Managing HVAC Systems to Reduce Infectious Disease Transmission

Panelist: Prof. William P. Bahnfleth, Ph.D, P.E., Fellow ASHRAE
Chair: Dr. Daniel Coakley, Secretary, ASHRAE Ireland
Date/Time: Thu, Jul 16, 2020 1:00 PM – 2:00 PM BST
Presentation Slides: [1] B. Bahnfleth [2] D. Coakley – Introduction
Further Information: Video | Flyer | Q&A

On Thursday 16th July 2020, ASHRAE Ireland was honoured to welcome Prof. William Bahnfleth, Chair of ASHRAE\’s Epidemic Task Force, to the third instalment of our COVID-19 Technical Seminar Series. The presentation covered a wide range of topics on HVAC-related infection control, and outlining practical guidance for the management of buildings and HVAC systems. Crucially, the guidance generally follows the precautionary principle – “One should take reasonable measures to avoid threats that are serious and plausible.”

A separate Q&A section has also now been published based on the attendee questions received during the webinar – see here

COVID-19 and Airborne Transmission

At the beginning of the seminar, Bill mentioned the growing acknowledgement amongst the scientific community regarding the role of buildings and HVAC systems in airborne infection transmission. On 6th July, a group of 239 scientists from 32 countries published an open letter to the WHO and other professional bodies, arguing that COVID-19 can be transmitted through the air. You can view the full text of this commentary here which was also covered in a recent news piece from MIT Technology Review.

For months, the WHO has insisted that Covid-19 is only transmitted via droplets emitted when people cough or sneeze. Droplets that do not linger in the air, but fall onto surfaces – that\’s why hand-washing has been identified as a key prevention measure.

Imogen Foulkes, BBC News (Link)

Based on the growing body of evidence presented by the scientific community, the WHO recently conceded that the risk of airborne spread \”cannot be ruled out\”. (VOX, \”The debate over “airborne” coronavirus spread, explained\”, 13 July 2020)

Masks and PPE

The topic of masks and PPE is also covered in detail, with a detailed analysis of the impacts of mask-wearing on the transmission rate of the virus among the general population (R0). The chart below presents an overview of the correlation between (1) Public adherence to mask-wearing; and (2) Mask Efficacy, to the overall transmission rate under the intervention.

Howard, J., et al. Face Masks Against COVID-19: An Evidence Review

Engineering Controls in Buildings and HVACS

In the context of buildings and HVAC systems, there are typically a number of engineering controls we have at our disposal to manage and mitigate the risks of disease transmission, and improve the health of occupants:

  • Ventilation – dilutes contaminants, increases exposure time required for exposure to an infectious dose;
  • Air distribution – may contribute to risk if it extends distance travelled by large droplets – avoid high velocity discharge in breathing zone;
  • Filtration – Can remove any aerosol contaminant (but not with 100% certainty). For indoor sources, requires re-circulation in space or system. Also, there is an interesting study on grades of filter vs. mean relative risk of infection, which shows that there is relatively little additional reduction of risk beyond a certain point (MERV 13 / EN779 F7, see Figure below);
  • Disinfection and air-cleaning – methods such as Ultravilot Germicidal Irradiation (UVGI), Photocatalyic Oxidation (PCO) etc.
  • Temperature and humidity control – Several recent studies recommend 40 –60% RH for infection risk, disease specific – and studies on coronavirus suggest they are more resilient than some
Azimi and Stephens, Building and Environment 70 (2013) 150-160 (Note, see Table here for MERV equivalence in EU filter classes)

ASHRAE Guidance

Bill concluded the presentation with an overview of the current work of ASHRAE and the Epidemic Task Force, which is made up of over 150 volunteers, who have been working tirelessly in disseminating technical guidance on topics from building re-opening to filtration, air-cleaning and transportation.

COVID-19 Resources

Seminar Information

About the Seminar: Managing our buildings to minimize the risk of infectious disease transmission is of paramount concern today. Every building and climate are different making it challenging to have one approach work in all situations. Prof. Bahnfleth of Pennsylvania State University provides a critical review of guidance published by ASHRAE and other built environment organizations and the relationship between them and scientists and health-focused organizations like WHO. Prof. Bahnfleth provides expert insight based on the best research for operations to help mitigate risks and what practices to avoid.


About the Speaker: William P. Bahnfleth is a professor of architectural engineering at Penn State University and is a fellow of ASHRAE, the American Society of Mechanical Engineers (ASME) and the International Society for Indoor Air Quality and Climate (ISIAQ).

His research interests cover a wide variety of indoor environmental control topics including chilled water pumping systems, stratified thermal energy storage, protection of building occupants from indoor bioaerosol releases, ultraviolet germicidal irradiation systems, and others. He is the author or co-author of more than 170 technical papers and articles and 14 books and book chapters.

Professor Bahnfleth currently serves as chair of the ASHRAE Epidemic Task Force.

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