The Importance of HVAC Systems in Mitigating Airborne Infectious Disease Transmission in Healthcare
By Goutham Challa
With the annual cold and flu seasons picking up, coupled with a current uptick in COVID-19 cases across the country , it is critical for health care facilities to review the Heating Ventilation and Air Conditioning (HVAC) systems in order to take preventative measures to minimize the spread of the airborne infectious diseases. As flu viruses are airborne and are highly infectious, occupant space conditions like temperature and humidity play a major role in how long flu and other airborne viruses can live and how far they can travel. In addition to providing comfort, HVAC systems play a major role in maintaining a healthy and safe occupant space. HVAC systems maintain good Indoor Air Quality standards by monitoring and controlling the moisture, temperature, airflow, and CO2 levels in the space.
Figure 1: The 9 Foundations of a Healthy Building 
While some of the measures to mitigate the spread of the airborne viruses can be common-sense measures like good housekeeping, maintaining good hygiene, washing our hands, covering face while coughing or sneezing, other measures require continuous monitoring of the HVAC systems, equipment upgrades, and good maintenance practices. HVAC Fault Detection and Diagnostics (FDD) tools can help facilities monitor, control, and maintain a healthy environment in the built space. FDD tools continuously collect building systems data and run them through algorithms and facility-specific rules to raise flags for any potential issues.
The spread of airborne infectious diseases can be minimized by maintaining good ventilation routines. Healthcare facilities should aim to operate as per the designed minimum Air Changes per Hour (ACH) as required by codes and regulations. If it meets mandatory codes and regulations facilities should try to achieve 12 to 15 ACH, as it was observed that the peak efficiency of the particle removal occurs in the air space between 12 and 15 Air Changes per hour . In Figure 2 we can see that the CO2 concentration on floor 8 is out of the recommended levels, while on floor 9 CO2 levels are within the acceptable range. Upon further investigation, it was found that the outdoor air damper serving floor 8 was stuck closed.
Figure 2: Monitoring CO2 concentration in different zones
Apart from providing sufficient ventilation, healthcare facility operators should make sure that the HAVC systems components like fans, filters, and ducts are free from damage, leaks, and are operating as intended. Good air filtration plays a key role in mitigating the spread of airborne infectious diseases . Inspect the exhaust streams from restrooms and other areas with harmful contaminants to make sure there are no leaks and are not mixed with indoor air streams. In Figure 3 we can see that the differential pressure across the air filter in an AHU is zero even though the fan command is on, typically in a good air filter, we will see a pressure drop anywhere between 0.5 w.g to 1.5 w.g. Upon further investigation with the maintenance team, it was found that the air filters were damaged. The maintenance team acted on this alert and resolved the issue leading to a healthier occupant space.
Figure 3: Differential Pressure is zero despite the fan running in an AHU indicating ineffective filter
Humidity levels must be continuously monitored and maintained within the recommended levels to avoid the creation of conducive environments for the spread of harmful microorganisms. The Environmental Protection Agency (EPA) recommends that the humidity level be kept below 60% - ideally between 30% and 60% . HVAC systems components like drain pans, cooling towers, chilled water pipes must be regularly maintained to avoid condensation and stagnant water to avoid the growth of harmful microorganisms. Humidity issues can also be caused by leaking roofs, plumbing systems, fire sprinkler systems, residual water from cleaning, etc. FDD tools are very effective and helpful in monitoring the zone conditions and raising alerts if the conditions are not within the recommended range across diverse spaces like patient rooms, operation theaters, office spaces, etc. In Figure 4 we can see an alert being raised when the zone RH is out of the recommended level for more than three hours.
Figure 4:Monitoring zone humidity levels and flagging zones out of the recommended range
Retrofitting HVAC units with ultraviolet germicidal irradiation (UVGI) can also help a great deal in mitigating the spread of airborne infectious diseases . By following industry standards and good practices, following good maintenance protocols, and a combination of the above suggested measures healthcare facilities can do their part in reducing the spread of airborne infectious diseases and contribute to a healthier and safer world.
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