There is ongoing concern about the potential toxicity of nanoparticles of various materials, and a new study in Chemical Research in Toxicology reports that certain sunscreens that contain zinc oxide nanoparticles may be toxic if ingested.

The research found that particle size affects the toxicity of zinc oxide. Particles smaller than 100 nanometers were slightly more toxic to colon cells than conventional zinc oxide and solid zinc oxide was more toxic than equivalent amounts of soluble zinc. Direct particle to cell contact was required to cause cell death, according to the study.

"Unintended exposure to nano-sized zinc oxide from children accidentally eating sunscreen products is a typical public concern, motivating the study of the effects of nanomaterials in the colon," the scientists note.

Experiments with cell cultures of colon cells compared the effects zinc oxide nanoparticles to zinc oxide sold as a conventional powder. They found that the nanoparticles were twice as toxic to the cells as the larger particles. Although the nominal particle size was 1,000 times larger, the conventional zinc oxide contained a wide range of particle sizes and included material small enough to be considered nanoparticles.

The concentration of nanoparticles that was toxic to the colon cells was equivalent to eating 2 grams of sunscreen — about 0.1 ounce. However, the study did not consider the changes to particles during passage through the digestive tract. The scientists say that further research should be done to determine whether zinc nanoparticle toxicity occurs in laboratory animals and people.

Citation: Moos et al., 'ZnO Particulate Matter Requires Cell Contact for Toxicity in Human Colon Cancer Cells', Chemical Research in Toxicology, April 2010, 100215135857018; doi: 10.1021/tx900203v