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Control Transmission of Airborne Diseases through Central HVAC System-Maintain Good Indoor Air Quality

Control Transmission of Airborne Diseases through Central HVAC System-Maintain Good Indoor Air Quality

In the face of incomplete rather emerging knowledge on COVID-19, we are well aware that infectious diseases spread by several different routes. The spread can be accelerated or controlled by Heating, Ventilation and Air-Conditioning (HVAC) systems. The purpose of this document is to provide information on how HVAC system can be a contributing factor to a good Indoor Air Quality System to keep airborne diseases under control.

The exposure through the air occurs mainly through droplets, which are released by an infected person and fall to surfaces about 1 m (3 ft)  and small particles, which stay airborne for hours at a time and can be transported long distances. See figure 1 below.

Droplet suspension: illustration of the aerobiology of droplets and small airborne particles produced by an infected patient. (Source: www.ashrae.org)
Droplet suspension: illustration of the aerobiology of droplets and small airborne particles produced by an infected patient. (Source: www.ashrae.org)
Figure taken directly from Welty 2009 presentation to FIC for IAQ, US EPA
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According to the WHO (World Health Organization), “The COVID-19 virus spreads primarily through droplets of saliva or nose discharge of an infected person.” Droplets generally fall to the ground or other surfaces in about 1 m (3 ft), while other particles (aka aerosols), behave more like gas and can travel through the air for longer distances, where they can transmit to people and also settle on surfaces. The virus can be picked up by hands that touch contaminated surfaces. 

Few actions related to HVAC Systems are suggested below to minimize the spread of airborne diseases/virus:

  1. Increase outdoor air ventilation presuming the outdoor air is not highly polluted will increase the effective dilution ventilation per person.
  2. If demand-controlled ventilation (DCV) is implemented to reduce energy consumption within the facility, either disable it completely OR cautious and calibrated use of DCV needs to be adopted.  
  3. Depending on the thermal comfort or humidity, the opening of a few outdoor air dampers to 100 % will help in eliminating air recirculation       
  4. Strengthening and improving  central air filtration will be helpful where systems deliver air to occupied space, room air returned in order to reduce contamination
  5. There is a need to keep systems running longer hours, preferably 24/7, to enhance the above actions.
  6. One may consider room air cleaners/ purifiers with HEPA filters.
  7. Very restricted and extremely controlled use of ultraviolet germicidal irradiation (UVGI) can be thought of in addition.

References:

  1. ASHRAE Position Document of Airborne Diseases January 2014.
  2. ASHRAE Journal Newsletter March 2020. Guidance for Building Operations During the COVID-19 Pandemic. By Lawrence J. Schoen, 

Apart from undertaking the above-suggested steps for HVAC System, it is important to monitor Indoor air quality continuously, preferably in real-time to identify the deterioration of air quality. The indoor air pollutants to be measured may be the presence of Carbon Dioxide (continuously emanated by humans), Temp., Relative Humidity, Particulate Matter (PM1, PM2.5, PM10) TVOC and Formaldehyde. www.indoorairquality.me

Airborne communicable infection can only be acquired by inhaling air that has been previously exhaled and because CO2 is a marker for exhaled breath, it would be useful to be able to relate infection risk directly to the rebreathed fraction, particularly now that continuous monitoring of CO2 concentration is relatively easy.

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