Warm air tends to rise. Cold air tends to settle. If air is not deliberately moved, the air will assume levels according to its temperature. This is called stratification. Air in an occupied space must be kept moving in order to eliminate stratification.
Thermostats and Humidistats must be placed at the proper levels because of stratification. Stratification tends to make smoke haze hover in layers. The layers make it difficult to get rid of the smoke.
Unfortunately some grills are poorly located. Then air moves only in certain parts of the room and becomes stagnant (not moving) in others. Furnishings also obstruct air movement. For this reason, some grills are located 6' high in the room or in the ceilings