Yeah it wasn't an OSHA study that I was referencing...

https://pubmed.ncbi.nlm.nih.gov/29283316

With military articles like https://taskandpurpose.com/news/military-beards-break-gas-mask-seal/ stating

The 2018 study showed that facial hair negatively influences the fit factor for half-face negative-pressure respirators as the hair gets longer and more dense. However, beard-wearers can still “achieve adequate fit factor scores even with substantial facial hair in the face seal area,” the study authors wrote. In fact, 98% of the study participants who had an eighth-inch of beard passed the fit test. Those results are encouraging because the respirators used in the study are pretty close to the M-50 gas masks used in the military today in terms of material and fit, Ritchie said.

So 2 out of 100 people using masks that are relatively similar to the military M50 would be at risk at 1/8th inch beard. Which is not a whole lot of hair... Like 3-4 days of growth (for me). 1/16 or less seemed to be 100% rates... But the big caveat here is that the fit-test doesn't adequately capture the rigor and activity that one might do in the military... So it seems logical that much more leakage will happen at every level.

But OSHA, ANSI, every branch of the DoD, and every study (though minimal) agrees with the fact that beard hair in of itself is a no go.

Example navy document... https://www.med.navy.mil/Portals/62/Documents/NMFA/NMCPHC/root/Industrial%20Hygiene/RESPIRATOR-SPECIAL-PROBLEMS.pdf?ver=Ng19UESJUtWmwvoHSABW-w%3D%3D There's a fun graph on table 2.