Negative Room Pressure

Negative Room Pressure to Prevent Cross-Contaminantion

A negative pressure room includes a ventilation system designed so that air flows from the corridors, or any adjacent area, into the negative pressure room, ensuring that contaminated air cannot escape from the negative pressure room to other parts of the facility.

Air naturally moves from areas of higher pressure to areas of lower pressure. When negative pressure exists, a continuous air current enters the room under the door, which prevents airborne particles generated in the room from escaping into the corridor. A common example of negative pressure is a bathroom with an exhaust fan. When operating correctly, and with the door closed, the fan prevents unwanted odours and moisture from escaping.

Negative pressure is created by balancing the room’s ventilation system so that more air is mechanically exhausted from a room than is mechanically supplied. This creates a ventilation imbalance, which the room ventilation compensates by continually drawing in air from outside the room. In a well-designed negative pressure room, this air is pulled in under the door through a gap (typically about half an inch high) for that purpose. Other than this gap, the room should be as airtight as possible to prevent air from being pulled in through cracks and gaps, for instance those around windows, light fixtures and electrical outlets. Leakage from these sources can compromise or eliminate room negative pressure, even if the system is balanced to achieve it.

The minimum pressure difference necessary to achieve and maintain negative pressure that will result in air flow into the room is very small (0.001 inch of water gauge). The actual level of negative pressure achieved will depend on the difference in the ventilation exhaust and supply flows and the physical configuration of the room, including the air flow path and flow openings. If the room is well sealed, negative pressures greater than the minimum of 0.001 inch of water may be readily achieved. However, if rooms are not well sealed, as may be the case in many facilities (especially older facilities), achieving higher negative pressures may require exhaust/supply flow differentials beyond the ventilation system's capacity.

To establish negative pressure in a room that has a normally functioning ventilation system, the room supply and exhaust air flows are first balanced to achieve an exhaust flow of either 10% or 50 cubic feet per minute (cfm) greater than the supply (whichever is greatest). In most situations, this specification should achieve a negative pressure of at least 0.001 inch of water. If the minimum 0.001 inch of water is not achieved and cannot be achieved by increasing the flow differential (within the limits of the ventilation system), the room should be inspected for leakage (e.g. through doors, windows, plumbing and equipment wall penetrations), and corrective action should be taken to seal the leaks.

Negative pressure in a room can be altered by changing the ventilation system operation or by the opening and closing of the room's doors, corridor doors or windows. When an operating configuration has been established, it is essential that all doors and windows remain properly closed in the negative pressure room and other areas (e.g., doors in corridors that affect air pressure) except when people need to enter or leave the room or area.

Smoke test

A smoke test is a simple procedure to determine whether a room is under negative pressure. The smoke tube is held near the bottom of the negative pressure room door and approximately 2 inches in front of the door. The tester generates a small amount of smoke by gently squeezing the bulb. The smoke tube is held parallel to the door, and the smoke is exhausted from the tube slowly to ensure the velocity of the smoke from the tube does not overpower the air velocity. If the room is under negative pressure, the smoke will travel under the door and into the room. If the room is not under negative pressure, the smoke will be blown outward or remain stationary.

This test must be performed with the door closed. The windows in the room must be closed. If room air cleaners are being used (including fume hoods or biosafety cabinets), they should be running. If the room has an anteroom, the pressure differential should be tested from the corridor to the anteroom, and from the anteroom to the room.


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