I’m always getting asked whether I prefer inorganic mineral-based sunscreens like titanium dioxide and zinc oxide, or inorganic ones like avobenzone and octinoxate. In order to come up with a more holistic and unbiased conclusion, the following characteristics will be considered in a series of FIVE articles:
Part I: Irritation Potential and Aesthetics
Part II: Photostability, Permeability, and Photoreactivity
Part III: Toxicity
Part IV: Level of Protection, and Practicality
Part V: Conclusion and Product Recommendations
***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.
The irritation potential is how likely a compound will innately trigger a negative reaction, whether in the form of redness, stinging, and/or an allergic response. Keep in mind that this negative reaction occurs without UV radiation. Negative reactions upon UV exposure are not included in this section, and will be discussed in subsequent weeks.
Now, it is a well-known fact that IOSs are much less irritating and trigger significantly fewer occurrences of contact dermatitis and allergic responses than OSs. IOSs like micronized zinc oxide (ZnO) are inert chemicals that simply reflect and scatter light. In fact, ZnO has been used in a petrolatum base to alleviate diaper rashes in infants (1). While the inclusion of petrolatum certainly reduced skin barrier damage and erythema (the markers used in the study), it didn’t do it alone. If it did, why wouldn’t they just use petrolatum as a cure-all for dermatitis? I won’t deny that the study would have been more conclusive if it were vehicle-controlled. Despite that fallacy, the study still suggests that ZnO didn’t increase irritation.
On the other hand, several commonly-used OSs have triggered negative reactions, due in-part to OSs' mechanism of action against UV light (they absorb UV light and transform it to some other energy form) and their similarities to other known irritants including fragrances such as those of the cinnamates family. Numerous cases of allergic reactions such as anaphylaxis have been documented for multiple types of OSs, including benzophenones like oxybenzone (2), and dibenzoylmethanes like avobenzone (3). While the total number of reported cases of irritation from OSs is small compared to the general population as a whole, as well as varying in intensity, why take that risk at all?
Based on the evidence and the historical tendencies of the general public, the win goes to iOSs.
iOSs = 1; OSs = 0
This characteristic considers how easily a sunscreen can be applied, and how cosmetically pleasing it looks on the skin.
Until the advent of nanotechnology in recent years, iOSs have always been thick, greasy, and opaquely white. This was because both titanium dioxide (TiO2) and ZnO, are minerals that have high refractive (ability to reflect light) properties, with the former being more white (4). The greasiness comes from the emollient vehicles that had to be used to properly and evenly disperse the particles in solution. Even now, with micronized versions, this whiteness can still be seen as a faint white/gray cast. Furthermore, while the micronized versions have reduced the level of opacity, the fact that thicker and more emollient vehicles disperse particles better than more elegant solutions, still holds true. Therefore, many of the available micronized inorganic sunscreens are still a bit greasy. For example, while the Blue Lizard Sensitive Skin SPF 30+ sunscreen provides excellent protection, the texture is still a bit goopy and thick.
OSs on the other hand, while being intrinsically greasy (since they’re oil-soluble), can easily be implemented into cosmetically elegant water and oil emulsions such as gels, serums, and lotions. They are completely translucent, and with a little help from solvents like ethanol, can have textures as light as liquids.
Overall, iOSs tend to be heavier and faintly white, while OSs are lighter and completely transparent.
iOSs = 1; OSs = 1
Right now the score is tied at 1-1. Stay tuned for next week’s continuation of this series. The discussion of the three P’s: photostability, permeability, and photoreactivity (which rhyme by the way), is sure to include some seriously heavy reading. Speaking of heavy reading, check out the 3,000+ post I did on Retinol FAQs HERE!
About the author: John Su is an established skin care expert and aspiring dermatologist. He also runs a blog, The Triple Helix Liaison, dedicated to providing unbiased, meaningful, and insightful information about skin care. For his full bio, please visit our About page.