When sourcing Zinc Oxide (ZnO) for cosmetics, the most critical factor is the particle size. You must choose between Non-Nano (USP Grade), which is preferred for 'clean beauty' and reef-safe claims, and Nano-scale ZnO, which offers better transparency on the skin. Additionally, ensure the purity level is ≥99% and that the heavy metal content (Lead, Arsenic, Mercury) is strictly within USP or EP pharmacopeia limits. For sunscreens, check the Specific Surface Area (BET) as it directly impacts the SPF efficacy and formulation stability.