03 Oct MINIMIZING THE TRANSMISSION OF AIRBORNE INFECTIOUS DISEASES THROUGH COMMERCIAL AIR-CONDITIONING SYSTEMS
The pathogens that cause infectious diseases are spread from a primary host to secondary
hosts via several different routes. Some diseases are known to spread by infectious aerosols;
for other diseases, the route of transmission is uncertain. The risk of pathogen spread, and
therefore the number of people exposed, can be affected both positively and negatively by the
airflow patterns in a space and by heating, ventilating, and air-conditioning (HVAC) and local
exhaust ventilation (LEV) systems.. However, the transmission of airborne infectious diseases with respect to commercial air conditioning systems can be reduced in so many ways. This write up is intended to provide general guidance but cannot cover every type of installation of air conditioning system installed throughout the range of buildings encountered.
In applying the guidance, due consideration of all potential consequences must be known, therefore, professional advice must be sought on a building by building basis prior to implementing any of these principles.
A common method for transmission of airborne infectious diseases is via large water droplets and small water particles, which are released by an infected person when coughing, sneezing, shouting and speaking. Shouting and speaking can produce water droplets which can travel up to 1 metre from a person. Water droplets are generally heavy, so do not
readily enter most ventilation systems.
Small particles (aerosolised water droplets), typically produced when a person coughs or sneezes. These particles can stay airborne for extended periods of time and can travel relatively long distances. These aerosols are buoyant and can be drawn into air conditioning units and ventilation systems.
Standard grade filters fitted to recirculating fan coil units or the indoor sections of typical air conditioning units can capture a portion of these aerosols. If the concentration of aerosols is high, the likelihood increases that more will pass through the filters and be re-distributed back into the occupied spaces.
An important role of a standard air conditioning system is to introduce adequate filtered, cooled outdoor air into the occupied spaces of a building, ensuring adequate oxygen is supplied to the building occupants and the dilution / removal of impurities released into the air by furnishings, as well as the removal of odours from respiratory functions. Inadequate quantities of filtered outdoor air or a lack of overall air movement means that the air within the occupied space is not removed and replaced at a sufficient rate. This allows dust particles or aerosols carrying pathogens to remain present within the space for a longer time or period, thus increasing the likelihood of contamination by direct contact or being inhaled by the occupants. This effectively increases the likely rate of transmission of airborne infectious diseases.
The strategies discussed in this document cover those which apply to common HVAC (Heating Ventilation and Air Conditioning) systems.
Strategies to Reduce Transmission of Airborne Infectious Diseases Via Air Conditioning Systems
Before the advent of COVID-19, there have been strategies used in reducing the spread of airborne infectious diseases in the HVAC applications in hospitals and which have been very effective. However, a lot of researches are going on regarding Sars-CoV 2, the virus that caused COVID-19.
These strategies are as follows:
• Configure Air Conditioning Systems to Full Outdoor Air System
• Increase Outdoor Air Flow rates to Occupied Spaces and extract the excess air from the spaces
• Undertake a Night Purge Cycle
• Clean All Air Conditioning Unit and Ventilation System Filters more Regularly
• Retrofitting HEPA Filtration in Ventilation Systems
• Installation of Portable Air Cleaners with HEPA Filters
• Retrofitting Ultra-Violet Germicidal Irradiation in Ventilation Systems
Any of the strategies stated above can be easily incorporated into a new installation, but for an existing installation, the air conditioning systems must be appraised so that the most practical strategies are deployed.
ASHRAE position document on airborne infectious diseases and a publication by Jackson Engineering Advisers.