 Currently "Paii's" are used as quality control and marketing tools in commerce and as a quick and easy way to assess and/or map the activities of new photocatalytic materials in research.In light of the need for "in situ" testing of commercial photocatalyst materials, "Paii" labels have been developed that can be applied simply in the field on any surface to be tested, in both a non-reusable and reusable form.The rate of the rapid colour change associated with photocatalyst activity indicator inks has been directly correlated with the photocatalytic oxidation of stearic acid, methylene blue  and NOx.