Safe on skin up to 25%, not recommended where it can be inhaled
Review (industry co-author)2019 review on nano titanium dioxideSCCS opinions summarized
Most human and animal studies show that nano titanium dioxide does not go beyond the outer layers of the skin and does not reach the systemic circulation, on healthy or damaged skin. The SCCS considers it non-sensitizing, at most weakly irritating, with no evidence of carcinogenicity by the skin route. It presents no risk on skin up to 25%, but the SCCS does not recommend its use in formulations that may expose the lungs (sprays, powders), because lung inflammation is reported in animals4.
Nanoparticles stay on the skin surface
Review2015 reviewMetal oxide nanoparticles in sunscreens
All the studies conclude that the overwhelming majority of nanoparticles stays on the surface. Any passage into living skin gives mixed results, because of very different protocols8.
Lung carcinogenicity: insufficient mechanistic evidence
Systematic review346 studies assessed, 206 of good qualityTitanium dioxide nanoparticles, in vitro and in vivo
The studies mostly give moderate to high confidence for genotoxicity, oxidative stress and chronic inflammation. The authors conclude that the nanoparticles could induce chronic inflammation and oxidative stress, but that mechanistic evidence for lung carcinogenicity is insufficient, because of the wide variety of particles and protocols9. It is for this route, inhalation, that the regulation sets restrictions.
No difference in profile between nano and micro, according to industry authors
Review (industry authors)2012 reviewAuthors working for a cosmetics manufacturer
According to this review, insoluble nanoparticles of titanium dioxide and zinc oxide do not cross human skin, with no safety difference between micro and nano forms10. The authors work for a cosmetics manufacturer.