MIT modelling study challenges indoor social distancing guidelines

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The risk of being exposed to COVID-19 indoors can be as great at 60 feet as it is at 6 feet in a room where the air is mixed – even when wearing a mask, according to a study by Massachusetts Institute of Technology researchers who challenge social distancing guidelines adopted across the world.

CNBC News reports that MIT professors Martin Z Bazant, who teaches chemical engineering and applied mathematics, and John WM Bush, who teaches applied mathematics, developed a method of calculating exposure risk to COVID-19 in an indoor setting that factors in a variety of issues that could affect transmission, including the amount of time spent inside, air filtration and circulation, immunidation, variant strains, mask use, and even respiratory activity such as breathing, eating, speaking or singing.

Bazant and Bush question long-held COVID-19 guidelines that recommend 6 feet of distance between people from the US Centres for Disease Control and Prevention in a peer-reviewed study. They say staying 6 feet away from another person may not be enough when people are inside for prolonged periods of time.

“We argue there really isn’t much of a benefit to the 6-foot rule, especially when people are wearing masks” since everyone in the room is breathing the same air, Bazant is quoted in CNBC as saying. “It really has almost no physical basis because the air a person is breathing while wearing a mask tends to rise and comes down elsewhere in the room so you’re more exposed to the average background than you are to a person at a distance.”

Without masks, however, Bazant said 6 feet would provide some additional protection against COVID-19 versus 3 feet. That’s particularly true if a person is breathing directly in your direction, creating what scientists call a “respiratory plume,” like a puff of smoke from a smoker. Their research was based on models that assume the air in the room is “well mixed” and that “the pathogen is distributed uniformly throughout,” they said in a follow-up statement to CNBC.

In well-mixed spaces, “one is no safer from airborne pathogens at 60 feet than 6 feet,” they said.

The important variable CDC and World Health Organisation guidelines have overlooked is the amount of time spent indoors, Bazant said. The longer someone is inside with an infected person, the greater the chance of transmission, he said.

Opening windows or installing new fans to keep the air moving could also be just as effective or more effective than spending large amounts of money on a new filtration system, he said.

Bazant also says that guidelines enforcing indoor occupancy caps are flawed. He said 20 people gathered inside for one minute is probably fine, but not over the course of several hours, he said.

 

Study details
A guideline to limit indoor airborne transmission of COVID-19

Martin Z Bazant, John WM Bush

Published in PNAS on 27 April 2021

Abstract
The current revival of the American economy is being predicated on social distancing, specifically the Six-Foot Rule, a guideline that offers little protection from pathogen-bearing aerosol droplets sufficiently small to be continuously mixed through an indoor space. The importance of airborne transmission of COVID-19 is now widely recognized. While tools for risk assessment have recently been developed, no safety guideline has been proposed to protect against it. We here build on models of airborne disease transmission in order to derive an indoor safety guideline that would impose an upper bound on the “cumulative exposure time,” the product of the number of occupants and their time in an enclosed space. We demonstrate how this bound depends on the rates of ventilation and air filtration, dimensions of the room, breathing rate, respiratory activity and face mask use of its occupants, and infectiousness of the respiratory aerosols. By synthesizing available data from the best-characterized indoor spreading events with respiratory drop size distributions, we estimate an infectious dose on the order of 10 aerosol-borne virions. The new virus (severe acute respiratory syndrome coronavirus 2 [SARS-CoV-2]) is thus inferred to be an order of magnitude more infectious than its forerunner (SARS-CoV), consistent with the pandemic status achieved by COVID-19. Case studies are presented for classrooms and nursing homes, and a spreadsheet and online app are provided to facilitate use of our guideline. Implications for contact tracing and quarantining are considered, and appropriate caveats enumerated. Particular consideration is given to respiratory jets, which may substantially elevate risk when face masks are not worn.

 

Full CNBC report (Open access)

PNAS study (Open access)


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