Under normal conditions, supply- and exhaust-air valves operate at "constant" airflow set points. The fixed differential between supply- and exhaust-airflow set points provides directional airflow and room pressurization. For example, an exhaust-airflow set point of 700 cfm and supply-airflow set point of 600 cfm provides 100-cfm inward airflow, maintaining a room at negative pressure. As long as the "measured" differential pressure across the supply- and exhaust-air valves is within an acceptable range (0.6 in. wg to 3.0 in. wg), air valves maintain their respective constant-airflow set points, maintaining directional airflow and room pressurization. Differential-pressure switches across each supply- and exhaust-air valve continuously monitor airflow.

If the differential pressure across either the supply- or exhaust-air valve in any room drops below 0.3 in. wg, the bubble-tight damper on the supply duct to that room closes immediately. The supply-airflow and exhaust-airflow set points are reset to emergency-mode minimum values. When the main supply-duct static pressure and main exhaust-duct static pressure rise above 0.85 in. wg, the bubble-tight damper opens, supply- and exhaust-airflow set points are reset to their normal values, and the room reverts to normal operation. The room control sequences are illustrated in figures 4-6.

Particle Dynamics

Clinically applicable distinctions are made between short-range airborne-infection routes (between individuals less than 3.28 ft apart) and long-range routes (within a room, between rooms, or between individuals greater than 3.28 ft apart). Small droplets may participate in short-range transmission, but are more likely than large droplets to evaporate to become droplet nuclei. True long-range aerosol transmission becomes possible when droplets of infectious material are small enough to remain airborne almost indefinitely and be transmitted over long distances. There is essential agreement that particles with an aerodynamic diameter of 5 ìm or less are aerosols, while particles with an aerodynamic diameter of 20 ìm are large droplets.