With all this panic buying of N95 masks for Coronavirus, I can't help but think that using NOS NBC cartridges in a real  gas mask will be better.

Using an NBC filter for the virus should be more effective than a N95 or 100 right? I know the activated carbon has a shelf life once unsealed, but for the virus does that really matter vs the filter media?

NOS filters and surplus masks are not really expensive, if a pandemic actually hits here I'd rather wear one of those than a paper mask on my face.

too many variables to say. How they are stored and the company that made them all have to do with how long they will last. One company, cant remember the name of had an issue with the glue or adhesive in the filters would break down and come loose from the inside of the filter casing allowing the filter to rattle around inside the casing thus making the filter useless.   Heat is a killer of all filters so is moisture.

Unless the filter element is physically degraded due to moisture or age, I believe they are good indefinitely for particulates. Which will still protect against things like droplets that will carry a virus or bacteria. I wouldn't 100% trust them to filter out chemicals of any kind if they are old either.

There are warnings out there not to use older cold-war filters because they may contain hexavalent chromium.

In my younger years a friend and I decided to pop smoke in an enclosed area using old M17 gas masks and filters. They worked perfectly, and the smoke was thick enough that we couldn't see our hands if we stretched them out in front of us.

In a training environment we used them for dozens of trips into the CS chamber over many months, and they still worked fine.

I just bought a lot of MSA Millennium CRBN filters that expired in 2016 BUT are still in their original boxes with the filter itself in a vacuum sealed Mylar bag. From what I’ve read, the filters will work great protecting against bio pathogens despite being expired.

What degrades is the charcoal when exposed to the air. But the charcoal is used for absorbing chemical agents.