Yes, the old gasmasks do,indeed, contain asbestos. This does not mean, however, that there is an instant and immediate warning of lung cancer. Think of the guys who used to grind asbestos brake pads all day in the brake shops, or the guys who used to blow in asbestos insulation into the walls of houses and into ships hulls. They worked every Day in clouds of the stuff. How about electricians who used to bite the insulation off of wires to strip them? They put asbestos right into their Mouths for who knows how many times in a lifetime. Yes, asbestos can and has, indeed, caused problems for people-sometimes lethal, but to have a gasmask in your house is not an enormous danger-why would it be? Use common sense and handle the mask responsibly. It's not a toy to be worn around by your kids and played in (although, I do know kids who did just that). If they are just sitting there on a shelf, I would not worry overly much. I lived in a house that had asbestos tile over the entire outside of it. Never a problem. The stuff is not healthy to be breathing in the dust, but it's not plutonium by any means.
"Much that once was, is lost. For none now live who remember it."
There was a recent article about this and many posts on other forums because the H&SE recommended to schools that gasmasks and WWI British helmets were a health risk and should be made safe or destroyed.
I think the answers given above cover it though, don't mess with it and there is very little chance of there being a problem.
Whatever its just an opinion.
As said, don't mess with it and you will be fine.
But you can make a solution of PVA wood glue and pour this into the filter which will seal it.
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