IMO, old gas masks should not be trusted.

Over time, rubber dry rots and cracks, making a complete seal an impossibility. Then there are the age and breakdown factors of the filters and chemical agents.

IMO, gas masks are another useful layer of personal protection IN A SHELTER.

Wearing a mask and moving about (walking, working, etc) is very laborious and HOT. A mask is used to get out of a hot area when you've been caught out in the open. The average person will have to move slow or else the act of breathing becomes very tiring, depending on your size and state of physical conditioning.

I have recently become more aware of the importance of having the right "fit" of your personal mask to your facial structure. The mask may be sealed, but if it is the wrong size for your face, then it will leak as you move around, moving your head to different angles, as you work.