The M5 was made of neoprene, a synthetic material....As stated above, they are subject to "cold setting".....This means they can deform under cold conditions, renderring them incapable of sealing properly....The sealing aspect is irrelevant, since it is an obselete mask for the purposes of gas protection any way....Get a cardboard form that were originally issued with the masks and store in the bag...Otherwise get a form that won't deform the piece, attack the rubber, and keep away from elements such as sun, heat, etc...Basically any thing that will further degrade and deteriorate the material....Bodes
^ Good advice...
Expanded polystyrene (aka "styrofoam" - which is a Dow Chemical Co. trade name) is essentially inert, so much so that it is practically non-biodegradable. It will dissolve in the presence of certain chemicals, e.g. acetone.
Any outgassing (or "off gassing") the product is likely to exhibit will be extremely minuscule and very slowly released... far beyond the normal storage environments of most artifacts. Apparently, any significant outgassing that may happen is more probable and rapid in very high temperature conditions.
Museums that have big budgets can afford to use head mannequins which have a cotton or synthetic cloth cover (i.e. barrier) over the polystyrene. But, in the likely storage period range of 0 - 100 years or more (?) polystyrene in and of itself will be highly unlikely to damage these gas masks.
As with artifacts of every kind, simply maintaining a stable temperature and humidity storage environment, away from direct sun light, heat, moisture and critters, under as-neutral pH conditions as possible will ensure long life for all but THE most delicate materials in nearly all preservation circumstances.
In some sense, then, one CAN be "too careful"... There are books upon libraries upon conferences upon experts upon professional organizations upon traditions upon personal opinions upon industries upon horror stories about the 100% perfect way to preserve objects. After 50+ years in the business, I've come to realize that all we can do is give it our best effort with good intentions and some common sense.