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Tuesday, September 22, 2020 — Houston, TX °

Critical fault lines in our reopening plan

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Photo Courtesy of Rice University Department of Chemical and Biomolecular Engineering

By Kyriacos Zygourakis     8/17/20 11:57am

Editor’s Note: This is a guest opinion that has been submitted by a member of the Rice community. The views expressed in this opinion are those of the author and do not necessarily represent or reflect the views of the Thresher or its editorial board. All guest opinions are fact-checked and edited for clarity and conciseness by Thresher editors.

Rice University’s president and provost’s message on July 28 summarized all the steps the university has taken to assure the safety of our community and our ability to fulfill our mission.  As the new academic year is about to begin, however, the rate of new COVID-19 cases is still very high and deaths in Harris County continue to increase. The pandemic is still spreading unabated in a community where approximately 1,500 of our off-campus undergraduates, our graduate students and our staff and faculty will live, dine, shop, party and risk being exposed to COVID-19. The same group of people will then come to campus to attend classes, teach, research, work and potentially infect more people. 

We are told that the protective measures we adopted will ensure the safety of our Rice community and that we will be able to manage the spread of the virus by wearing face masks, maintaining six-foot separation, washing our hands, sanitizing surfaces and following a testing and tracing regimen. However, epidemiology does not fully support this assertion. Experts tell us that while these measures may help slow the spread of COVID-19, more drastic steps like lockdowns are needed to contain the disease down to a level that would allow for safe reopening of schools, colleges and businesses — one to 10 new cases per million people per day.  But currently, Harris County has more than 230 new cases per million people per day



Worse still, these measures may not be effective against all modes of transmission of SARS-CoV-2. In the beginning of the pandemic, we thought that the primary mode of SARS-CoV-2 transmission was by respiratory droplets larger than five to 10 microns that fall rapidly to the ground. This led to the recommendations for six-foot separation, surface sanitizing and, eventually, wearing masks. Evidence has been building over time, however, that SARS-CoV-2 can also be transmitted via aerosol droplets smaller than five microns in size. This mode of transmission was confirmed in a hot-off-the-press preprint publication that reports the presence of viable SARS-CoV-2 virus in aerosol droplets collected 16 feet away from a patient. Viral transmission via aerosols is a serious threat to people who work or teach inside because droplets in the one to five micron size range stay suspended in the air and circulate for long periods of time. While a 10-micron droplet with unity density will settle five feet in eight minutes, a three-micron droplet takes an hour and a half to settle from the same height and one-micron droplets will require 12 hours. Currents created by ventilation systems will keep aerosol droplets suspended in air for even longer times. If no fresh air is introduced into a room where an infected individual sheds one micron virus-laden droplets, it may take 36 hours for 90 percent of these droplets to settle or clear.

Since SARS-CoV-2 may also be transmitted via aerosol droplets, the protective measures we have adopted may not be good enough. Cloth masks may not fully protect us from infection, since aerosols can both penetrate and circumnavigate them. Face shields will provide only partial protection because there are open gaps between the shield and the wearer’s face. And six feet of separation will not necessarily protect from aerosols that remain suspended in the air or are carried by currents over much longer distances. There are only two additional mitigation strategies for lowering the risk from aerosol transmission. We can either dilute the concentration of aerosol droplets by exchanging air in the interior of a building with air from outside (thus lowering the viral load a person may inhale) or we can remove aerosol droplets from the air with filters.

The president and provost’s letter mentions that laboratory spaces typically have more than 10 air changes per hour. This will definitely lower the risk for those working in lab buildings.  But most Rice buildings have HVAC systems that recirculate old air with only a fraction of fresh air pumped in at any time. Here, the letter only says that “campus buildings are modified to … increase outside air ventilation by reprogramming systems designed to vary air flow for energy conservation ... and enhance air filtration through the use of higher efficiency filters.”  

These statements, however, do not directly address the aerosol transmission risk. To assess the effectiveness of these HVAC improvements, we should know how often the air in every building is replaced with fresh air, or, equivalently, the percentage of fresh air introduced into the HVAC system of every Rice building after the reprogramming. We also need to know the minimum efficiency reporting values rating of the new filters because only filters rating higher than 12 can remove 80 to 90 percent of aerosol particles in the one to three micron range, and more than 90 percent of particles in the three to 10 micron range. The July 28 letter also mentions the incorporation of UV-C lights into the HVAC systems. More details and specifications are needed, however, to determine whether such add-ons are effective and can deliver the dose of UV-C irradiation required to kill viruses. Incidentally, different systems of germicidal UV lights are used to sanitize the air in hospital rooms

The persistently high community transmission in Houston together with the mounting evidence about the importance of aerosol transmission of SARS-CoV-2 raise serious doubts about the overall effectiveness of the measures our university has taken to keep our community safe. Our reopening plan seems more and more like a risky experiment, something that other universities like Princeton and Stanford have ultimately decided not to undertake.

Kyriacos Zygourakis is the A.J. Hartsook Professor of Chemical and Biomolecular Engineering.



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