Although it seems to be common sense and even routine to some consumers to use sunscreens to protect the skin from the harmful effects of the sun, many still do not use any sunscreens in America. This is especially true in the BIPOC (Black, Indigenous, and People of Color) community. As the demography in USA has become more diversified over time, many cosmetic brands have recognized the needs of consumers of diverse skin tones. In recent years, there has been a push to wear sunscreen for this BIPOC demographic.
Among many reasons for the lack of use of sunscreens in this demographic, aesthetics and safety of sunscreen products are most worth noting. For decades, organic sunscreens have been dominating the sunscreen market. They could be irritating to sensitive skin and sometimes sting the eyes. There has been a shift in recent years to the use of inorganic UV filters due to several reasons:
- Mineral based ingredients are deemed to be inert, sustainable, and well associated with personal wellbeing.
- ZnO was approved in 2016 as a safe and effective sunscreen active in EU
- More importantly, TiO2 and ZnO are the only two actives assigned with GRASE status by FDA in its 2019 proposal1.
However, formulating mineral sunscreens for consumers with dark tone, especially skin types V and VI on the Fitzpatrick scale, has remained a challenge. As it can be imagined, the major challenge to consumer acceptance is whitening or white cast on skin after application. This is because inorganic UV filters are particulate materials with high refractive index, and thus, can scatter the visible light strongly.
Although material technology has much advanced to allow TiO2 and ZnO particles to be made as small as 10 – 20 nm and highly transparent on light skin types, whitening and/or bluing on very dark skin remains problematic for sunscreen formulators. Below, will review a few formulating strategies for mitigating this undesirable side effect.
Use ZnO only
ZnO has a refractive index of 2, much lower than rutile TiO2 which has a refractive index of 2.7. According to Mie’s theory on scattering, light scattering by ZnO is just about one third that of TiO2, meaning it is much more transparent. Use of TiO2 even at a low level could spoil the aesthetics. Therefore, it is imperative to use ZnO only for dark skin tones.
There are many grades of ZnO powder on the market with primary particle sizes in the range of 20 – 300 nm. Obviously, the smaller the size, the higher the transparency. For dark to very dark skin tones, a primary particle size in the range of 20 – 30 nm should be used.
ZnO is a moderately effective UVB sunscreen active, and thus, is often needed at very high level (15 -25%) to achieve SPF 30 or higher. Such high use level presents another reason why a very small particle size must be chosen to maintain high transparency.
There are many ZnO-only sunscreen products marketed for consumers with dark skin types especially African American. One example is On-The-Defense Sunscreen SPF 30 from Eleven by Venus Williams. It contains 25% ZnO and claims “Sheer mineral sunscreen that melts onto skin, leaving a semi-matte, non-chalky finish.’
Disperse ZnO powder well
Just finding a ZnO with a small primary particle size does not mean a complete solution yet. ZnO particles at this size scale have a very large specific surface area and surface energy and tend to aggregate heavily. In reality, what really interact with the light are the aggregates or even agglomerates. Therefore, proper dispersion to remove or minimize the population of large aggregates is important. Keep in mind, a small portion of large particles play a significant role in scattering visible light (whitening) due to their relatively large mass. While dispersing ZnO with high-speed mixer or homogenizer may be sufficient for skin type I to IV, milling ZnO powder using a bead mill is necessary for higher transparency requirement. In the absence of an efficient mill, the use of a ZnO pre-dispersion is a simple and effective approach.
Mitigating Whitening/Bluing
At high use level, ZnO will show some whiting on skin types V and VI even when it is very fine and well dispersed. Moreover, even if the whitening is made unnoticeable, scattering of light in the range of 380 – 450 nm cannot be avoided, leading to bluing. To mitigate the whitening/bluing and make sunscreen blend into dark skin well during application, pigments of warm colors can be used, as follows:
- Red iron oxide pigment
The red color of typical iron oxide pigment used at a level of 0.2 – 1.0% is able to neutralize ll the whiteness and bluing of ZnO sunscreen. Many mineral sunscreens tinted with red iron oxide are available on the market and are marketed for ethnic skin style. However, red iron oxide pigment is highly opaque, and its texture on skin can be chalky. When it comes to skin type V and VI, the finish with such pigment just cannot be as natural as consumer would expect.
