What is "stuffy"/"stale" air composed of? When a room is not ventilated for a long time (days if people are present, weeks if not), humans perceive a change in air quality commonly described as "stale" or "stuffy" air.

What is going on here chemically? How is "stale" air different from normal air?

Note that the difference cannot be the mere presence of biological exhalations from the inhabitants, as the phenomenon definitely also occurs when nobody lives in the building at all (though it takes a lot longer in that case).
It may not be a complete answer but I can think of a few distinguishable features that may or may not be chemical reasons:


chemicals created by bacteria and organisms that are present in everyday households (mold, light activating some production from bacteria and micro-organisms, $NH_3, S^{2-}$ related compounds (they smell bad), etc). 
Compound this with the acknowledged lack of circulation and you have funky smelling air.
high humidity: how one can "feel" "stale, muggy" air.
high carbon dioxide concentration can make the air feel heavier (from interior design sources).