When in a room where coronavirus pathogens have been "well mixed" throughout the air, physical distance alone is not always sufficient to reduce transmission, according to a new study in the Proceedings of the National Academy of Sciences.
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For the study—which underwent three rounds of peer review before being published—MIT professors Martin Bazant, who teaches chemical engineering and applied mathematics, and John Bush, who teaches applied mathematics, developed a model to calculate Covid-19 exposure risks in a variety of indoor settings. For instance, the model could take into account factors such as how much time someone spent indoors, overall air circulation and filtration, a person's vaccination status, the variant strains in circulation, mask use, and the level of respiratory activity, such as breathing, eating, talking, or singing.
Using those factors, the researchers then calculated the exposure risk from "well mixed" air—as opposed to direct exposure to large respiratory droplets—in various hypothetical scenarios. For example, in a hypothetical classroom that could hold 19 students and a teacher with either open windows or an adequate HVAC system, the researchers determined that, "[f]or normal occupancy and without masks, the safe time after an infected individual enters the classroom is 1.2 hours for natural ventilation and 7.2 hours with mechanical ventilation."
However, the researchers said those estimates assumed that the students were sitting quietly. "Extended periods of physical activity, collective speech, or singing would lower the time limit by an order of magnitude," the researchers wrote.
Similarly, the researchers applied to New York City nursing homes, where the law mandates a "maximum occupancy of three and recommends a minimum area of 80 square feet per person." In that scenario, the researchers found that, with natural ventilation, keeping people six feet apart would protect them from exposure for just three minutes after an infected person entered the room, or for 17 minutes if the infected person stayed in the room only momentarily.
And in that scenario, mechanical ventilation would provide little improvement, the researchers wrote. "With mechanical ventilation in steady state, three occupants could safely remain in the room for no more than 18 minutes," they wrote.
Based on their calculations, the researchers said exposure risk in a room where the virus has been "well mixed"—or, in other words, when "the pathogen is distributed uniformly throughout" the room—depends more on an individual's time spent in the room and the overall occupancy rate than occupants' physical distance from each other.
"Efficient mask use is the most effective safety measure, followed by room ventilation, then filtration," Bush said. "And risk increases with the number of occupants and the exposure time, so one should try to spend as little time as possible in crowded indoor spaces."
However, the researchers pushed back on interpretations of their findings that suggested they found social distancing to be an ineffective precaution.
"A common misinterpretation of our study is that it 'finds' no effect of social distancing, when in fact, our safety guideline for airborne transmission is derived from the standard assumption of well-mixed indoor air, where distance plays no role," Bush explained. In those scenarios, with well-mixed indoor air, Bazant said, relying on distancing alone could "giv[e] you a false sense of security" against transmission.
But physical distance can still help protect against coronavirus transmission from larger respiratory droplets that tend to fall to the ground within a short distance. As Bush and Bazant stated in the study conclusion, "Adherence to the Six-Foot Rule would limit large-drop transmission, and adherence to our guideline ... would limit long-range airborne transmission."
Using cigarettes as an example, Bazant added, "Turbulent puffs of exhaled smoke help us visualize respiratory jets [or large-drop transmission] from coughing, speaking, or breathing through the mouth or nose, whose concentration decays quickly with distance." In comparison, he explained, "secondhand smoke in a well-mixed 'smoke-filled room' is analogous to long-range airborne transmission, which depends more on time and occupancy than on physical distance."
Likewise, Bazant said, unmasked people would have some additional protection against Covid-19 if they were to stand further away from each other indoors—say, six feet rather than three—so as to minimize large-drop transmission. But if everyone is wearing a mask, and the air in the room is well mixed, people could better protect themselves against transmission by focusing on ventilation, filtration, level of occupancy, and overall time in the room.
Ultimately, the researchers said their findings aim to demonstrate when social distancing guidance may—in certain hypothetical scenarios—be either too restrictive or too lax. "Our guideline also quantifies the relative benefits of improved ventilation, filtration, and mask directives, which can inform decisions to reopen or upgrade specific indoor spaces, such as schools and churches," they added (Bellware, Washington Post, 4/27; Fox, CNN, 4/27; Mendez, CNBC, 4/27).
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