I have heard carbon filters declines by just laying around unless its in a bag sealed. Is this true?
If in a sealed bag you should be fine to go, carbon in the filter has been lying underground for 100-900 thousand years,
as per RM, humidity is your enemy, if storing for years keep wrapped in plastic and warm and dry, better to store if not in use
Can one put it in a low temp oven to 'dehumidify' back to newness?
I'd be tempted to store the filter in a hot dry place like an attic over a summer season, rather than try a quick fix that will screw upCan one put it in a low temp oven to 'dehumidify' back to newness?