Brian has wisely advised that some WWll gas mask filters contain asbestos and should never be worn.
The filters of the British General Service respirator contained crocidolite: the most dangerous type of asbestos. These filters are easily isdentified: they are a flattened oval in shape, are a reddsih brown colour and connect to fthe acepiece by a corrugated hose. These filters are dangerous to handle, far less wear. In addition, the canvas haversack used with these gas masks can also be contaminated.
I have tested such filters and haversacks and was able to measure hazardous fibre concentrations being released.