Calculating SPF for Physical Sunscreens with TiO2 and ZnO

Calculating the exact SPF value for a physical sunscreen formulation using Titanium Dioxide (TiO2) and Zinc Oxide (ZnO) is complex and cannot be done with a simple linear calculation based solely on the percentage of these ingredients.

Several factors significantly influence the final SPF value:

• Particle Size and Type: Different grades of TiO2 and ZnO (e.g., micronized, non-micronized, coated, uncoated) have varying efficiencies in blocking UV radiation.
• Concentration: Higher concentrations generally provide higher SPF, but the relationship is not linear and can plateau.
• Formulation Matrix: The other ingredients in the cream base (emulsifiers, oils, thickeners, etc.) can affect the dispersion of the UV filters and the film formed on the skin, impacting performance.
• Dispersion: How well the TiO2 and ZnO particles are dispersed in the formula is crucial for even coverage and effective UV protection.

While there are general guidelines or starting points provided by raw material suppliers for specific grades of TiO2 and ZnO (e.g., a certain percentage might typically contribute a certain amount of SPF), these are only estimates for formulation guidance. They are not a substitute for actual testing.

Accurate SPF determination requires in-vivo testing on human subjects according to standardized international protocols (like ISO 24444). This is the only way to reliably determine the SPF value that can be claimed on a product label.

Therefore, you cannot calculate the precise SPF value just from the percentages of TiO2 and ZnO in your formula. You would need to formulate the product and then send it for professional in-vivo SPF testing.

• (Note: I was unable to find specific product information for cosmetic grades of TiO2 and ZnO in the database with the search terms used.)*