Q&A with Prof. Bill Bahnfleth, Epidemic Task Force Chair

On Thursday 16th July, we were delighted to host Prof. Bill Bahnfleth, Chair of the ASHRAE Epidemic Task force on our COVID-19 webinar series – you can view the write-up and webinar recording here. During the webinar, attendees were given the opportunity to ask questions, but due to time constraints and some connections issues towards the end of the webinar, we were unable to answer all of them live. Prof. Bahnfleth kindly agreed to answer these queries offline and has provided detailed responses below.

I also suggest referring to Bill\’s commentary on the latest July 2020 issue of the ASHRAE Journal (pp. 6-7, see below), as well as the COVID-19 FAQ page which has been created based on the aggregated 500+ questions received and answered by the 150+ volunteers on the Epidemic Task Force.

Questions and Answers

Q. Whats the advice on ventilation of clean rooms and using air re-circulation through AHU\’s?  This is been done traditionally.  Is it best to avoid this?

A. REHVA came out strongly in favour of eliminating recirculation in its initial guidance. ASHRAE somewhat followed suit. However, as more consideration was given to the beneficial effects of recirculation through high efficiency filters – which would be defeated by putting the kind of systems we typically have in the US into 100% outdoor air mode + the energy cost of doing that + the inability to maintain space conditions during hot or cold weather + the fact that there is no evidence yet of transmission from space to space due to recirculation, we began to feel that this might not be the right recommendation. Certainly, ensuring at least the intended standard for outside air flow is essential as analysis of super-spreading cases has shown, and some increase would be beneficial for a variety of reasons, but when recirculated air is being filtered by high efficiency filters that are properly installed and maintained, it seems possible to have a high level of protection from COVID-19 with a lower impact on performance and cost. A colleague on the ASHRAE task force who works for a major international consultancy had his staff model risk in a typical office building with 100% outside air and the same system with minimum outside air and MERV 13 filters and found that the relative risk reduction in both cases was about the same. If I were thinking about increase outside air, I would probably be looking at up to 2X minimum for ASHRAE 62.1 because there is quite a bit of evidence that the impacts of increase ventilation within this range will lead to significant IAQ benefits aside from infection risk.

Q. Thank you very much for your talk. Guidance for naturally ventilated buildings often focuses on opening windows to increase ventilation rates. The ASHRAE position document on aerosol transmission and guidance more generally recognises that ventilation rates may not be guaranteed due to the driving forces for natural ventilation. Is there any evidence that suggests the naturally ventilated buildings present a greater risk in transmission terms than mechanically ventilated buildings, and are any additional measures or strategies that could be easily implemented to reduce risk in these types of buildings outside of health care settings?

A. There are certainly those who believe that outdoor air brought in through a window is a better way of ventilating than the same amount of outdoor air brought in by a mechanical ventilation system. The reasons for that point of view would require a lengthy explanation. On the other hand, as you note, the outdoor air ventilation rate from natural ventilation can be highly variable and may also result in changes in flow direction within a building. In a naturally ventilated building, I think the best option for ensuring at least a minimally acceptable flow of outdoor air in the current circumstances would be to use exhaust fans, placed in windows, if necessary. Naturally ventilated buildings are also candidates for supplementary engineering controls like portable HEPA filtration units or air disinfection systems

Q. Request for comment regarding recirculation through HEPA filters

A. With some airborne pathogens that are highly infectious (e.g., measles), recirculation could be a significant hazard. Evidence so far indicates that while SARS-CoV-2 exists in aerosols, it is not as infectious, which means that if it is significantly diluted by recirculation in a large volume, risk may not be high. When high efficiency filtration is combined with recirculation, which is a typical practice in healthcare facilities, it can be protective rather than a risk factor and HEPA filters don\’t seem, again from available and incomplete evidence, to be necessary. This is why ASHRAE is recommending MERV 13 for non-healthcare applications – for central filtration. If portable filter units are used, they should be HEPA to maximize their impact.

Q. So you touched upon UV light filters. It seems to me that UV Filters are quite effective. What are the hindrances to making UV filters more commonplace and how could we address these issues?

