**1. Introduction**

A systematic search with the keywords "titanium dioxide" and "sunscreen" all in the title was conducted using the Google Scholar database. No restrictions on year of publication, language, full-text availability, human- or animal-based studies were applied to the initial search. The results were then manually filtered using a systematic approach. Articles that did not pertain to the topic of titanium dioxide and sunscreens were excluded. Articles which were published only in a language other than English or in which full text was not available were excluded. A similar search and systematic review was conducted using the PUBMED database with the same keywords. However, the filters "clinical trials," "review," "full text," and "humans" were applied to result in 133 results which were then manually filtered excluding articles in non-English languages and not pertaining to the topic.

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Titanium dioxide (TiO2 ) is a Food and Drug Administration (FDA)-approved sunscreen, which is considered broad spectrum (protects against UVB radiation as well as UVA2 radiation 320– 340 nm). Previously, TiO2 had a suboptimal cosmetic profile, appearing thick and white on application. Current formulations are micronized or nanoparticle formulations, which blend in with the skin tone and attain better cosmesis. Concern about dermal penetration of these smaller particulate formulations and safety has been raised. However, current data are controversial. Several studies demonstrate negligible penetration beyond the stratum corneum, whereas other studies demonstrate cytotoxicity and oxidative stress in cell models. More studies are needed to definitively comment on long-term use of TiO<sup>2</sup> sunscreens and health effects. TiO2 has interestingly been implicated in allergic contact dermatitis in gold allergic patients in recent years. We discuss a newly hypothesized mechanism of TiO2 and gold particulate matter interaction on the surface of the skin.
