"Grampa R." wrote in, asking about changing filters in a contaminated environment. I agree with you on the constant exhale method. I've also seen military NBC folks cover the opening after removing the filter while changing it to a new one. This seemed a little complicated to me, even with the other filter prepped for install. Also, this method would necessitate deconing your gloves or whatever you would use to cover the hole before covering the hole (or you would risk inhaling contaminant that might be on your gloves.) I like the method you described much better. It is always good to seek overhead protection before changing canisters if you are still receiving agent.
The M17 series of masks should be considered "Tier 2" masks in my opinion, due to the problems changing the filters in a contaminated environment.
Regarding masks and filters: Your mask has a series of valves that control intake and outlet. Hence, you should be alright to keep a filter installed in your mask as long as you keep the opening to the filter covered with something like duct tape. Roll one end of the duct tape well past the opening and make a small "handle" by putting it back on itself. Then when you don the mask all you need do is to pull the duct tape off the opening of the filter and your good to go. The inlet valve on the mask (which only works one way) and the tape covering the opening of the filter are keeping dust, pollen, etc. from getting into the filter. The older issue C2 filters came in a metal can that took approximately 1 minute to open. These are great for storage, but would take some time to open without practice. The new issue C2A1 come in a quick open plastic can type container. Very durable, I've stood on them to no effect. Micronell M95 filters are another good choice if you can't find C2A1s.
I encourage readers to learn symptoms of the various chemical agents as well as treatment. Hope this helps. - R.H.