Cosmetics containing nanoparticles (NPs) are surrounded by controversy. It is unclear how NPs in cosmetics can affect us and to what extent they are used. A concern is that NPs might penetrate the skin and interact with DNA and other macromolecules, due to their small size
NPs in cosmetics are especially used as UV filters in sunscreens. Nano-sized titanium dioxide (TiO2) and zinc oxide (ZnO) have been widely used for years. The safety of ZnO NPs is being evaluated by the European Committee, however their use is not yet permitted in the EU with the exception of Germany. Last year Dr. Vinodpravin Sharma and colleagues from the Indian Institute of Toxicology Research showed that ZnO NPs even at low concentrations have a DNA-damaging potential in human skin cells. This result is not enough to claim that ZnO NPs in sunscreens would damage DNA under normal conditions of use.
Lang Tran, Director of Computational Toxiciology at the Institute of Occupational Medicine, is coordinating the EU-project ENPRA (risk assessment of engineered nanoparticles). “At the moment a lot of work is being done on TiO2 nanoparticles used in cosmetics. The particles stay on the other layer of the skin, they don’t go deep enough. You also have to bear in mind that the particles are part of mixtures in cosmetics so they tend to agglomerate and that means that they are hindered in this way to go far into the skin. No risks at all have been shown from this route of exposure, but that doesn’t mean that there are no risks. We still have to look at injured skin because that’s different. Through a wound or a scratch you have access to the blood right away. People are trying to device injured skin models so that they can study it better. But that is all work in progress.”
Tillman Butz has seen similar results as a coordinator for the EU-project Nanoderm. “We did not observe penetration of nanoparticles into vital tissue for healthy skin. In very few cases we observed penetration through the entire horny layer. We also found nanoparticles in greater depth in hair follicles, but never nanoparticles in contact with vital skin cells. Thus there were no toxic effects. In-vitro studies where vital skin cells were deliberately in contact with nanoparticles exhibited various responses depending on the cell type, for example uptake in the cytoplasm only or uptake into the cell nucleus.”
Most previous results come from in vitro studies. Professor Brian Gulson at Macquarie University and colleagues tried to shed some light on ZnO NP penetration under normal conditions of sunscreen use. Through stable isotope tracing with 68Zn, the researchers showed that human volunteers’ skin was penetrated by small amounts of zinc from ZnO NPs. Unfortunately they could not show if 68Zn went through the skin as ZnO NPs or soluble Zn, or both.
The non-profit organization Environmental Working Group (EWG) highlights that TiO2 and ZnO NP oral intake through sunscreen lip products is also a potential concern. The two reports quoted showed NPs-induced lesions after large doses had been given to mice. In the TiO2 report the authors chose a large dose due to the low toxicity. The effects in humans from normal condition of use are unclear. The EWG has investigated different sunscreens with and without TiO2 and ZnO NPs. It balanced factors such as proven cancer risks from sun exposure against potential dangers of sunscreen ingredients. Contrary to the investigators’ own expectations, they concluded that among the safest and most effective sunscreens are the creams containing TiO2 and ZnO NPs. On the other hand, the environmental network Friends of the Earth refers to in vitro studies showing DNA damage and advises consumers against NP-containing sunscreens.
Most experts seem to agree that TiO2 and ZnO NPs are too large to get through the gaps in healthy skin, but research about injured, sunburned, diseased, flexed and thin skin of the elderly and the young may give different results. There is a lack of information on oral and inhalation exposure as well. Meanwhile the gaps are filled in basic research about potential toxicity, mandatory labeling of permitted NP-containing cosmetics including clear information can empower consumers to make informed choices.