Scientists are reporting that particle size may affect the toxicity of zinc oxide, a material widely used in sunscreens. Particles smaller than 100 nanometers are reported as slightly more toxic to colon cells than conventional zinc oxide when ingested. Their study is in the ACS Chemical Research in Toxicology, a monthly journal. Zinc Oxide is used in sunscreens as a highly effective agent to reduce and prevent sun burns. Part of its effectiveness is due to its particle size.