Designing for healthcare patient and critical environment spaces is strongly dictated by strict environmental and safety standards. However, possibly one of the most important components that must be taken into consideration is the one you can’t see. Effective airflow design (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. When designing for healthcare facilities, it is important to abide by airflow and air quality standards, in particular for three priority rooms for EADs: Hybrid operating rooms, patient rooms and isolation rooms.
Standards and Approaches
To determine what airflow plan is right for a space, engineers first meet with a designer and give them a general layout for the room, diffuser size and placement, requirements for airflow and other details. Although the designed layout has a big impact, the effectiveness of an airflow design boils down to the velocity of the air through the space and what direction it is flowing. In the majority of spaces within the Healthcare environment the primary objective it to ensure the cleanest air is supplied first to the patient then into the remainder of the room and that it’s filtered before it circulates back into the area. As for requirements, most states (42) have adopted some version of the Facility Guidelines Institute (FGI) recommendations for healthcare facilities, but each administration has their 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, which has been the standard practice. The International Code Council (ICC) has formed an Ad Hoc Committee on Healthcare that is working to ensure standards and codes are not increasing the cost of construction and operation purely based on assumptions or outdated practices. This is a key area of focus, since 9 percent of the annual energy usage in the United States is dedicated to healthcare spaces; of that usage, HVAC is responsible for half.
Finally, thermal comfort for is addressed in ASHRAE Standard 55. Since most regulations are concerned with airflow and air quality, thermal comfort is not a priority. However, a room’s temperature and humidity is important because it can impact recovery time of patients as well as the performance of the facility’s staff – an overly cold or warm environment makes it difficult for surgical staff to perform at the highest level. ASHRAE Standard 170 also has stipulations as far as minimum and maximum humidity temperatures. While 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 step must be taken to realize thermal comfort in the space for patients, as well as for visitors.
Hybrid Operating Rooms
Hybrid operating rooms (hybrid 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 have 30 percent more air changes per hour (ACH) than catheterization labs. The increased airflow and type of procedures in the space 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 right above where critical work is happening. This air then expands out and pushes the contaminants away. A body’s natural convection can also protect itself from unclean air, so it is a best practice for diffusers to have very low velocities that do not disrupt the wound’s convective plume. 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 very low velocity is critical to minimizing entrainment of contaminates since this natural defense does not always occur.
Design specifications for hybrid ORs call for diffusers to be located right over operating tables, and to satisfy ASHRAE Standard 170 the diffusers must cover at least one foot beyond the table and emit no more than 25-35 CFM/ft2. ASHRAE Research Project 1397: EXPERIMENTAL INVESTIGATION OF HOSPITAL OPERATING ROOM (OR) AIR DISTRIBUTION results showed that the unidirectional airflow collapses in towards 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. Titus recommends that the diffuser array extend two to three feet. 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. This practice helps reduce costly surgical side infections (SSIs), which make up about 30 percent of all healthcare acquired infections (HAIs).
Like hybrid ORs, patient rooms are critical spaces that require a high standard of air quality. Designers do not typically have many 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 fails 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 potentially leading to HAIs, and thereby raising costs. An EAD in these spaces means lower costs because patients recover more quickly and there is a higher turnover rate. Facilities also do not have to treat or retreat patients for something they acquired during their stay.
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 2-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 deigns, placement over the occupant seeks to minimize air velocity and create a uniform temperature around the patient for thermal comfort. Recently the result a CFD study (Comparative Analysis of Overhead Air Supply and Active Chilled Beam HVAC Systems for Patient Room) showed that placement of a 1-way beam over the head of patient could potentially create an airflow pattern that results 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 HAIs.
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 towards 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 very effective at getting air into spaces at the right temperature, exhausting and/or recirculating it without bringing contaminants back into the occupied space – the primary goals of EAD. In addition, displacement ventilation systems are extremely effective in removing pathogens from patient’s 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. PE specifically designed to prevent patients with suppressed immune systems ( i.e. chemotherapy patients, bone marrow or other organ transplant recipients, AIDS patients). AII rooms are designed to minimize transmission of airborne infectious diseases from an infected patient to staff, visitors, and other patients.
To prevent infections in Isolation rooms, ASHRAE Standard 170-2013 stipulates requirements to help achieve EAD. These requirements include room pressurization, filtration, air change rate, and use specific diffuser type and their location. To prevent migration of particles into the isolation rooms a minimum requirement the room must maintain differential pressure +/-0.01 in wc 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 wc particles can migrate into the room as people enter and exit the rooms. To minimize transmission of particles into or out of isolation rooms differential pressurization of at least +/- 0.04 in wc 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 Healthcare Acquired Infections (HAIs) use of non-aspirating, unidirectional diffusers are to be installed directly over the patient with exhausts/returns grilles located near the door the patient room. This is to create an airflow pattern within the space where the cleanest air possible flows over the patient first before moving into the rest of the room. However, to achieve effective airflow design in PE room thermal comfort of the patient must also be considered. Patients are going to have very low clo levels and met rates, so additional diffusers must be used to keep the volume and velocity of the air flow 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 effective air distribution in AII rooms, supply diffusers should be installed near the entrance to the room with throw patterns directed towards the patient.
Combination AII/PE Isolation rooms are allowed by ASHRAE Standard 170-2013. Combined Isolation rooms 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 wc, 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 Effective Airflow Design across many different critical and non-critical spaces. 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 readmission rates and reducing the number of Healthcare Acquired Infections are goals all healthcare buildings should strive for; EAD helps make that happen. Be sure to consult a designer before embarking on your next project to determine which layout makes the most sense for your spaces.