Improved indicator system for useful life of products which release active agents into the atmosphere

摘要

An indicator comprising a substance which becomes oxidised and so changes colour on exposure to the atmosphere and means for inhibiting the rate at which such oxidation occurs, whereby the said colour change takes place over a predetermined time.;A product containing such an indicator and an active agent for example a volatile air freshener, insecticide or vapour-phase corrosion inhibitor, which is gradually released into the atmosphere, so that the change in colour of the indicator is related to the depletion or exhaustion of the active ingredient.