(Corvallis, OR) -- A new study out of Oregon State University shows sunscreen with the common ingredient zinc oxide loses much of its effectiveness and becomes toxic after two hours of exposure to the sun. Researchers at OSU say zinc oxide or titanium dioxide are effective at blocking UV rays, but they change and turn toxic after two hours of exposure and they lose 80-percent of their effectiveness. The study was published in Photochemical & Photobiological Sciences.