I'm split on gas masks.
Most war gasses are dangerous (or deadly) even with tiny amounts of skin exposure.
Staying in MOPP 4 for extended lengths of time doesn't sound realistic or practical.
To give you an idea of what you're dealing with, about two gallons of the newer Novachok nerve agents are enough to kill every person in Manhattan, and much of Queens and Brooklyn.
The tinest gap in your all-over suit will be a death sentence.....despite having perfect respiratory protection.
A gas mask may be practical in an epidemic of airborne disease.....but other precautions, like frequent, strict discipline about quarantine, decon with bleach water, physical isolation, and so forth must also be understood and adhered to.....with no exceptions.
A gas mask might be a more realistic precaution against the riot gas and nausea gas that are sometimes used during civil unrest. If you are a rioter and/or into protests, then a gas mask may be a good idea.
Practice using it, store extra filters (being mindful that filters have an expiration date), and make sure that you aren't getting an older, outdated model.....as some older models are heavily contaminated with asbestoes (this isn't bullshit. Look it up).
When I consider all of these points, I've decided that a gas mask probably won't be of much help to me.....but I could be wrong.
If I was going to prep for chemical warfare, I'd create an airtight RV with its own filtered air system. I'd be able to drive out of the contaminated area with multiple people and a good quantity of supplies.