives are probably what's causing that to happen. There are also safety concerns with how much of the chemical is absorbed by your skin, which is why a number of the photostable chemical UV filters have not been approved for use by the FDA in the US (these sunscreens are usually available in Europe, Asia, and Australia).
Aside from skin irritation and potential safety issues, the other problem with chemical UV filters is their photostability. This is actually mainly an issue with US sunscreens. Since many of the photostable chemical UV filters are not approved by the FDA, this leaves the chemical UV filters that are approved and unfortunately, they tend to be unstable (like avobenzone).
Unstable sunscreen actives are not a lost cause though. Photostabilizers, which are inactive ingredients in sunscreen, can be added. Many UV filters also have a stabilizing effect on other UV filters (but some of them can actually have a destabilizing effect, such as using octinoxate and avobenzone together). Because usually more than one UV filter is added to a sunscreen to make it more protective, formulating a photostable sunscreen with synergistic UV actives can get tricky.
The degree to which photostabilizers (either other UV actives or inactive sunscreen ingredients) actually help to stabilize unstable UV filters is also unknown. Therefore, the photostability of sunscreens with chemical UV filters (especially chemical UV filters known to be unstable that sunscreen manufacturers attempt to make more stable with photostabilizers) are unknown and it makes you wonder whether or not your sunscreen will protect you enough.
Keep in mind that even though photostable chemical UV sunscreens are stable and highly UV proof, they can potentially cause skin irritation. Like with the cosmetic finish of physical sunscreens, if your sunscreen burns your skin and eyes or you simply don't like using it, you probably won't use it and then what's the point?
How balancing photostability works
To achieve effective sun protection and good cosmetic finish, along with photostability, various combinations of chemical and physical UV filters are used to try to balance each other out. Here is an example of the thought process that goes behind this:
Zinc oxide might be added for UVA and UVB protection, but since it's kind of thick and white, the sunscreen manufacturer might not use as much of it in the formulation. Not using as much zinc oxide will make the sunscreen protect less against UVA and UVB rays, but it will make the sunscreen feel less thick and look less white. To compensate for the loss of UVA and UVB protection, the sunscreen manufacturer might add a chemical UV filter (titanium dioxide is another physical UV filter that can be added to boost UVB protection, but it doesn't offer adequate UVA protection). If they add avobenzone for more UVA protection and to keep skin irritancy low, they risk making a sunscreen that will destabilize in sunlight. So, to make the avobenzone more stable, they add other UV filters like octocrylene or include other inactive photostabilizers like SolaStay or polyester-8.
If they were in Europe, they could add something like Tinosorb S (which is photostable but not FDA approved) and then tinker with improving the cosmetic finish of the sunscreen. If they add something like Mexoryl SX, which is photostable and FDA approved, they run into another problem: a sunscreen active like Mexoryl SX is water-soluble, so it makes it more difficult to be water-proof. And if when you sweat, your sunscreen comes off, then that will make wearing it less effective as well.
So you see, making a good, photostable sunscreen means you have to compensate for a lot of small interacting details. If you get the UVA and UVB protection down, you still have to work on the texture of the finished product. If you get the texture down, you might not have the best sun protection. If you get both the protection and texture down, you might have the FDA to contend with. Or if the FDA gives you a pass, you still have to tinker with skin irritancy, substantivity (how well the sunscreen sticks to your skin and keeps up its SPF protection), water-proof qualities, and potential safety concerns. See why it's so hard to make a good, photostable sunscreen?
Solutions to photostability problems
As of yet, there is no perfect sunscreen that protects against the full range of UVA and UVB rays, does not destabilize, has no safety issues, and feels great on the skin. However, sunscreen manufacturers are experimenting with different sunscreen technologies to improve stability issues without compromising other aspects of the sunscreen.
Because the main issues with physical sunscreen actives is their cosmetic finish, many sunscreen companies are experimenting with nanoparticle sized zinc oxide and titanium dioxide. These microfine UV filters don't leave as bad a white cast as regular sized zinc and titanium and also seem to protect just as well. The downside to nanoparticle sunscreen actives (there always seems to be one!) is how much is absorbed by skin and how safe they are to use. Nanoparticles in sunscreens are often coated (Z-cote is a popular coating) to make them less reactive and less penetrating to the skin. However, more research must to be done to determine the efficacy and safety of these new agents.
Sunscreen manufacturers are also experimenting with microencapsulated actives. Chemical UV filters can be irritating to the skin, but microencapsulating the actives can make them less irritating and penetrating to the skin (and therefore safer) while retaining their ability to protect against UV rays. Microencapsulated avobenzone may also degrade less than regular avobenzone, which would make the active more stable and easier to formulate in sunscreens. Again, more research must be conducted.
Sunscreen manufacturers are also getting better with their formulations so not all total physical sunscreens feel bad on your skin and not all chemical sunscreen cause irritation. It really depends on your skin, how it reacts, and what it likes.
Is it pointless to wear unstable sunscreens?
What do you do if you find out your sunscreen is photounstable? Just because your sunscreen isn't stable doesn't mean it's completely useless. If you wear and re-apply the sunscreen correctly, you will still get some kind of protection, certainly more sun protection than not wearing sunscreen at all! The only thing is, if you are out in the sun for long periods of time and fail to re-apply an unstable sunscreen frequently, you will not be as protected. Photounstable sunscreens may also potentially damage your skin by releasing free radicals; however, this warrants further study. All in all, a sunscreen that isn't photostable is less reliable and potentially unsafe for use, so it's just best to find a sunscreen that is photostable.
More than just photostability
Whatever combination of ingredients in a sunscreen, the level of sun protection, cosmetic finish, and photostability must constantly be taken into account. How you use the sunscreen, how much sunscreen you wear, how often you re-apply it, if you are outdoors for long periods of time etc., all impact how effective a sunscreen will be.
Sunscreen manufacturers will tinker with the formulation of a sunscreen until they find one as close to perfect as they can get, but perfect still seems like a long ways away. Furthermore, because everyone's skin is different and reacts differently to different things, this makes developing a sunscreen that is accepted by the masses quite challenging. It is ultimately up to the individual to choose what sunscreen qualities are more important. Are you okay with using a sunscreen that is less protective but feels better? Or are you fine with having your sunscreen feel a little heavy on your skin as long as it has a high SPF and PPD? You have to decide what you can live with and what you can live without.
But if you can find a photostable sunscreen that agrees with your skin, you are definitely better with it than without! A good sunscreen is one of the best preventative products you can use on your skin.
Last updated: September 13, 2012
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