Not all N95 masks have the exhale valve. Years ago I went to the N95 masks with the exhale valve because I was doing some rigorous sanding grinding on fiberglass
Exhale valves is the only way to get rid of most of the CO2 buildup. And on warm to hot days, it becomes necessary for that level of filtration to force air back through.
My wife and I have P100 masks, and I can’t fathom ever not having an exhale valve to bypass the filter. Using that in a 130 degree hot attic space doing work is terrible (I’ve done it).
I did not know that.