The problem of describing quantitatively the effective ventilation in a room when the air within the room is imperfectly mixed is discussed. It is suggested that the protection afforded by the ventilation to any given position against airborne contamination liberated at any other position can be best expressed in terms of the integrated exposure at the first point following liberation of a tracer substance at the second. This quantity is called the transfer index and its reciprocal the equivalent ventilation approaches numerically to the rate of supply of ventilating air as the mixing of air within the room approaches completeness. Nitrous oxide is a convenient tracer gas for making such measurements.

A method is also described whereby estimates of the transfer index can be made employing only such apparatus as is easily available in a reasonably well-equipped laboratory. This method employs acetone vapour as a tracer substance. The concentration of this vapour in the air is measured by the pH change produced in a dilute solution of hydroxylamine hydrochloride as it flows down a cotton wick exposed to the atmosphere.