A dilution ventilation control system for use in a one-pass critical environment (20A, 20B, 20C) in which air is supplied to the critical environment using a supply airflow control device (51 A, 51B, 51C), and airflow out of the critical environment is exhausted using an exhaust airflow control device (41 A, 41 B, 41C), without any recirculation of exhausted air back to the critical environment (20A, 20B, 20C), the dilution ventilation control system comprising: a facility monitoring system that uses at least one air contaminant sensor to measure the level of an air contaminant in the critical environment (20A, 20B, 20C) and the level of the same air contaminant in the supply air, and generates output signals indicative of the level of the contaminant in the critical environment (20A, 20B, 20C) and the level of the contaminant in the supply air; and a signal processing controller responsive to the facility monitoring system output signals, characterised in that: the signal processing controller determines a contaminant differential measurement equal to the difference between the measured contaminant level in the critical environment (20A, 20B, 20C) and the measured contaminant level in the supply air, and in response generates a dilution ventilation command signal that is provided to a critical environment airflow controller (30A,30B,30C) that uses said dilution ventilation command signal to control at least one of the supply airflow control device (51 A, 51B, 51C) and the exhaust airflow control device (41 A, 41B, 41C), thereby controlling the flow of at least one of the air supplied to the critical environment (20A, 20B, 20C) and the air exhausted from the critical environment (20A, 20B, 20C) so as to control the quantity of air changes per hour in the critical environment (20A, 20B, 20C); there is no re-circulation of the exhausted air back into the critical environment (20A, 20B, 20C); the dilution ventilation command signal under normal conditions causes a minimum quantity of ACH in the critical environment (20A, 20B, 20C), and when a trigger event occurs the dilution ventilation command signal causes an increased quantity of ACH in the critical environment (20A, 20B, 20C); and wherein a trigger event comprises at least one of: (i) the contaminant level in the critical environment (20A, 20B, 20C) exceeding an input high trigger level; and (ii) a rapid increase of the contaminant level in the critical environment (20A, 20B, 20C) even if the actual level is below the input high trigger level. a facility monitoring system that uses at least one air contaminant sensor to measure the level of an air contaminant in the critical environment (20A, 20B, 20C) and the level of the same air contaminant in the supply air, and generates output signals indicative of the level of the contaminant in the critical environment (20A, 20B, 20C) and the level of the contaminant in the supply air; and a signal processing controller responsive to the facility monitoring system output signals, the signal processing controller determines a contaminant differential measurement equal to the difference between the measured contaminant level in the critical environment (20A, 20B, 20C) and the measured contaminant level in the supply air, and in response generates a dilution ventilation command signal that is provided to a critical environment airflow controller (30A,30B,30C) that uses said dilution ventilation command signal to control at least one of the supply airflow control device (51 A, 51B, 51C) and the exhaust airflow control device (41 A, 41B, 41C), thereby controlling the flow of at least one of the air supplied to the critical environment (20A, 20B, 20C) and the air exhausted from the critical environment (20A, 20B, 20C) so as to control the quantity of air changes per hour in the critical environment (20A, 20B, 20C); there is no re-circulation of the exhausted air back into the critical environment (20A, 20B, 20C); the dilution ventilation command signal under normal conditions causes a minimum quantity of ACH in the critical environment (20A, 20B, 20C), and when a trigger event occurs the dilution ventilation command signal causes an increased quantity of ACH in the critical environment (20A, 20B, 20C); and (i) the contaminant level in the critical environment (20A, 20B, 20C) exceeding an input high trigger level; and (ii) a rapid increase of the contaminant level in the critical environment (20A, 20B, 20C) even if the actual level is below the input high trigger level.