Salubrious Simulation: Modeling Airborne Virus Dispersion to Reduce Transmission Risks

by Marcy Marro | February 1, 2021 12:00 am

As we move beyond the now-familiar reactive responses to reduce virus transmission risks, we can begin to explore how to better design buildings and building systems to promote wellness and reduce transmission of known and anticipated pathogens. Elevated pathogen concerns and increased awareness of indoor air quality will make enhanced ventilation and air cleaning systems desired attributes for buildings. In response, several simulation software tools are emerging as useful means to support important health and wellness design decisions. There are limitations to each of these tools, and experts should fully recognize these when they apply them.

The building industry has increasingly turned to simulation tools to inform the design of buildings, addressing performance aspects including energy use, enclosure design, ventilation optimization, fire safety and smoke control. Code Unlimited[2] has been a leader in some of these applications, guiding unique solutions to exiting, atrium smoke evacuation and other fire and life safety design challenges. With the onset of the COVID-19 pandemic, the Code Unlimited team began to extend these capabilities to help clients with the new health and life safety threat from the virus. Some of the tools used for air and fire/smoke movement modeling can be applied to simulate the movement of virus-laden respiratory particles in occupied spaces as they act similar to soot particles in smoke. NIST developed a new software application called Fate and Transport of Indoor Microbiological Aerosols (FaTIMA). This application is used to create low-resolution models of single, mechanically ventilated zones and to show the fate of respiratory droplets and particles from a simulated sneeze, tracking whether particles exited the zone, were filtered out, deposited on surfaces or remained airborne. Of the primary ways that virus-containing droplets and particles are expelled by infected people, sneezes are considered more expulsive than talking, singing or coughing.

Prompted by client requests, Code Unlimited initiated a pilot investigation of respiratory droplet dispersion using FaTIMA in hypothetical spaces to test its application. The simulation considered an operating room and an open office. In both cases, a 530 square foot (23-foot by 23-foot) room with a 13-foot-high ceiling was modeled with one unmasked infected person sneezing in the space. Currently published research established that particle sizes in a sneeze have a bimodal distribution, with mean peak values centered around 74 microns and 360 microns, so the simulated sneeze droplet were modeled based on this presumption.

With a typical operating room ventilation rates of 2800 CFM of supply air and 2000 CFM of exhaust, the simulated airborne droplets remain suspended in the air for only a few minutes. However, in an office environment with 10% of the operating room ventilation rates (300 CFM supply and 240 CFM return) some of the particles remained suspended for several hours, especially the smaller ones, which are still infectable.

These simulations also uncovered some important differences in particle fate depending on the location of supply diffusers and return grilles (e.g., ceiling or floor). Future simulations will focus on identifying optimal supply and return locations to limit occupant exposure to airborne pathogens.

Simulation of airborne pathogen dispersion is a new and evolving practice. The Code Unlimited team plans to take what they have learned and do another round of analysis comparing these results against other common space types like meeting rooms, classrooms and other assembly spaces, using FaTIMA and higher resolution computational fluid dynamic (CFD) software tools. As we emerge from the pandemic and imagine healthy indoor environments where face covering and other ad hoc infection control strategies are no longer required, ventilation that quickly and effectively remove pathogens will still be desirable.

Reliable simulation tools are needed to inform the design of ventilation systems that limit pathogen transmission risks from SARS-CoV-2, rhinovirus (common cold), seasonal flu, measles and future viral threats. These applications will become indispensable tools in the design of healthy buildings.

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Alan Scott, FAIA, LEED Fellow, LEED AP BD+C, O+M, WELL AP, CEM, is an architect with over 30 years of experience in sustainable building design. He is a senior consultant with Intertek Building Science Solutions[3] in Portland, Ore. To learn more, follow Scott on Twitter @alanscott_faia[4]. Samir Mokashi is a founding principal of Code Unlimited, a minority-owned firm in Beaverton, Ore. IG: @codeunlimited[5]. He has more than 30 years of building code, fire and hazardous materials consulting experience. Learn more at www.codeul.com[6]. The Code Unlimited advanced computing team of Amedeo Gallucci, Caitlynn Holmes, Deyan Aydarski, and Vincent Collins, made significant contributions to this article.

Source URL: https://www.metalarchitecture.com/articles/salubrious-simulation-modeling-airborne-virus-dispersion-to-reduce-transmission-risks/

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