All standards that assess the photocatalytic activity of a material, whether they be for air or water purification, self-cleaning or disinfection, provide just a snapshot of the activity of the sample. These measured activities should NOT be assumed to be everlasting, since there are many substances that can deactivate semiconductor photocatalysts, mainly by forming inert (or at least highly recalcitrant), and/or UV-blocking, coatings. These species may be: (i) photocatalytically generated metal oxides and hydroxides, such as SiO2 (e.g. from any silicone-based cleaning solution or sealing compound), (ii) deposited metal oxides/hydroxides, from metal ions in solution (e.g. Fe2O3 from the Fe(III) ions in waste water), (iii) polymeric, coloured, UV-blocking aromatics (e.g. polyaromatics from toluene in air purification) and (iv) precipitated carbon-containing materials (such as: soot, carbonates and even dead cells).
IPS can test activity with repeated sample use if commissioned to do so to address any wear issues. Price on application.