Contaminants on gas system components can cause incurable damage in ionization chambers, and that, particularly in the environment of the LHC. The most dangerous (but not the only) contaminants which are known to cause severe ageing are trace pollutants from Si lubricants or greases.
It is impossible to establish a complete list of contaminants that are harmful, or to write down quantitative limits on the cleanliness of components that would guarantee safe operation for the life of the LHC. Some ATLAS groups, together with PH-DT1-GS and EST-SM-CP, have started a campaign to perform component validation tests, which consists of either: a full ageing test in a wire chamber or of a chemical analysis searching directly for contaminants.
The outcome of these tests are made available here: