An indicator paper for the detection of ozone. Ozone paper is impregnated with potassium iodide and starch (LaBarre 1969). It will turn blue on exposure to ozone. Alternate detection solutions are composed of thallous hydroxide, which turns brown, or indigo, which lightens.
E.J.LaBarre, Dictionary and Encyclopedia of Paper and Paper-making, Swets & Zeitlinger, Amsterdam, 1969.