MIT researchers say you’re no safer from Covid indoors at 6 feet or 60 feet in new study
Customers dine at Picos Restaurant, which received threats following their announcement of continuing to require masks, as the state of Texas prepares to lift its mask mandate and reopen businesses to full capacity during the coronavirus disease (COVID-19) pandemic in Houston, Texas, March 9, 2021.
Callaghan O’Hare | Reuters
The risk of being exposed to Covid-19 indoors is as great at 60 feet as it is at 6 feet — even when wearing a mask, according to a new study by Massachusetts Institute of Technology researchers who challenge social distancing guidelines adopted across the world.
MIT professors Martin Z. Bazant, who teaches chemical engineering and applied mathematics, and John W.M. 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, immunization, variant strains, mask use, and even respiratory activity such as breathing, eating, speaking or singing.
Bazant and Bush question long-held Covid-19 guidelines from the Centers for Disease Control and Prevention and the World Health Organization in a peer-reviewed study published earlier this week in Proceedings of the National Academy of Science of the United States of America.
“We argue there really isn’t much of a benefit to the 6-foot rule, especially when people are wearing masks,” Bazant said in an interview. “It really has 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.”
The important variable the CDC and the WHO 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 1 minute is probably fine, but not over the course of several hours, he said.
“What our analysis continues to show is that many spaces that have been shut down in fact don’t need to be. Often times the space is large enough, the ventilation is good enough, the amount of time people spend together is such that those spaces can be safely operated even at full capacity and the scientific support for reduced capacity in those spaces is really not very good,” Bazant said. “I think if you run the numbers, even right now for many types of spaces you’d find that there is not a need for occupancy restrictions.”
Six-feet social distancing rules that inadvertently result in closed businesses and schools are “just not reasonable,” according to Bazant.
“This emphasis on distancing has been really misplaced from the very beginning. The CDC or WHO have never really provided justification for it, they’ve just said this is what you must do and the only justification I’m aware of, is based on studies of coughs and sneezes, where they look at the largest particles that might sediment onto the floor and even then it’s very approximate, you can certainly have longer or shorter range, large droplets,” Bazant said.
“The distancing isn’t helping you that much and it’s also giving you a false sense of security because you’re as safe at 6 feet as you are at 60 feet if you’re indoors. Everyone in that space is at roughly the same risk, actually,” he noted.
Pathogen-laced droplets travel through the air indoors when people talk, breathe or eat. It is now known that airborne transmission plays a huge role in the spread of Covid-19, compared with the earlier months of the pandemic where hand-washing was considered the leading recommendation to avoid transmission.
Those droplets from one’s warm exhalation mix with body heat and air currents in the area to rise and travel throughout the entire room, no matter how socially distanced a person is. People seem to be more exposed to that “background” air than they are by droplets from a distance, according to the study.
For example, if someone infected with Covid-19 is wearing a mask and singing loudly in an enclosed room, a person who is sitting at the other side of the room is not more protected than someone who is sitting just six feet away from the infected person. This is why time spent in the enclosed area is more important than how far you are from the infected person.
Masks work in general to prevent transmission by blocking larger droplets, therefore larger droplets aren’t making up the majority of Covid infections because most people are wearing masks. The majority of people who are transmitting Covid aren’t coughing and sneezing, they’re asymptomatic.
Masks also work to prevent indoor transmission by blocking direct plumes of air, best visualized by imagining someone exhaling smoke. Constant exposure to direct plumes of infectious air would result in a higher risk of transmission, though exposure to direct plumes of exhaled air doesn’t usually last long.
Even with masks on, as with smoking, those who are in the vicinity are heavily affected by the secondhand smoke that makes its way around the enclosed area and lingers. The same logic applies to infectious airborne droplets, according to the study. When indoors and masked, factors besides distance can be more important to consider to avoid transmission.
As for social distancing outdoors, Bazant says it makes almost no sense and that doing so with masks on is “kind of crazy.”
“If you look at the air flow outside, the infected air would be swept away and very unlikely to cause transmission. There are very few recorded instances of outdoor transmission.” he said. “Crowded spaces outdoor could be an issue, but if people are keeping a reasonable distance of like 3 feet outside, I feel pretty comfortable with that even without masks frankly.”
Bazant says this could possibly explain why there haven’t been spikes in transmission in states like Texas or Florida that have reopened businesses without capacity limits.
As for variant strains that are 60% more transmissible, increasing ventilation by 60%, reducing the amount of time spent inside or limiting the number of people indoors could offset that risk.
Bazant also said that a big question that is coming will be when masks can be removed, and that the study’s guidelines can help quantify the risks involved. He also noted that measuring carbon dioxide in a room can also help quantify how much infected air is present and hence risk of transmission.
“We need scientific information conveyed to the public in a way that is not just fearmongering but is actually based in analysis,” Bazant said. After three rounds of heavy peer review, he said it’s the most review he’s ever been through, and that now that it’s published he hopes it will influence policy.