Rubber rots sealed or not.
Filters have a shelf life for a reason sealed or not, some turn toxic.
Theres more to a gas mask than the rubber. The mouth piece, eye pieces are all conections that will leak over time.

There is only one! filter that does it all with a 10 yr shelf life and provides full protection against all known biological and chemical agents.
The NATO M-95 filter.

You can use dust masks to hardware store repirators the list is long. Some will filter out some agents but not others.

Ask yourself this question. Did you spend more time and money shopping for a good pc that is just a pc than you are /will a life saving device like a gas mask?

Should you buy one at all? Who knows.

But if you do, buy a good one that you know will do the job when you need it too.

This is not the place to cheap out people.

Mike