Pretty much everything has an expiry date because the manufacturer has guaranteed its still effective up to that date. Doing that requires time and money. This

Pretty much everything has an expiry date because the manufacturer has guaranteed its still effective up to that date. Doing that requires time and money. This doesn't mean the masks doesn't work after the expiry date (although it might be less effective) simply that the performance isn't guaranteed and most likely hasn't been tested sufficiently.