Answer copied in part from: Does sun screen cause cancer?
A critical review of the existing evidence for sun-screens has been published.
- Burnett, M. E. and Wang, S. Q. (2011), Current sunscreen controversies: a critical review. Photodermatology, Photoimmunology & Photomedicine, 27: 58–67. doi: 10.1111/j.1600-0781.2011.00557.x
They looked at:
- Whether sunscreens stop skin-cancers.
- Whether sunscreens hinder the production of Vitamin D.
- Whether the controversial ingredients were shown to cause harm, specifically:
- oxybenzone and hormonal homeostasis,
- retinyl palmitate and cancer
- nanoparticles of zinc oxide (ZnO) and titanium dioxide (TiO2) and toxicity through the skin.
For this question, only the last issue is relevant.
The nanoparticle debate extends past just sunscreens, into other personal care and cosmetic products. They are widely used and have had a good safety record.
Nonetheless, some studies in rats suggested they may be photogenotoxic.
Considerable data assessing the potential toxicity of these materials in sunscreens has been published to date, and the studies referenced above were performed in controlled environments on healthy, undamaged skin. It has been established that the stratum corneum is an effective barrier preventing the entry of nano-ZnO and -TiO2 into deeper layers of the skin. Nonetheless, it remains to be determined whether a greater degree of penetration occurs through skin that is damaged, diseased or otherwise compromised. At the present time, however, the available data do not provide conclusive evidence demonstrating that damaged skin leads to an increased penetration of nanoparticles.
In summary, if your skin is undamaged, the nanoparticles can't get through. If your skin is damaged, there is no good evidence they get through, but more research is required.