A study published in Cosmetics tested the stability and skin penetration of two types of zinc oxide, following recent concerns regarding the safety of this ingredient. The authors found that both types of zinc oxide tested did not penetrate the skin to the deepest layers.
The authors used 20% coated zinc oxide and 20% uncoated zinc oxide nanoparticles. The penetration was assessed via confocal laser microscopy.
Sunscreens
There are two different types of sunscreens: physical ones that use inorganic filters, and chemical ones that use organic filters. Only zinc oxide and titanium dioxide are approved for use as physical sunscreens, which help to reflect or scatter UV light. The alternative, organic filter sunscreens, often have poor spreadability and other appearance concerns.
The use of nanoparticulate formations creates transparent sunscreen formulations, which is desirable to most consumers. However, in prior studies zinc oxide nanoparticles have had more adverse effects on keratinocytes than titanium dioxide. These effects include disruption of mitochondrial function, the production of reactive oxygen species and oxidative DNA damage, along with genotoxic potential within epidermal cells.
It is worth noting that most studies analyze sunscreens available on the market rather than the key ingredients, leading to uncertainty regarding which ingredient may be causing issues. The authors of this study made a simple sunscreen formulation in order to test zinc oxide itself, rather than other active and inactive ingredients found in sunscreens.
Study Results
The coated zinc oxide formulation accumulated on skin furrows, while the uncoated did not. Both formulations were found around the hair and in the hair follicles. “The coated ZnOn nanoparticles incorporated into the formulation accumulated in the furrows and hair follicles, but were not found inside the keratinocytes, which suggests that they do not penetrate in the viable epidermis,” the authors write.
Regarding UVA/UVB radiation absorption, the coated nanoparticle zinc oxide formulation showed better absorption than the uncoated.
The authors found the presence of small-sized nanoparticles in both the coated and uncoated samples that “could result in the cell uptake of these nanoparticles, which could lead to cell damage.”
After 12 hours, there were no signs of inflammation, but the authors do note that penetration could have been underestimated for the uncoated zinc oxide formulation. “The increased reflectance in the skin treated with the formulation containing the coated ZnOn was observed only in the skin furrows and hair follicles, but not within the keratinocytes, which could suggest that the nanoparticles did not enter the cells.”
This study used one application of the formulas. Daily application could lead to an increase of zinc in the skin and therefore possible safety concerns. The authors recommend that prolonged use should be tested for biochemical markers of oxidative stress along with inflammation.
Source:
Cosmetics 2024, 11(1), 6; https://doi.org/10.3390/cosmetics11010006
“Development of Topical Formulations Containing 20% of Coated and Uncoated Zinc Oxide Nanoparticles: Stability Assessment and Penetration Evaluation by Reflectance Confocal Laser Microscopy”
Authors: Assis Dias Alves, G., et al.