Airflow and AII Rooms: Why Supply and Exhaust Location Matters

Airborne infection isolation (AII) rooms are negative pressure rooms designed to prevent airborne pathogens released from patients with illnesses like tuberculosis from recirculating in the air¹ (read more on how TB spreads from the CDC here).

The ANSI/ASHRAE/ASHE Standard 170-2008 makes recommendations for hospital critical care spaces in regards to negative and positive pressure, air changes per hour (ACH), relative humidity, and temperature. The  recommendations for negative pressure AII rooms are²:

• Minimum of 2 outdoor air changes per hour
• Minimum of 12 air changes per hour
• All air exhausted outdoors
• No air recirculated within the room
• Max of 60% of relative humidity
• Room temperature between 70-75° F

Here is a quick overview of why these aspects are an important for critical care patient rooms, including AII rooms:

• A certain number of ACH are important because they keep the air from getting stagnant, dilute microbial contaminants in the air, and improve the general indoor air quality to promote a more healing and therapeutic atmosphere.¹

• Critical care rooms are positive pressure to adjacent rooms and hallways to prevent dangerous pathogens from entering the room (i.e. rooms for patients who have recently had surgery or with compromised immune systems).³

• AII patient rooms are kept at negative pressure to keep pathogens from exiting the room (i.e. rooms for patients with tuberculosis or MERS).³

• Relative humidity is important in a critical patient environment because high humidity is associated with respiratory illnesses, but low humidity can reduce the antiseptic layer of mucous that lines the respiratory system, also causing respiratory illnesses. Optimal humidity levels are important as they reduce the risk of other illnesses, and decrease the risk of mold growth and dust mites.⁴

• Temperature is important for patient comfort and controlling the humidity levels⁴

It’s generally accepted that multiple air changes per hour aid in controlling the spread of airborne pathogens.

However, a concern that a high number of air changes per hour could exacerbate the distribution of contaminants based on the location of the supply and exhaust in a patient room prompted Dr. Kishor Khankari, Ph.D. and ASHRAE Fellow to conduct a study on the optimal location for supply and exhaust components.⁵

Dr. Khankari published his study “Patient Room HVAC: Airflow Path Matters” in a recent edition of the ASHRAE Journal. Dr. Khankari utilized computational fluid dynamics (CFD) simulations to evaluate the impact of various HVAC configurations on airflow patterns, thermal comfort, and the probable flow path of airborne particles released from the patient’s face.

His CFD simulations showed that airflow patterns can greatly differ from patient room to patient room, based on the placement of supply diffusers and return (or exhaust) grilles.

The different design configurations Dr. Khankari simulated included:

• Ceiling supply diffuser over patient’s head with ceiling return near the entry door (control example)

• Ceiling supply diffuser moved away from patient over opposite wall with ceiling return near entry door

• Ceiling supply diffuser moved away from patient over opposite wall with a low wall return placed behind the patient’s head

• Ceiling supply diffuser over the patient’s head with a large ceiling return over the patient’s head and behind the supply diffuser

Dr. Khankari concluded that placing a return grille behind the supply diffuser and over the patient’s head provides a steady flow path for pathogens released from the patient’s face to exit the room without recirculating.⁵

Patient room airflow paths are complex, but with the right supply diffuser and return grille configuration, higher ACH can be a great asset in directing the flow of airborne pathogens, protecting healthcare workers, and providing a better healing environment for patients.

You can check out Dr. Khankari’s air flow simulation graphics, along with his full article in the June 2016 edition of the ASHRAE Journal (must be a member to log in).

So, where do Triatek products fit into controlling AII rooms?

The FMS-1655 Room Pressure Controllers can control the air changes per hour, room pressure,  temperature,  humidity, and alert hospital staff if conditions change to create a safe and therapeutic patient room.

¹“Air Change Rate/Dilution.”  HVAC Design Manual for Hospitals and Clinics. 2nd ed. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, 2013. 27. Print.

²“Air Exchange Rates and Ventilation.” Table 3-3. “Ventilation Design Parameters.” HVAC Design Manual for Hospitals and Clinics. 2nd ed. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, 2013. 77. Print.

³“Air Movement and Pressurization.”  HVAC Design Manual for Hospitals and Clinics. 2nd ed. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, 2013. 32. Print.

⁴“Humidity.”  HVAC Design Manual for Hospitals and Clinics. 2nd ed. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers, 2013. 30-31. Print.

⁵Khankari, Kishor, PH.D. “Airflow Path Matters: Patient Room HVAC.” ASHRAE Journal June 2016: 16-26. Print.