I would be cautious about sealing previously unsealed rough-sawn wood as you may change its ability to absorb and release humidity which could lead to dimensional changes. It could warp, twist, shrink or crack.
You could try hoovering the ceiling of the first floor space to remove loose surface dust and then suspend some white sheets (textile or paper) or sticky mats underneath as much of the ceiling as you can, especially in the most trafficked areas above (doorways). Get as many people as you can to walk around in the space above and then look at the papers. How much dust is there? Any pests ? Please let us know the results !