@PTRB— [ul]The relevant
significance of a share here is whatever significance is imputed by Ueno; she might be excited by any positive number of molecules that one might suggest.
Your hypothetical 50% humidity would be considered
at or above the upper bound of what is desirable for an indoor environment. The lower bound is usually given as 30%. The room might plausibly be below that lower bound.
Those water molecules whose sources were the human bodies in the room would not be distributed uniformly; the closer some person were to the device, the greater the share of molecules contributed to the ice.
Increased heat would cause increased perspiration; assuming a roughly equal propensity to perspire, the greater share of perspiration would come from the operator of the device because he or she would be more proximate to the source of heat. [/ul]