Canada has answered this question. It tossed out millions of masks a year or two ago (the vast majority of its stockpile IIRC) 'cause they were just past the ex

Canada has answered this question. It tossed out millions of masks a year or two ago (the vast majority of its stockpile IIRC) 'cause they were just past the expiry date (the real reason was that a sharp-penciled bureaucrat wanted to save the storage fee.) Post-COVID they tested and found the degree of degradation in such cases to be very small. The expiry date is set by the manufacturer, after all; and it's in their interest that it comes up sooner, rather than later. Not the only business plan to ever cause mass death, I'm sure. My own 10-year old plus stock of (industrial) N-95 masks were mostly sealed from oxygen (just in plastic bags) and did fine. Good as new as far as I could tell, certainly. A couple that had been left out for a decade had foam seals that crumbled away entirely, and rubber bands that were fragile. But those in held in bags were pristine.