Negative room pressure
Negative room pressure is an isolation technique used in hospitals and medical centres to prevent cross-contaminations from room to room. It includes a ventilation system that generates negative pressure to allow air to flow into the isolation room but not escape from the room, as air will naturally flow from areas with higher pressure to areas with lower pressure, thereby preventing contaminated air from escaping the room. This technique to used for isolation of patients with airborne contagious diseases such as tuberculosis, measles, or chickenpox.
Negative pressure is generated and maintained by a ventilation system that removes more exhaust air from the room than air is allowed into the room. Air is allowed into the room through a gap under the door (typically about one half-inch high). Except for this gap, the room should be as airtight as possible, allowing no air in through cracks and gaps, such as those around windows, light fixtures, and electrical outlets. Leakage from these sources can compromise or eliminate room negative pressure.
Smoke test 
A smoke test can help determine whether a room is under negative pressure. A tube containing smoke is held near the bottom of the negative pressure room door, about 2 inches in front of the door. The smoke tube is held parallel to the door, and a small amount of smoke is then generated by gently squeezing the bulb. Care is taken to release the smoke from the tube slowly to ensure the velocity of the smoke from the tube does not overpower the air velocity. If the room is at negative pressure, the smoke will travel under the door and into the room. If the room is not a negative pressure, the smoke will be blown outward or will stay stationary.
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