Designing HVAC systems for health care patient and critical environment spaces is dictated by strict environmental and safety standards, and one of the most important components that must be taken into consideration is effective airflow design (EAD). EAD not only helps meet airflow change and industry standards but is critical in limiting the contraction of airborne illnesses and can reap considerable cost savings for facilities. Therefore, when designing for health care facilities, it is important to abide by airflow and air quality standards. This is particularly true for three priority rooms for EADs: hybrid operating rooms, patient rooms, and isolation rooms.



The effectiveness of an airflow design in a health care environment boils down to the velocity of the air through the space and what direction it is flowing.

In the majority of spaces, the primary objective is to ensure the cleanest air is supplied first to the patient, then to the remainder of the room, and that it’s filtered before it circulates back into the area. Most states have adopted some version of the Facility Guidelines Institute (FGI) recommendations for health care facilities, but each administration has its own rules and regulations, so it’s critical for those involved to be aware of what standard(s) they’re designing to. Though it would not be a drastic shift, this individualistic approach among states to regulations may change within the next two decades as results of research projects are adopted into code. This research, commissioned by ASHRAE, FGI, and others, entails determining how much airflow is needed to prevent contamination in certain spaces based on evidence rather than conjecture. The International Code Council (ICC) formed an ad hoc committee on health care that is working to prevent standards and codes from increasing costs for construction and operation based on assumptions or outdated practices.

Finally, thermal comfort for health care spaces is addressed in ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy. Most health care regulations are concerned with airflow and air quality, but a room’s temperature and humidity are also important because they can impact recovery time of patients as well as the performance of the facility’s staff. ASHRAE Standard 170, Ventilation of Health Care Facilities, also has stipulations regarding minimum and maximum humidity temperatures, but although the scope of Standard 170 includes occupancy comfort, it should not be assumed that meeting the prescriptive design minimums will ensure compliance with ASHRAE Standard 55. Appropriate steps must be taken to realize thermal comfort in the space for patients as well as for visitors.

Let’s take a look at the EAD needs of three specialized health care environments.



Hybrid operating rooms (ORs) are surgical areas equipped with advanced medical imaging devices, such as CT and MRI scanners. Incoming air should be HEPA-filtered to minimize the pathogens entering the space. Hybrid ORs typically have 30 percent more air changes per hour (ACH) than catheterization labs. The increased airflow and type of procedures performed in these spaces dictate a different approach to EAD. Rather than conventional or radial flow diffusers, hybrid ORs utilize unidirectional diffusers, so air comes straight in one direction. These diffusers introduce highly filtered air into a space directly above where critical work is happening. This air then expands and pushes the contaminants away.

A human body’s natural convection also can protect itself from unclean air, so it is a best practice for diffusers to have very low velocities that do not disrupt a wound’s convective plume. Howver, recent studies have shown that in some surgery types, there is not a thermal plume generated at the wound site. In these instances, delivering clean air at a very low velocity is critical to minimizing entrainment of contaminants since this natural defense does not always occur.

Design specifications for hybrid ORs call for diffusers to be located directly over operating tables, and to satisfy ASHRAE Standard 170, the diffusers must cover at least 1 foot beyond the table and emit no more than 25-35 cfm per square foot. Results from ASHRAE Research Project 1397, Experimental Investigation of Hospital Operating Room Air Distribution, showed the unidirectional airflow collapses in toward the table and accelerates into the operating room as a result of buoyant and gravitational forces. The amount of collapse and acceleration is affected greatly by the temperature difference between supply air temperature and room air temperature. Therefore, it’s recommended to extend the diffuser array 2-3 feet beyond the table. Doing so will allow for a smaller temperature difference, limiting the collapse and acceleration, so patients, nurses, surgeons, and all surgical instrumentation are covered by the sterile field.



Like hybrid ORs, patient rooms are critical spaces that require a high standard of air quality. Designers typically do not have major issues designing these rooms; however, when using chilled beams and displacement ventilation systems, intuitive designs can lead to airflow patterns that are less than ideal. This can be a concern as an inefficient airflow design may fail to minimize the amount of potential particles and pathogens in the air being circulated or re-circulated through the room, translating to higher levels of airborne contaminants. EAD in these spaces means lower costs because patients recover more quickly, and there is a higher turnover rate — facilities do not have to treat or retreat patients for an infection or other condition acquired during their stay.

