Inlet and exhaust ducting

The main problem with this analysis is the assumption of complete mixing . The ability of the air to mix in the room will be dictated by the positions of the air inlets and exhaust ducts (chemical hoods or ducts in labs without hoods) and whether doors are opened or closed. (Most labs do not have windows that can open.) Even in a fast-flowing river, for example, there can be very quiet pools of water at the edges, depending on the outline of the bank of the river and the placement of rocks or other obstructions. These disturbances are sometimes called eddies. Similarly, even in a high-turnover room, there are pockets of air that go virtually undisturbed. Little can be done about this, so it is good to know, for example, that at some weighing station in the corner of a lab, there may be little air turnover. 

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