Because zinc oxide and titanium dioxide are perfectly safe sunscreen ingredients and the fears over nano-particles are unfounded.
The concern is that nanoparticles can be absorbed by the skin and harm living skin tissue. However, current research indicates that fears about absorption are unwarranted: Sunscreen is applied to the stratum corneum, the outermost layer of skin, which is made up of dead cells, and multiple studies have shown that nanoparticles do not penetrate living skin.27,28,29,30,31,32 Additionally, in sunscreen formulas, it appears that nanoparticles tend to clump together to form larger-than-nano-sized particles.
Lansdown AB, Taylor A. Zinc and titanium oxides: promising UV-absorbersbut what influence do they have on the intact skin? Int J Cosmet Sci 1997; 19:167-172.
Laderman J, Weigmann H, Rickmeyer C, Barthelmes H, Schaefer H, Mueller G, Sterry W. Penetration of titanium dioxide microparticles in a sunscreen formulation into the horny layer and the follicular orifice. Skin Pharmacol Appl Skin Physiol 1999; 12:247-256.
Pflucker F, Wendel V, Hohenberg H, et al. The human stratum corneum layer: an effective barrier against dermal uptake of different forms of topically applied micronised titanium dioxide. Skin Pharmacol Appl Skin Physiol 2001; 14 (Suppl. 1):92-97.
SchulzJ,Hohenberg H, Pflücker F,etal.Distribution of sunscreens on skin.Adv Drug Deliv Rev 2002; 54 (Suppl. 1): S157-S163.
GamerAO, Leibold E,vanRavenzwaay B.The invitro absorption ofmicrofinezinc oxide and titanium dioxide through porcine skin. Toxicol In Vitro 2006; 20:301-307.
Mavon A, Miquel C, Lejeune O, Payre B,Moretto P.Invitro percutaneous absorption and in vivo stratum corneum distribution of an organic and a mineral sunscreen. Skin Pharmacol Physiol 2007; 20:10-20.