Mineral-based sunscreens, often described as “physical” sunscreens, rely on inorganic compounds to provide ultraviolet (UV) protection. This type of formulation functions by sitting on the skin’s surface, creating a protective layer that defends against solar radiation. The effectiveness of these products depends entirely on the specific mineral compounds included as active ingredients, which are subject to rigorous regulatory oversight.
The Two Primary Active Mineral Compounds
Only two inorganic compounds are recognized by the U.S. Food and Drug Administration (FDA) as Generally Recognized As Safe and Effective (GRASE) for use as active ingredients in sunscreens: zinc oxide and titanium dioxide. These metal oxides are naturally occurring minerals that are milled into fine particles for cosmetic application. Zinc oxide (ZnO) is widely regarded for offering superior broad-spectrum protection, meaning it effectively covers both UVA and UVB portions of the solar spectrum. It is highly efficient against the longer UVA wavelengths, which penetrate deeply and contribute to aging.
Titanium dioxide (TiO2) is the other approved mineral filter, and it is particularly effective at blocking the shorter UVB rays, which are the primary cause of sunburn. While it also provides good protection against short-wave UVA radiation, it is less comprehensive than zinc oxide across the full UVA spectrum. For this reason, titanium dioxide is frequently combined with zinc oxide in formulations to ensure the sunscreen achieves true broad-spectrum coverage.
How Mineral Compounds Block UV Radiation
The mechanism by which these mineral compounds work is more complex than the simple “physical barrier” concept often described. While historically taught that zinc oxide and titanium dioxide primarily reflect and scatter UV rays, modern photochemistry reveals a different primary function. In reality, these mineral particles function mainly by absorbing ultraviolet radiation, similar to chemical sunscreens. Studies demonstrate that approximately 85% to 95% of the UV energy is absorbed by the semiconductor properties of these metal oxides. This absorbed energy is safely converted and released as heat, preventing it from penetrating the skin and causing cellular damage.
Navigating Ingredient Size and Surface Treatments
Mineral sunscreen formulations are complex, requiring manufacturers to control the size and surface chemistry of the active particles to balance efficacy with cosmetic appeal. Particle size is measured in nanometers (nm), and this dimension directly influences the sunscreen’s transparency on the skin. Traditional, larger non-nano particles are highly effective at scattering visible light, which is why they leave a noticeable white cast. To improve the aesthetic finish, many modern sunscreens utilize nano-sized particles, typically less than 100 nm, which scatter less visible light and appear transparent on the skin.
To ensure stability and safety, the mineral particles are often treated with inert surface coatings before being added to the final formula. Common coating materials include silica, alumina (aluminum oxide), and silicone-based polymers like simethicone. These coatings prevent the mineral particles from reacting with other ingredients in the formula and reduce their photocatalytic activity, which could otherwise generate reactive oxygen species. Furthermore, these surface treatments improve the minerals’ dispersion within the lotion base, preventing clumping and ensuring the sunscreen film is uniform and highly photostable on the skin.