The limits Of WO3 as an optical gas sensor were derived by establishing the change in optical constants induced by 2% H-2 in At. Using Langmuir's adsorption equation, it was found that at low H-2 concentrations a high sensitivity is predicted, but the coloration could saturate at 57.9% of the material's maximum ion adsorption. The air poisoning problem observed in these devices was shown to be remedied by coating with a permeable polydimethylsiloxane layer, thus eliminating common atmospheric gases as the possible poisoning agents. (C) 2009 Elsevier B.V. All rights reserved.