I can buy carbon filters that will filter out chemical vapours including those from epoxy.

The filters won't be compromised just because they have been hanging on a hook. There is enough resistance to flow through the filters that air will not pass through them and reduce effectiveness, especially if it is stored in a bag, loose or not. Your breathing is what forces the air through the filters. Unless you live in a very badly polluted area, like in one of those russian steel towns, a respirator in a loosely closed plastic bag will be fine.

I do not know what living in a humid area will do to the filter but I can imagine that the humidity will get into the carbon and reduce its effectiveness, bagged or not, worn or not. Persoanlly, except for a dust filter, I would doubt any filter that wasn't purchased in an air-tight package.

I hope this helps a little.

Robert N Pruden