for reference - air exchanges vs N95

https://www.medrxiv.org/content/10.1101/2022.08.09.22278555v5

realization

I had the house's insulation improved last year. It suddenly occurred to me this morning that for the whole time I have lived in this house there has essentially been an open hole between the outdoors and one of the rooms downstairs. It's the fireplace, which has not been used for a fire at least since 1987, when the previous owners lived here. There are well-fitting glass doors, but it's still not completely tight. The same flue goes from the furnace to the outdoors, so I can't have it blocked up at the top (there is a cap on the chimney to keep creatures out, but it lets air through). Probably there is a damper? What should I wear to try to reach into a grubby chimney to sure it's tightly closed? I have learned from the web that there are insulated blankets that can be placed over the glass doors, held in place by magnets. Magnets don't stick to the whatever type of metal it is that holds the glass in place here, so that won't work, but maybe I could just tape some sort of insulation?
It is also true that there are vents between each bathroom and the outdoors, but they are much smaller and t least the downstairs one has some sort of flap (?)