Yeah... I expected those here as well... thus the "and to preempt an argument" section.

I'm out of the military now... I oftentimes let my beard get longer... I can promise you(anyone) that masks don't fit nearly as well. I have a personal full face respirator for a number of reasons. It doesn't seal nearly as well when my beard is anything beyond basic stubble for me. First the beard changes your face shape, second hair doesn't compress well unless you really crank on the straps and thus by nature the seal becomes uneven since hair moves and clumps, third when you crank on the straps... it fucking hurts after a while. Turns out people in general don't like having their face compressed.

Now you want to apply those problems to a warzone... Where the first and second will make donning your mask considerably harder when you're under fire... and the third will make it more likely that people will want to remove the mask or make worse choices because they're in literal pain wearing the mask.

It's one thing if you're only wearing the mask in a fire or something and a nominal amount of carbon monoxide gets through... Mustard gas or other agents could be outright deadly at very low doses.

Edit: Oh another difference... Consumer shit isn't meant to be worn for days on end... So it tends to be softer/pliable. Which can contour and fit more shapes/beards and such... Military NBC equipment isn't this way. It's mean to be worn for considerably longer and perform to a much different standard. They much more rigid, which adds to the problem a bit... less flexible overall because it needs to be a more resistant rubber/plastic. A respirator for a ~~firefighter gets~~ civilian purposes are typically used for a few hours before being replaced... A soldier could be wearing the same mask for weeks or months only replacing the cartridge when expended.