ISIAQ Webinar: Ventilation Impacts on Indoor Aerosol Transport and Current HVAC Recommendations for Re-Opening Buildings

Dr. Persily and Dr. Ng gave a great webinar. Andy Persily is a mentor of mine and is the Division Chief of the Energy & Environment Division, National Institute of Standards and Technology (NIST), and Lisa Ng, is a NIST scientist in the Indoor Air Quality & Ventilation Group. you had to be there to glean all the wisdom, but here are some bullet points I wrote down in summary. The webinar is available at the ISIAQ website for members (and it is only $100 to join, totally worth it).

Ventilation and building airflows:

  • infiltration rates in individual buildings vary 5 to 1 with weather
  • Mechanical/residential systems can provide between 0.1 to 0.5 ACH outdoor air, not many homes have these systems (yet)
  • Commercial outdoor air is around 1 per hour, and up to 5 per hour. Supply air is around 3-5 per hour. Note this is just for the US!
  • Characterizing ventilation: 
    • What is system intended to do? what are the outdoor air intakes rates?  Supply air flow rates? when is it suppose to operate?
    • Air flow rates – is it on or off, mode of operation, whole building outdoor air change rates need to be measured and not just one time. Room air change rates hard to measure.  
    • investigate the pressure differences: indoor to outdoor inter zone
  • ASK: Why are you measuring ventilation? What is the weather, what is the system operation? One measurement doesn’t tell you much
  • Take a lot of measurements, over range of weather and system operation. This hasn’t been done very often
  • Systems do not always run as intended
  • Reducing aerosol exposure with airflow?
    • Build tight, ventilate (& filter) right
    • Minimize uncontrolled infiltration
    • Overpressure the buildings – careful with moisture
    • Air flow from clean spaces to dirty
    • Commission, operations and maintenance procedures must be in place and followed

Recommendations for Outdoor ventilation – from APPA, ASHRAE, REHVA, BOMA, DOE, etc:

  • recommend increasing outdoor air or recirculation rates (I add that if increasing recirculation, need to make sure you have good filtration)
  • Maintain 24/7 outdoor ventilation
  • Disable demand control ventilation
  • Check heat recovery devices for leaks
  • Check airflow directions and pressures
  • Clean/disinfect intakes and returns (AIHA)
  • Ensure proper filtration (OSHA, AIHA)
  • Continue routine maintenance
  • MERV-13 minimum (MERV-14 preferred, HEPA better) for filtration
  • HEPA filters are very good because:
    • See this paper by Zhao et al. 2020
    • Peak concentration of COVID appears in two distinct size ranges 0.25-1 microns and >2.5 microns; HEPA is 95% effective for 0.25-1 um and and ~100%> 2.5 um
  • RH? Between 40-60% RH
    • Evidence does not support that moderate humidity will be beneficial for COVID thus humidification is NOT a method to reduce viability of SARS-CoV-2
  • Close lid of toilet when flushing, maintain under pressure, exhaust fans 24/7, keep windows/doors closed
  • germicidal UVC consider as supplementary (ASHRAE, DOE, APPA)
  • No PPE recommendations
  • Schoen L.J. 2020 Guidance for building operations during COVID-19 pandemic, ASHRAE J 62 (5) 72-74.
  • Persily and Wargocki, P (2016) how to evaluate ventilation in IAQ studies, proceedings, 14th Int Conference on Indoor Air Quality and Climate, Indoor Air 2016 Ghent