Understanding Negative Room Pressure

Negative room pressure is created when the air exhausted out of the room is greater than the amount of air coming into the room. Our experts explain how to get rid of it!

What Is A Negative Room Pressure

Negative room pressure is created when the air exhausted out of the room is greater than the amount of air coming into the room. This creates lower air pressure, which allow air to flow into the environment but not from the environment out. In short, it prevents air and bacteria from flowing out of the room. This is because air will naturally flow from areas of higher pressure to areas of lower pressure.

If you were to open a door in a negative pressure room, the air from outside the room would be sucked into the space. This can be used to prevent infectious or harmful particles within the space from leaving the area. Medical facilities often isolate patients with aerosol based infections in negative pressure rooms to prevent cross contamination from room to room.

BalCon Air Balancer in a high visibility shirt adjusting a damper in the ceiling

The number of air changes in the room will determine how high of a negative pressure the room has. One air change occurs in a room when a volume of air equal to the volume of the room is supplied and/or exhausted. Negative pressure rooms are most commonly found in hospital setting. A patient with an airborne infection may be placed in an airborne infection isolation (AII) room, which has a high negative pressure. This prevents droplets in the air caused from coughing, sneezing or breathing from leaving the room. Patients with COVID-19, tuberculosis(tb), SARS-CoV, influenza, measles, chickenpox, and MERS-CoV are often placed in negative pressure rooms. Most recently these rooms have gained attention due to use during Covid-19 and Ebola.

Testing Negative Room Pressure

This is most commonly done with a smoke test to determine the airflow pattern around the room. By placing a smoke capsule or tissue outside the door, you can visually see if air is being pulled in or pushed out.

Continuous electronic pressure monitors gauging the differential air pressure While this is a good way to visually inspect room pressure, it does not provide an exact pressure reading. Continuous electronic pressure monitors, gauge the differential air pressure and are used when an exact reading is necessary. ASHRAE’s Standard 170 requires that, isolation rooms for example, have a pressure differential monitoring devices installed. Some common devices include manometers, calibrated ball-in-tube indicators, or electronic pressure monitors. Many of these devices tie directly into the existing control system.

According to an article written by HFM, requirements for isolation rooms are as follows:

“Negative-pressure isolation rooms require a minimum of 12 air changes of exhaust per hour and must maintain a minimum 0.01-inch WC negative-pressure differential to the adjacent corridor whether or not an anteroom is utilized. Typically, a setpoint closer to minus 0.03-inch WC is used. When not required for use with an infectious patient, the negative-pressure AII room may be occupied by noninfectious patients. The negative-pressure relationship to the corridor should be upheld; however, it is not required to be maintained at the minimum of minus 0.01-inch WC."

"When an anteroom is provided, airflow should be from the corridor into the anteroom, and from the anteroom into the patient isolation room. To maintain the required pressure differential, the exhaust air quantity must always be higher than the supply airflow. Depending on such factors as room size and the room’s heating and cooling loads, more than 12 air changes per hour may be necessary. Typically, a minimum airflow difference of 150 to 200 cubic feet per minute (CFM) is adequate to maintain pressure differential in a well-sealed room.“

Room Integrity And Maintaining Pressure

During the building process of negative pressure rooms it is extremely important to consider the overall integrity of the room. These negative pressure rooms are designed to move a certain amount of air based on the number of square feet the room. If the room is not sealed properly then these calculations will be off and cause the room to have problems maintaining its pressure.

Areas above drop ceilings for example, are commonly unfinished and unsealed because they won’t be seen by people using the space. This creates a serious problem. Holes in the drywall above the ceiling and unsealed penetrations allow outside air to seep into the space. This throws off the design and the room is unable to maintain pressure.

An exposed ceiling with duct, wires, and large wholes in the wall

This leads to ceiling tiles being siliconed downed to reduce airflow from outside the room. But this creates a problem when the system needs to be serviced. Silicone will have to be removed and reapplied just to access the equipment. It is far easier to finish the room correctly on the front end instead of trying to seal following construction.

Exhausted Air

When exhausting air from a negative pressure room, the air is generally run through a Hepa Filter. These filters are designed to remove any harmful particles before the air is pumped outside or recirculated.

Some medical facilities will use UV lights as part of the filtration process. UV Lights work to sterilize and kill active viruses, which can protect healthcare workers who are working with quarantined patients. This technique is commonly used in hospital settings to prevent infectious diseases from escaping the room.

Common Negative Pressure Rooms

It is common to have negative pressure rooms in bathrooms, autopsy rooms, pharmacies, laboratories, research facilities, decontamination spaces and in infectious isolation rooms.

According to the Facility Guidlines Institute (FGI), the following types of rooms in hospitals and outpatient facilities should also be nega­tively pressurized:

  • ER waiting rooms
  • Radiology waiting rooms
  • Triage
  • Airborne infection isolation (AII) rooms
  • Darkrooms
  • Cytology, glass washing, histology, microbiology, nuclear medicine, pathology, and sterilizing laboratories
  • Autopsy rooms
  • Soiled workrooms or holding rooms
  • Decontamination room for central medical and surgical supply
  • Soiled linen and trash chute rooms
  • Janitors’ closets

Bonus Section: Positive Pressure

Positive rooms exhaust less air than is being supplied, creating a net positive pressure. This means when you an open a door in this room, air flow outward into the hallway or adjacent room. These type of rooms are designed to protect patients with compromised immune systems and sterile environments housing surgical supplies. FGI says the following are rooms that should have positive pressure in a hospital:

  • Operating rooms
  • Delivery rooms
  • Trauma rooms
  • Newborn intensive care
  • Laser eye rooms
  • Protective environment rooms
  • Pharmacy
  • Laboratory, media transfer
  • Central Medical and Surgical Supply Clean workrooms
  • Central Medical and Surgical Supply Sterile Storage