The use of chilled beams can be a useful means of developing an EAD within patient room spaces. The most intuitive design is to place a two-way active beam near the patient bed with the throw introduced into the room perpendicular to the patient’s bed. This is typical for most active beam designs: Placement over the occupant seeks to minimize air velocity and create a uniform temperature around the patient for thermal comfort. Recently, the result of a computational fluid dynamics study, Comparative Analysis of Overhead Air Supply and Active Chilled Beam HVAC Systems for Patient Room, showed placement of a one-way beam over the head of a patient could potentially create an airflow pattern that results in a single-pass system in regards to airborne particulate in the room. A single-pass airflow pattern, or reduced pass airflow patterns, strive to minimize the airborne particulates in the space to reduce the risk of health care-acquired infections.

Displacement ventilation design also presents a challenge in some cases. The size and floor level installation of these diffusers can lead to their installation in corners where they can be easily blocked by furniture or belongings, significantly reducing their efficacy. Placement of diffusers on the wall adjacent to the foot of the bed results in the most effective airflow pattern. Placing the exhaust above the patient’s bed at a 15-degree angle away from the head of the bed and toward the foot will be most effective in removing aerosolized saliva containing potentially viable viruses and bacteria from the space. Additionally, it is critical to have the transfer grille to the toilet space installed at least 6 feet above the finished floor to prevent short circuiting. Since the toilet room is to be negatively pressurized and has a high air-change rate, a low-level transfer grille could lead to the low-velocity air discharged from the displacement ventilation unit being exhausted from the patient room without addressing the load in the space.

So, why are more facilities implementing displaced ventilation and chilled beams for projects? Both systems are effective at getting air into spaces at the right temperature and exhausting and/or recirculating it without bringing contaminants back into the occupied space — the primary goals of EAD. In addition, displacement ventilation systems are highly effective in removing pathogens from patients’ bedside areas.



There are some specialized types of patient rooms that rely heavily on EAD to achieve their individual goals. These are airborne infection isolation (AII) rooms and protective environment (PE) rooms. AII rooms are designed to minimize transmission of airborne infectious diseases from an infected patient to staff, visitors, and other patients. PE rooms are specifically designed to protect patients with suppressed immune systems (such as chemotherapy patients, bone marrow or organ transplant recipients, and AIDS patients).

To prevent infections in isolation rooms, ASHRAE Standard 170 stipulates requirements to help achieve EAD. These requirements include room pressurization, filtration, air-change rate, and use of specific diffuser types and their location. To prevent migration of particles into isolation rooms, a minimum requirement is that the room must maintain differential pressure +/-0.01 inches water column (in. w.c.) to the adjacent spaces. However, ASHRAE Research Project 1344, Cleanroom Pressurization Strategy Update — Quantification and Validation of Minimum Pressure Differentials for Basic Configurations and Applications, has shown that even when maintaining a pressurization of +/-0.01 in. w.c., particles can migrate into the room as people enter and exit. To minimize transmission of particles into or out of isolation rooms, differential pressurization of at least +/- 0.04 in. w.c. or use of a anteroom is recommended.

All air supplied to PE rooms must be HEPA filtered. To further develop air distribution to reduce the chance of health care-acquired infections, non-aspirating unidirectional diffusers are installed directly over the patient with exhausts/returns grilles located near the door. This creates an airflow pattern within the space where the cleanest air possible flows over the patient first before moving into the rest of the room.

In addition, to achieve EAD in PE rooms, thermal comfort of the patient must be considered. Patients are going to have very low clothing insulation levels and metabolic rates, so additional diffusers must be used to keep the volume and velocity of the airflow out of the non-aspirating diffusers to a comfortable level. Displacement ventilation would complement the non-aspirating diffusers best in this space as it would not disrupt the airflow pattern that is to be developed by the non-aspirating diffuser.

In AII rooms, the goal is to prevent transmission of infections from the patient to staff, visitors, or other patients. As such, the location of the exhaust is to be directly over the patient bed or in the wall at the head of the bed, and all air must be exhausted out of the building. To establish EAD in AII rooms, supply diffusers should be installed near the entrance to the room with throw patterns directed toward the patient.

Combination AII and PE Isolation rooms are allowed by ASHRAE Standard 170. They must have an anteroom and must be pressurized to both the corridor and the isolation room itself. The differential pressure must be at least 0.01 in. w.c., and can be either positive or negative. In combined isolation rooms, air distribution must follow the same guidelines as PE rooms with diffusers located over the patient and exhaust by the anteroom door. And, as with the AII rooms, all of the air must be directly exhausted out of the building.



Appropriate use of chilled beams, displacement ventilation, and non-aspirating diffusers play a pivotal role in establishing EAD across many critical and non-critical spaces in the health care environment. Designing a system that utilizes each piece in the best way possible not only creates an environment that is safer and more comfortable but is also good for a facility’s bottom line.

Lowering re-admission rates and reducing the number of health care-acquired infections are goals all health care facilities should strive for; EAD helps make that happen.

Publication date: 1/22/2018

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