In the past, I’ve briefly touched on the subject that octinoxate degrades avobenzone. Specifically, in Are Inorganic Sunscreens Better Than Organic Ones? Part III: Toxicity and Spotlight On: Vitamin B3 (Niacinamide and Nicotinic Acid), readers have seen the phrase “2 + 2 addition of cinnamates and alkenes” repeatedly used to explain this phenomenon. And in subsequent weeks, I’ve received many comments and messages inquiring about what exactly does this math mean? Therefore, this post will document what happens when octinoxate and avobenzone come together.
“But didn’t I learn this in elementary school?”
Well, the “2 + 2 addition of cinnamates and alkenes” is a bit more complicated than adding two groups of fruits or vegetables.
But getting back on topic, this phrase contains two elements: “2 + 2 addition of cinnamates” and “2 + 2 addition of alkenes.” The first refers to when an octinoxate molecule forms a dimer with another octinoxate molecule. The resulting structure or dimer does not allow the octinoxate molecules to be photostable and therefore, meaningfully absorb UVB light. The top row shows the two main dimers that are formed from this “2 + 2 addition of cinnamates.”
The second element, the “2 + 2 addition of alkenes” is when an octinoxate molecule binds to the double bond of the dominant form of avobenzone, resulting in the formation of a cyclobutane, which then undergo ring opening to form structures that don’t allow either the octinoxate or avobenzone molecule to properly function. One of these structures can be seen in the bottom row of the picture shown.
As you can see, this degradation is caused by a structural transformation that’s precipitated by the presence of the UV filter octinoxate. Furthermore, note that these two reactions only occur in the presence of UV light. Therefore, if your sunscreen contains both octinoxate and avobenzone, you don’t have worry about them interacting in their container, assuming that it’s completely opaque. But, you do still have to worry about this interaction after you actually apply the sunscreen!
“Okay that makes sense. But wouldn’t photostabilizers like octocrylene reduce or even eliminate this octinoxate-induced degradation of avobenzone?
To answer this question, we need to understand how photostabilizers like octocrylene bring stability to avobenzone in the presence of UV light. When avobenzone absorbs a photon of UVA light, its electron goes into a triplet energy state. But avobenzone itself has no way of dissipating or quenching this excited electron. That’s where octocrylene comes in. It accepts this “excited” energy, which allows avobenzone to return to its previously ground or “un-excited” state, where it’s ready to receive another photon of UVA light. If there is no photostabilizer present, the excited electron will either destroy the avobenzone molecule, or avobenzone will pass it to whatever is nearby including the lipid bilayers of the skin, resulting in the generation of reactive oxygen species (free radicals), which leads to oxidative damage in the form of lipid peroxidation.
Now, as I stated in the last section, octinoxate degrades avobenzone by changing its very structure, which only occurs in the presence of UV light. Therefore, octocrylene would have some positive effect on the octinoxate-induced degradation of avobenzone because at times, it’d be able to accept the “excited” energy; other times, octinoxate accepts the energy first, leading to the inevitable “2 + 2 addition of alkenes.”
A metaphor for all of this would be if there are hundreds of three-man squads–comprised of Privates Avobenzone, Octocrylene, and Octinoxate; trying to disarm a field of landmines (photons of UVA light). Only Pvt. Avobenzone knows how to disarm the mines. But by himself, Pvt. Avobenzone would blow up after a few successful disarmaments because he would tire and no longer move fast enough; land mines have timers, right? But with Pvt. Octocrylene at his side, the two would work as a team and disarm the mines without loss of life.
So how would Pvt. Octinoxate fit into the metaphor? He’d be the renegade soldier that binds Pvt. Avobenzone’s and his hands together. So what good would Pvt. Octocrylene do then? He has no one to assist anymore. The landmines would then explode. Across the hundreds of squads, sometimes Pvt. Octinoxate would succeed in sabotaging the disarming process, and sometimes he would fail.
While this metaphor is far from perfect, (I mean how can 1 out of every 3 soldiers be a renegade?) I hope it gives you a better idea of what I’m talking about.
“Okay it does. But then why would manufacturers even include octinoxate and avobenzone in the same formulation?”
Here’s where it gets a bit less certain. I can only speculate on why formulators include both ingredients; there are no hard facts.
- They are lazy, uninformed, and/or simply don’t care. This is the most unlikely reason, but it can’t be ruled out.
- Because avobenzone is the only organic UV filter approved in the US to adequately absorb UVA rays of all wavelengths, and because octinoxate is the most potent and effective UVB-absorbing organic filter, they include both because it’s the most cost-effective combination.
- They figure that because the octinoxate-induced degradation of avobenzone doesn’t occur often enough to result in an overall or net loss in the level of protection that avobenzone and octinoxate inherently provide, it’s preferable to use that combination rather than something with avobenzone and another less-potent UVB filter like homosalate.
- They’ve found a way to prevent the octinoxate and avobenzone molecules from ever coming into contact with each other. This may be possible, though I doubt it since both molecules are so similar in terms of solubility; they’re both very oil-soluble. Furthermore, why would manufacturers put in the extra effort to have these two similar compounds separated? They’d have to have the formulation be an oil-water-oil emulsion, which isn’t very easy or cost-effective to make. Furthermore, how can they guarantee that these compounds stay separated during the most crucial time: once they’ve dried and set on the skin, which of course is exposed to UV light?
- They assume that all their consumers follow the 2-hour reapplication rule decreed by the various dermatologic and medical associations. “So who cares if products aren’t that photostable, since they’re supposed to be reapplied every 2 hours anyways?” Obviously, very few consumers actually follow that rule. And like I’ve stated in the comments section of Are Inorganic Sunscreens Better than Organic Ones? Part IV: Level of Protection, and Practicality, the 2-hour rule was created because these medical associations want to cover all the bases. They know that there is such a heterogeneity or variety of behavioral and formulation-related aspects that factor into how much protection the average person achieves. Therefore, it’d be irrational for manufacturers to use this reason to justify the inclusion of octinoxate and avobenzone in a single formulation. But again, it can’t be ruled out.
- Finally, perhaps octinoxate is more cosmetically acceptable, cheaper, and/or easier to obtain than other organic UVB filters. I highly doubt that this is true since you can find UV filters in products from all price ranges. But once again, I can’t completely rule this out.
“Okay that was a lot of information. What should I take away from this post?”
- Octinoxate degrades itself and avobenzone via 2 + 2 addition of cinnamates and alkenes.
- While photostabilizers like octocrylene do mildly reduce this octinoxate-induced structural degradation of avobenzone, but why would you want to deal with this anyways?
- Because there are so many unknowns when considering the reason behind why octinoxate is used in conjunction with avobenzone, look for sunscreens that don’t contain both octinoxate and avobenzone. It’s just one less thing to worry about.
Thank you for reading through this rather dense post!