Which of the following best describes positive pressure in a room?

• A. More air is exhausted than supplied
• B. More air is supplied to the room than is exhausted
• C. Equal supply and exhaust
• D. There is no ventilation

Answer: (B)

Positive pressure means the air inside a space is at a higher pressure than the surrounding environment. This happens when more air is supplied to the room than is removed. The extra supply raises the internal pressure, causing air to flow outward through cracks, doors, or other openings. The result is that outside air or contaminants are kept from entering the space, which is why positive pressure is often used in rooms that must stay clean or contain contaminants.

If more air were exhausted than supplied, the room would experience negative pressure, drawing air in from adjacent spaces. If supply and exhaust are equal, the room is at neutral pressure with no net flow. If there were no ventilation at all, there would be no controlled pressure relationship to rely on.