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GAS CANISTER The Canisters(s) must be changed at regular intervals as determined by a competent person or when any breakthrough of contaminate is detected by smell or taste. We recommend the following MAXIMUM environmental contamination levels for filters as defined by EN 12941/2. Class 1 Gas/Dust filters (e.g. M09A1/P2) = 500 ppm 200mg/m3 Class 2 Gas canisters (e.g. M23A2) = 1000ppm N/A Class 2 Gas combination/Dust canisters (e.g. M23 A2B2/P3) = 1000ppm 100mg/m3 Dust filters TH2P (e.g. M23P2/5)