- Transparent iron oxide pigments
Transparent iron oxides are an improvement from standard iron oxide pigments and were initially developed for varnish formulation. They typically have a primary particle size of < 30 nm and are as transparent as nano ZnO. Boots Co. PLC first disclosed the use of nano red iron oxide in inorganic sunscreen formulation in the early 1900s2. A few premium brands started to use both transparent red and yellow iron oxides in their daily wear sunscreen products since the mid-1990s. However, the use of such pigments remained very limited to this day. In addition to high cost, one technical hurdle is that transparent oxides are very difficult to disperse. With this in mind, I highly recommend the use of a pre-dispersion.
Typically, 0.2 -0.5% of transparent red is sufficient in an all-ZnO sunscreen formulations. Because dark skin can have different undertones (red, yellow or grey, etc.), a combination of transparent yellow and red iron oxides provides a more complete solution. At this use level, the transparent iron oxides impart almost no texture to the skin, and the finish is completely natural.
- Use of Earth tone or dark pearl pigments
The basic optical principle of using Earth tone pearl is similar to using iron oxides. As we know, pearl pigments often refer to mica with layers of metal oxide coating. They usually have good transparency, especially when the substrate is highly pure synthetic mica. As a result, its finish on the skin can be much more natural than a typical red iron oxide pigment.
It is preferred that pearls have red iron oxide as coating and that their particle size be below 15 microns. Any larger size may generate a pearlescent sheen on skin that will be deemed unnatural. Typical use level is about 0.1- 1.0 %. For very dark skin, grey or dark pearl pigments with a coating of black iron oxide or a combination of red and black iron oxides can be used at a level of 0.05 – 0.5% for further adjustment.
Like transparent oxides, a blend of Earth tone pearl and dark pearl pigments will provide a good balance for dark skin types with various undertones. Formulators at Kobo Products applied this technology to its 4 in 1 Multi-Purpose Sunscreen Cream and won CEW Award in 2019 for the category of Ingredient and Formulation3.
- Use of SPF boosters
The direct way to reduce whitening is to reduce the use level of ZnO. It can be done by selecting the right SPF boosting agents. Below are some strategies presented in the 2015 Sunscreen Symposium4. Here are a few highlights:
a) Film former: This technology is well known in our industry. Film formers can be oil soluble, water soluble or water dispersible (like latex). Many have been shown to boost SPF by 20% or more.
b) Antioxidants/anti-inflammatory: Many of them are proven to suppress the generation of erythema and can boost SPF very effectively.
Conclusion
Formulating mineral sunscreens for skin types IV to VI requires special considerations for very high transparency. An all zinc formulation should be the first consideration. The use of transparent oxides, and a blend of Earth tone pearl pigments can help to further reduce the whiten and/or bluing of sunscreens on dark skin tones and make them blend into skin more naturally.
References
- https://www.federalregister.gov/documents/2019/02/26/2019-03019/sunscreen-drug-products-for-over-the-counter-human-use
- NA Fardell et al., EP 0616522: Sunscreen compositions
- https://www.cew.org/award/4-in-1-multi-purpose-sunscreen-cream/
- Y Shao et al., Practical tools for boosting sunscreen efficacy, Sunscreen Symposium 2015
Acknowledgements
The author is grateful to Tatyana Tabakman and Cheres Chambers for insightful and helpful discussions.
Dr. Yun Shao joined Kobo Products Inc. in 1996 and currently serves as the vice president of R&D. He has over 20 years of experience in micro TiO2 and ZnO development and in inorganic sunscreen technology and regulations. He is also experienced in pigment surface treatment, wet grinding, specialty cosmetic ingredients, color cosmetics and global cosmetic ingredient regulations. He has presented his work in various scientific conferences including IFSCC congress and FLSCC Sunscreen Symposium. Dr. Shao holds 8 patents. He has co-authored several chapter books and technical papers on surface treatment and inorganic sunscreen formulations. Dr. Shao earned his Ph.D. in Polymer Chemistry from Rensselaer Polytechnic Institute and his B.S. in Applied Chemistry from University of Science and Technology of China. He is the founding member of Chinese American Cosmetic Professional Association and the President during 2011-2012. He is also member of Society of Cosmetic Chemist and Chinese American Cosmetic Professionals Association and Tristate CACS. Dr Shao joined the NYSCC Scientific Committee in 2020.