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Experts Embrace “Equivalent Air Changes” Approach to Reduce COVID Transmission in Buildings

Published 04/26/2021
By Christian Weeks
enVerid HEPA Air Purifier with Controller

In the early days of the COVID-19 pandemic, ASHRAE released initial recommendations based on our limited understanding of the virus at that time, focusing heavily on controlling airborne transmission by maximizing outside air. Many experts echoed ASHRAE’s recommendations, calling for as much outside air ventilation as possible. For example, on March 4, 2020 Dr. Joseph Allen of the Healthy Buildings program at Harvard’s T.H. Chan School of Public Health wrote a NY Times Op-Ed summarizing the best practices at that time, which called for ventilation as the first line of defense with filtration as a backup if that was the only option.

ASHRAE Core Recommendations for COVID-19

Fast forward to early 2021 and ASHRAE has developed new “core recommendations” reflecting a more informed understanding of the virus and how its airborne transmission can be controlled. The new guidance also recognizes the limitations of many building HVAC systems to handle more outside air and the significant energy costs associated with bringing in more outside air. Now ASHRAE and experts like Dr. Allen recommend control strategies based on the concept of “equivalent” or “effective” air changes.

Translation: increasing outside air is not the only way to achieve safe, clean indoor air. In fact, the new guidance acknowledges that there are more cost-effective ways to achieve target effective air changes with filtration and local air cleaning.

ASHRAE’s updated core recommendations from January open with a statement saying that they are:

“…based on the concept that within limits ventilation, filtration, and air cleaners can be deployed flexibly to achieve exposure reduction goals subject to constraints that may include comfort, energy use, and costs.”

With this concept in mind, ASHRAE’s core recommendations are now to:

  • Provide and maintain at least required minimum outdoor airflow rates for ventilation as specified by applicable codes and standards.
  • Use combinations of filters and air cleaners that achieve MERV 13 or better levels of performance for air recirculated by HVAC systems.
  • Only use air cleaners for which evidence of effectiveness and safety are clear.
  • Select control options, including standalone filters and air cleaners, that provide desired exposure reduction while minimizing associated energy penalties.

Since releasing these core recommendations, ASHRAE has also updated its Building Readiness Guide to align with the core recommendations. The Building Readiness Guide now describes increasing outside air above code minimum as a “mitigation strategy to be evaluated” rather than a recommendation and says:

“The equivalent outdoor air calculation indicates that the outdoor air can be calculated by using the combination of the actual outdoor air, impact of filtration or air cleaning technologies on recirculated air, and the impact of air cleaning technologies in the space.” (pg 26)

ASHRAE is not alone in updating its guidance. Other experts including Dr. Allen have also updated their recommendations and have gone a step further by providing an “effective air change” or ACHe target for COVID-19 mitigation. In an April 16, 2021 peer-reviewed paper in JAMA, the Journal of the American Medical Association, Dr. Allen wrote:

“To reduce far-field airborne transmission of SARS-CoV-2 in small-volume indoor spaces (eg, classrooms, retail shops, homes if guests are visiting), the suggestions include targeting 4 to 6 air changes per hour, through any combination of the following: outdoor air ventilation; recirculated air that passes through a filter with at least a minimum efficiency rating value 13 (MERV 13) rating; or passage of air through portable air cleaners with HEPA (high-efficiency particulate air) filters.”

Translation: one can achieve the recommended 4-6 ACHe through a combination of control strategies that may include MERV 13 filtration, local HEPA filtration, and outside air ventilation, as well as upper-room UVGI (Ultraviolet Germicidal Irradiation).  

enVerid HEPA Air Purifier with Controller
The enVerid HEPA Air Purifier with Controller

For example, if your existing HVAC system has a MERV 13 filter, you can use it to change the air 3 times per hour and use additional outside air or local HEPA filtration to achieve an additional 1-3 ACH. Similarly, if your HVAC system cannot provide MERV 13 or better filtration, rather than replacing the HVAC, you might choose to install local HEPA filtration that filters the air to achieve the target 4-6 ACHe in the space.

This shift from recommending outside air as the gold standard to a recognition that filtered air can be just as clean and safe is significant because it allows for a pragmatic and cost-effective approach to providing safe indoor air, enabling more ways to arrive at that goal. Even more significant on the heels of Earth Day and ever more ambitious carbon neutrality goals from leading real estate companies, using filtration and local air cleaning is far less energy- and carbon-intensive than increasing outside air.

This was highlighted during a recent ASHRAE NY webinar on “Energy Use in the Pandemic Era.” Speakers included Molly Dee and Jonathan Li from Jaros Baum & Bolles (JB&B), who presented a study on “Energy and Carbon in the COVID Era: Lessons Learned on Balancing IAQ and Building Performance.”

As part of the study, JB&B performed a theoretical set of calculations using ASHRAE’s initial COVID guidance from a year ago and then compared energy use, carbon emissions, and IAQ using alternative ASHRAE-approved solutions now included in the “core recommendations” including UV, MERV filtration, and HEPA filtration. To perform the calculations, JB&B used an open-source COVID aerosol transmission modeling tool developed by Prof. Jose Jimenez and Dr. Zhe Peng at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder.

What the JB&B study found was that using increased MERV filtration, reducing flushing, and using UVC+ portable air cleaners is more cost-effective than following ASHRAE’s initial guidance for COVID mitigation. The study showed that following the initial maximum outdoor air approach across 10 properties studied would result in an average 14% annual energy increase, 10% annual cost increase, and 11% annual carbon emissions increase, all of which become very large numbers over time. The equivalent air changes methods used 4 to 8% more energy than baseline. These findings align with building-specific analysis conducted by enVerid since the start of the pandemic.

enVerid COVID-19 Energy Estimator Infographic
Click to expand

So how is an engineer to know how to balance outside air, MERV filtration, and HEPA filtration to achieve 4-6 ACHe? To calculate the effectiveness and costs of these approaches, enVerid published the enVerid COVID-19 Energy Estimator, a free, open-source tool developed by enVerid in partnership with Dr. Marwa Zaatari, P.E., an ASHRAE Distinguished Lecturer and member of ASHRAE’s Epidemic Task Force Commercial Buildings Subcommittee. The Energy Estimator helps building owners, mechanical engineers, and facility managers evaluate the risk, costs, and carbon impacts of different ventilation and filtration approaches. The Estimator confirms that installing high-efficiency filtration can be as effective and lower cost than increasing ventilation rates to achieve target air exchange rates to reduce COVID-19 exposure risk. According to Barry Abramson, P.E., Principal at Servidyne,

“The COVID Energy Estimator is a timely tool to help engineers and building managers assess different ventilation and filtration strategies in terms of risk mitigation, cost and carbon impact… It is the most useful HVAC-focused risk estimator that we’ve seen, and even in this early iteration it will be very helpful for developing re-entry strategies for our clients’ buildings.”

Now that a variety of equivalent approaches has been proven viable and endorsed by ASHRAE and other experts, healthy indoor air can be more easily achieved in any space. To quote Dr. Joseph Allen,

“Increasing air changes per hour and air filtration is a simplified but important concept that could be deployed to help reduce risk from within-room, far-field airborne transmission of SARS-CoV-2 and other respiratory infectious diseases. Healthy building controls like higher ventilation and enhanced filtration are a fundamental, but often overlooked, part of risk reduction strategies that could have benefit beyond the current pandemic.”

Click here to learn more about the enVerid COVID-19 Energy Estimator.

Christian Weeks

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