Limiting the Spread of Airborne Pathogens Through Mechanical Design

Article By: Kevin Sharples (B.A.Sc., P.Eng.), Kurt Monteiro (B.A.Sc., P.Eng., LEED AP, HFDP, HBDP), Douglas Smith (BA.Sc., P.Eng.), Dan Larson (B.A.Sc., P.Eng., Eng. / ing., LEED AP BD+C), Andrew Caller (B.Eng., P.Eng., LEED AP), Greg Trypuc (P.Eng., LEED AP), Tony Spina (C.E.T.)

 

Life was very different four months ago. Practices like “social distancing”, “self isolation”, and “community spread” were not considered mainstream in January. In the wake of COVID-19, concern about the invisible is now visible on the face of every participant in every virtual meeting that (seemingly overnight) has become the norm.

Invisible threats—airborne pathogens like viruses and bacteria—have always been a central consideration in each of our mechanical designs. Limiting the spread of airborne pathogens plays a big role regardless of the building type and, in many spaces (such as healthcare and laboratory facilities) this is actually a design driver. Technology regularly leveraged in such spaces is now up for discussion in every building type—hotels, office buildings, restaurants, shopping centres—as building managers, operators, and developers examine every possibility in the context of opening up shared spaces in a safe and healthy way.

 

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“Is the spread of SARS-CoV-2 through HVAC systems something I should be worried about in my building?”

There is no conclusive, peer-reviewed research that confirms that SARS-CoV-2 is distributed by the HVAC system and, based on current infection patterns, the HVAC system is likely not a high transmission pathway for SARS-CoV-2. However, research is ongoing, and the solutions under consideration (some of which are outlined here) have demonstrated efficacy for other airborne pathogens.

Based on the research available to date, the most significant threat surrounding contracting COVID-19 is presented through direct contamination within the workplace and not airborne concerns. Insufficient social (physical) distancing, contact with contaminated surfaces, and subsequent contact with the hands, eyes, nose, and mouth are the more prevalent concerns in shared spaces during this pandemic. That being said, HVAC design is always evolving, and never more so than after a global pandemic. We must review the technology available with an open mind and a level head. A closer examination of preventative measures for this specific health concern may result in design and operations practices that further support overall wellness in indoor environments.  

 

“What technology should I consider?”

Before any decisions are made, a thorough understanding of the building systems involved, and the risks and benefits each part provides in the indoor air environment, is required. An under floor air distribution (UFAD) system circulates air differently than displacement ventilation or overhead air distribution. Airflow in partially or naturally ventilated buildings is variable and unpredictable, so the ability to actively manage the spread of airborne pathogens in buildings that implement these systems is not straightforward. 

There is conclusive research demonstrating the direct relationship between the number of airborne particles and the rate of transmission among airborne pathogens. Most large particles in the air stream drop to the ground within two metres of where they were expelled—dropping outside of the “breathing zone”. Medium-sized particles move through spaces and HVAC systems, and are captured by the appropriate filters. Smaller particles—particles that can support the transfer of a virus like SARS-CoV-2—largely stay suspended in indoor spaces, and are not impacted by HVAC system air currents. It is, therefore, less likely that these particles would be captured by conventional HVAC media filters. 

Drawing from our experience with healthcare design across the country, some possible design solutions that may help limit the spread of airborne pathogens include:

  • Increased Ventilation Air: Undoubtedly, increasing outside air helps dilute recirculated air, which is the air that presents a greater risk to building occupants (as other occupants could be infected). Increasing ventilation air would also increase energy consumption, and would need to be reviewed in conjunction with a filtration solution appropriate for the space. We support dilution as a possible benefit, but only during the hours when a space is occupied. Dilution, when there is no source, would only benefit the overall health of the indoor air, and would not address the specific concern of airborne pathogens.

  • Bi-Polar Ionization Filtration: This technology uses polarizing particles to initiate coagulation of smaller particles and pathogens into bigger particles that can be more easily captured by filters. This form of active filtration could be used whether or not you bring outside air into your space, and would integrate relatively easily into existing commercial and residential HVAC systems. Bi-polar ionization removes both medium and small particles from indoor spaces, reducing transmission vectors. There is also the benefit of sending ionized particles into the occupied zone, where they act like sponges when contacting airborne particles.

  • High Efficiency Particulate Air (HEPA) Filtration: HEPA filters utilize much higher levels of filtration than standard medium-efficiency filters, and are proven to capture and remove medium and large particulates and (to some degree) infectious contaminants from the air. Most on-floor systems in office buildings do not have HEPA filters in place, and discussion around these filters is ongoing as they play a central role in the recirculated air path. HEPA filters have high air pressure drop, resulting in higher energy use, and are more expensive than traditional filters. If HEPA filters are used to replace the existing final filters, they will load more quickly, but will require more frequent replacement at significant operational cost. In most traditional air handling units or compartment units, modifications would have to be made to the fan system to overcome the added pressure drop, as well as to the ductwork or air handling system to accommodate HEPA filters that do not require frequent replacement.

  • Ultraviolet (UV) Filtration: Using UV light inside air handling units to disinfect coil and duct surfaces has been implemented in buildings for years, though often not prioritized. The use of UV, in any capacity, requires proper operations and maintenance procedures in place, an understanding of life cycle costs, and a thorough risk assessment conducted with the owner.

  • Increased Relative Humidity: There is mounting support for an increase in relative humidity in buildings to decrease the transmission of some infectious diseases. Regional climate (a significant factor in Canada) and building envelope would play a significant role in this discussion, as well as scheduling considerations to account for occupant comfort. In the case of COVID-19, some studies have shown that the SARS-CoV-2 is susceptible only at very high temperatures (e.g., 30oC) and very high relative humidity (e.g., 80%), which is not widely-accepted for occupant comfort in indoor environments.

 

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“Where do we go from here?”

Without a cure or a vaccine for COVID-19, it is tempting for many to want to point to a single solution for the answer. But, if there’s one thing that 55 years of HVAC design has taught us, it’s that there is no “one size fits all” solution. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) released a Position Document on Infectious Aerosols (Approved by ASHRAE Board of Directors on April 14, 2020). In this document, ASHRAE emphasizes that mitigating infectious aerosol dissemination should be a consideration in the design of all facilities, yet reminds that:  

“The design and operation of HVAC systems can affect infectious aerosol transport, but they are only one part of an infection control bundle.” (ASHRAE Position Document on Infectious Aerosols, April 14, 2020)

We need to rely on one another’s strengths, balance innovation and skepticism, and work together to clear the air. 

 

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Editor's Note: This article was originally published on May 13, 2020. As of June 9, 2020, research on the airborne nature of SARS-CoV-2 remains ongoing, with an increased amount of research conducted within a healthcare setting indicating the likelihood of airborne transmission. Much of this research remains inconclusive, though precautions are encouraged. Smith + Andersen continues to review available technology and provide our clients with appropriate solutions in conjunction with the latest research and technology available.