Do beards actually break the seal of gas masks?

I will tell you for a fact from direct observation and extensive MIST (Man in Simulant) testing that beards will absolutely compromise the mask seal. The Air Force doctor they are citing can call up to Aberdeen Proving Grounds and, assuming he has a clearance, get all the test data he needs.

The standard issue military "gas mask" is a negative pressure air purifying respirator in that it operates by the wearer breathing which creates a vacuum (inhale - negative pressure) inside the mask. Nature abhors a vacuum and air will enter from ANY break in the seal no matter how slight. Ask any mechanic or anyone that has ever done a vacuum test on anything.

When working with SOF, this can be offset by using a powered air pump along with a biased outlet valve installed on the mask. These are generally battery powered, so not provided to the rank and file because you would need to keep truckloads of batteries on hand. Logistics nightmare. Or, as an expedient, we would put a small package of Vaseline in the mask carrier to quickly slap on any facial hair and help affect a seal, but it certainly isn't perfect.

Regardless, yes, even a few days stubble can compromise a mask seal and a full beard will most definitely compromise the seal. And you can take that to the bank.

I used an OBA with my regulation beard for hours during the Trident Sea Foods factory fire in Akutan, Alaska back in '82 (I think), and I had the taste and smell of smoke coming in my mask. Just sayin.................................................

If I were going into a situation where a gas mask was required I wouldn't take any unnecessary chances, even if the odds were extremely slim. My luck is I would be the exception and it would cost me my health!