.. Picking the correct floor

Did you find the leaflet from the roadblock? It mentions something about air density.

"Hide on the upper floor if the contaminated gas density is higher than normal air density. Otherwise, hide on the ground floor."

So, if the air density of the gas is over 1.61kg/m³, go upstairs. Otherwise stay downstairs.

Check out the drone you find at the broken bridge. It tells you the air density.

If the air density of the gas is over 1.61kg/m³, go upstairs. Otherwise stay downstairs.