Since our post on toxicity last week was so tedious, I’ve decided to keep this one relatively short and sweet. Okay, it’s more that there aren’t too many conclusive articles specifically pertaining to these characteristics!
***In this series, I will refer to inorganic sunscreens as iOSs (not Apple, mind you) and organic sunscreens as OSs. While there are many individual compounds in each group—particularly OSs, for the purposes of this post, I will only refer to individual iOSs and OSs as parts of their respective groups. Complete and comparative ingredient profiles will not be seen in this series. Furthermore, I will attempt to discuss technologies that are more relevant and reflect the sunscreen technologies and tendencies of the current market. For example, while para-aminobenzoic acid (PABA) has many documented negativities, it will be ignored in our discussion, due to the fact that it is hardly used nowadays in sunscreen formulations.
Level of Protection
These days, because adequate protection against UVB rays is virtually guaranteed, I’ll mostly be focusing on protection against UVA rays.
For OSs, the two readily available ingredients (in the US) are oxybenzone and avobenzone, with the former protecting against UVA2 rays, and the latter protecting against UVA1 rays. Both, when properly formulated and stabilized, provide excellent protection.
On to the iOSs: the biggest “controversy” or question that I get asked is whether or not zinc oxide is better at absorbing UVA1 rays (>340 nm) than titanium dioxide. So that will be the primary focus of this section. To start off, the answer is a resounding YES!
IN MOST WAYS, ZINC OXIDE IS BETTER THAN TITANIUM DIOXIDE
Various studies have indicated that ZnO is better than TiO2 at protecting against UVA1 rays. For example, this study (1) demonstrated that both (3%) avobenzone and (5%) ZnO increased PFA values by nearly 3-fold when added to 6% oxybenzone; 18.2 and 16.0 respectively. Titanium dioxide at various concentrations (2.4%-9.1%) and with or without oxybenzone, only increased PFA values marginally (8.4-10.5). This is further supported by the fact that in the past, both the FDA and the Skin Cancer Foundation have not recognized TiO2 to provide adequate protection against UVA1 rays. In fact, the latter doesn’t even consider TiO2 to block UVA1 rays (3). However, it is known that TiO2 does provide some protection in the UVA1 range; it does well against UV rays that are < 350 nm.
Keep in mind that the PFA values were determined via the delayed erythema method, which may be used interchangeably with the persistent pigment darkening (PPD) method. The results from both of these methods are considered comparable (2).
However, some have claimed that when formulated properly, TiO2 will provide adequate UVA1 protection and specifically, will satisfy the critical wavelength test of providing protection against UV rays that are >370 nm. This qualification is one of the rules that the FDA plans to implement and enforce beginning December, 2012.
While the FDA is very rigorous when testing the safety and toxicology profile of a UV filter, it’s not so stringent when it comes to HOW MUCH protection is seen. Yes, TiO2 does provide some protection beyond the critical wavelength of 370 nm, but it doesn’t provide nearly as well as ZnO in that range. However, if you still insist on using just TiO2 to provide full UVB, UVA2, and UVA1 protection, then that is your choice. But as study (2) suggests, the difference in protection is not as small as previously thought. Study (4) further substantiates this claim, in addition to acknowledging that ZnO is less “white” than TiO2 due to the former (2.0) having a lower refractive index than the latter (2.6).
However, in terms of UVB protection, TiO2 is significantly better than ZnO! This study (5) suggests that ZnO can’t achieve an SPF of higher than 10, while TiO2 reached an SPF of 38. While these numbers aren’t absolute, they do support the above mentioned claim.
Like I said, there are pros and cons to ZnO and TiO2.
Overall and for the first time ever, I’m going to award both sides a point each, just because exact levels of protection will vary based on a variety of factors; many of which were discussed in parts II, and III. But relatively speaking, both iOSs and Oss can provide excellent levels of protection.
iOSs = 6; OSs = 2
This will be about the need of reapplication and how practical that application is in terms of iOSs and OSs.
Reapplication is deemed necessary for two reasons: one, because UV filters degrade upon irradiation; and two, the sunscreens might be wiped off, sweated off, and/or otherwise removed.
In terms of photostability, as we noted in part II, iOSs are more stable than OSs. However, at a nano-scale level, it was also noted that iOSs begin to act as semi-conductors and absorb as well as scatter UV light. Therefore, they aren’t completely stable and reapplication is necessary… in certain situations. If you’re going to a place where you expect to constantly be direct sunlight, then reapplication every two hours is necessary, and more frequently if you’re swimming. However, on a day-to-day day basis, if you’re wearing sunscreen under makeup and aren’t outdoors for a long period of time, it isn’t practical to take off your entire face and reapply everything. So in this sense, because iOSs are more stable than OSs, the former group is more “practical;” because iOSs need to be less frequently reapplied, if at all, on a day-to-day basis.
What about the sunscreen being removed factor? Well, that doesn’t really depend on the UV filters themselves. Rather it’s mostly the vehicle that determines how transfer-, water-, and/or sebum-resistant a sunscreen is. While OSs can help make a product more water-resistant due to their lipophilic nature, there are other ingredients that can be incorporated into a iOS to function in a similar fashion. Therefore, in terms of this, I’d say both groups are tied.
For this somewhat subjectively evaluated characteristic, I’d give the point to iOSs because, while stabilized OSs can work just as well as iOSs, many products on the market are not stable. And the ability to stay on the skin or to not be removed, depends more on the vehicle rather than the specific UV filters, which is independent of sunscreen type. Therefore, iOSs have a slight edge over OSs.
iOSs = 7; OSs = 2
We’re done! Finally!
This concludes the (mostly) evidence-based interpretation of why I think iOSs are better than OSs. For the final part next week, I will summarize all the major points of each post written thus far, and make product recommendation based on elements such as skin type, level of protection, and many other aspects! So keep checking back!
While you’re waiting, join in the lively discussion on my blog asking, “What type of product do you think is still missing on the current market?” And of course, you can let me know what you think of this post, too!