A. While very effective when needed, upper room UV can be expensive. Equipment costs alone are on the order of US$30-40/m^2 of floor space served without including installation costs. Air-handling unit installations are much less expensive, perhaps US$6 – 10/m^2. They are certainly a good investment in facilities where they are needed continuously – e.g., healthcare. Whether it makes sense to operate these systems continuously in other occupancies requires more thought. At least one study has shown reductions in sick building syndrome symptoms from use in an office building and there could be a beneficial effect on seasonal illnesses like influenza that would have a quite significant impact on productivity and healthcare costs. From my perspective, doing the necessary studies to better determine the monetary benefits of an investment in UV will help overcome current reluctance. Also, emerging technologies, particularly LEDs that produce different, safer wavelengths than the 254nm produced by Hg vapour lamps could make use widespread because of the longer life and flexibility of application of LEDs. We now have excellent flashlights in our phones thanks to that technology. Who would have thought that would be possible before LEDs came along?

Q. I really like the combination advice of ventilation, HVAC plus UVCGI and Air Cleaners. If the existing system can\’t be upgraded with high efficiency PM1 or even PM2.5 filters

A. It is very logical if one thinks through it. We need some ventilation to handle the wide range of – often unidentified – gaseous contaminants in a building. Ventilation helps removes particles, but filters provide a lower cost way to do it if high efficiency is needed, and disinfection of air is a third layer of protection that can supplement a level of filtration that represents a good balance between performance and cost. We are certainly in a performance-first mind-set at the moment for obvious reasons, but in the future, economics and other factors will influence what changes we make to design to make buildings safer.

Q. Do you have any comments on the Amoy Gardens infection in the 2003 SARs incident where it appears that air transmission was the route cause?

A. While SARS-1, MERS and SARS-2 are not identical, I think it is reasonable to consider the experience with SARS when thinking about how to deal with COVID-19. Amoy Gardens, in that epidemic, has about the same significance as the Guangzhou restaurant in the current pandemic. Indeed, our friend Yuguo Li (same one who has investigated Diamond Princess and the restaurant) was a central figure in the analysis of SARS outbreaks and Amoy Gardens in particular. Forensic studies concluded that defective plumbing allowed re-entry of faecal aerosols into apartments within a block and that movement of infectious aerosol in the building due to natural driving forces contributed to additional infections. Potential for transmission from one block to another was also suggested by modelling. I think this is a further reason for us not to dismiss the possibility of aerosol transmission of COVID-19. Someday we will probably know, but for now, prudence dictates that we protect against it in reasonable ways.

Q. Have there been any other recent published studies regarding airborne transmission other than the Chinese restaurant and US choir practice?

A. There is a new analysis based on modelling of the Diamond Princess incident by Azimi, et al. (2020). It is somewhat at odds with the findings of Li, et al. (2020) in an earlier study. Li, et al (same Li who analysed the Guangzhou restaurant) concluded \”We infer that the ship central air conditioning system did not play a role, i.e. the long-range airborne route was absent in the outbreak. Most transmission appears to have occurred through close contact and fomites.\” Azimi, et al. believes that long range transmission did occur.


I would like to once again extend our gratitude to Prof. Bahnfleth for his enormous effort in Chairing the Epidemic Task Force (ETP), and to all the ASHRAE volunteers who have helped assemble this wealth of technical guidance, and continuously provided answers to questions and concerns raised by practitioners and the general public. The work of the ETP really exemplifies the values and ethos that ASHRAE stands for: Excellence, Commitment, Integrity, Collaboration, Volunteerism and Divesity.

I include some of the comments below from those who attended Prof. Bahnfleths webinar, which were unanimously positive.

  • Super presentation Bill and looks like equally good technical guidance.
  • I\’m delighted with the quality and practical guidance.
  • Well presented topic! Looking forward to the next event.
  • The presentation was clear and delivered on the presentation topic. The logic of the presentation was coherent, well-reasoned and informative
  • This was an excellent presentation and certainly very useful in the current context.
  • A wealth of information presented professionally by Bill which pulled together all the hard work undertaken by ASHRAE Covid committees in a very short period. Interesting that the information is continually evolving as more experience is gained which highlights the need for all HVAC professionals to keep regularly updating themselves through the auspices of ASHRAE, REHVA and CIBSE.
  • The presentation was precise and exact and a tonic in an uncertain time.

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