How long can you safely be in a room with someone infected with COVID-19?

MIT model offers finely tuned answers
Posted at 10:19 PM, Dec 11, 2020
and last updated 2020-12-12 17:50:27-05

SAN DIEGO (KGTV) — Nearly three-quarters of the students in San Diego County are currently learning from home, and those numbers don’t make a lot of sense to MIT professor Martin Bazant.

Bazant is one of the creators of the COVID-19 Indoor Safety Guideline. The free online tool simulates the fluid dynamics of respiratory droplets in various kinds of rooms to show users the risk of different indoor environments.

“If you run the numbers for a typical classroom, especially if the students and the teachers are wearing masks, then the amount of time in the room can be very significant,” Bazant said. “Essentially those people could spend a week in the classroom and not have a problem.”

Bazant says the six-foot rule that has largely determined which businesses can stay open is overly simplistic. In some cases, he says the rule creates a false sense of security. In other situations, he says it causes the closure of businesses or schools that could safely operate.

“If you place nursing home beds six feet apart in a shared room, they are absolutely not safe,” he said. But he said well-ventilated classrooms, with teachers and students in masks, are another story.

“It's only the United States where we follow the six-foot rule. That's given by the CDC. The World Health Organization all along has been saying one meter, which is about three feet. If you simply followed the one-meter rule, all our schools would be open, as they are in most of the world right now,” he said.

Bazant and his colleague John Bush said they set out to design a tool that would reveal the risk of transmission based on science, not guesswork.

Users start by selecting a room type, like a classroom or an airplane. From there, they can control a wide array of variables to dial in the risk, like the room size, the ceiling height, and the ventilation and filtration systems.

From there, users can further tune the model by human behavior. How many people are in the room? Are they whispering, shouting or singing? How many are wearing masks?

Mask usage is a critical component, Bazant says. The model quickly reveals the challenges of maskless activities in rooms with poor ventilation. Put five people without masks in a 400 square foot room with closed windows, and it becomes unsafe after 33 minutes.

Bazant says studies on super spreader events show in indoor environments, the distance from an infected person doesn’t matter. Infected droplets spread and mix throughout the room like second-hand smoke from a cigarette.

But with masks trapping droplets and diverting them upwards like a chimney, an environment like an airplane could be safe for several hours, according to the model. With more than 95% of the passengers and crew wearing surgical masks, a Boeing 737 could support 200 people -- around full capacity depending on the configuration -- for 18 hours.