Ventilation helps make public transit safer from spread of Covid-19, but masks

CNN
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Although a federal judge struck down the Biden administration’s mask mandate for public transportation Monday, some experts say you don’t want to throw out your mask just yet. No matter the form of public transportation – planes, trains, trams, subways, buses or even rideshares – good ventilation can help reduce the spread of Covid-19, but masks work best.

“You can’t engineer your way out of a problem like this,” said Krystal Pollitt, an assistant professor of epidemiology and assistant professor in chemical and environmental engineering at the Yale Institute for Global Health.

Someone infected with Covid-19 releases aerosols that contain the coronavirus when they talk, laugh or simply exhale. If the person isn’t wearing a mask to block those aerosols, they can hang in the air and be inhaled by other people nearby.

Outdoors, viral particles may disperse with even a light wind. But indoors, where there is no wind, particles tend to concentrate and hang around. Good ventilation can help break up the concentration of viral particles, but it can’t do everything.

“Ventilation is great, but we know if we are outside, the risks are lower. Being able to re-create the same amount of airflow that you would have with just your natural wind patterns in a confined indoor space that’s heavily occupied is incredibly hard to do,” Pollitt said. “Outside of putting yourself into, say, a wind tunnel, which obviously wouldn’t be comfortable for many reasons or energy-wise, we have to think about what’s realistic.”

On public transportation of any kind, a heating, ventilation and air-conditioning system sucks in air from outside, treats it and pushes it into the cabin or car. It’s a closed-loop system that can pull a little bit of fresh air.

A bus or subway car can have 10 to 18 air exchanges per hour on the low side, according to Jim Aloisi, a former Massachusetts secretary of transportation who is now a lecturer on transportation policy planning at MIT’s Department of Urban Studies and Planning; on the high side, it might have 40 to 50.

“Everything depends on the age of the equipment,” Aloisi said. Older systems are not as efficient as newer ones.

Ventilated air that is well-mixed has good circulation and is relatively safe, said Varghese Mathai, an assistant professor in the Department of Physics at the University of Massachusetts-Amherst who has done studies on how the coronavirus spreads in an environment.

But when ventilation is not as good, the air is not so well-mixed, and there can be zones inside a room with a higher concentration of particles.

“One can’t really predict where these zones are not well-mixed in a room. Really, it’s a multidimensional problem, and it’s not easy to predict in a not-so-well-mixed room how safe it is to stay for an extended duration of time,” Mathai said.

And if the transportation system wants maximum efficiency to cool or heat the air in a cabin, it can shut off the air intake and use what’s already inside, explained Aly Tawfik, director of the Fresno State Transportation Institute and an associate professor in the Department of Civil and Geomatics Engineering at California State University, Fresno.

“Buses have filters, just like the ones inside our vehicles,” Tawfik said. “But they are not designed for viruses like the coronavirus or the flu virus.”

In May 2020, Tawfik and his team did an experiment to see how a virus could spread through buses with a typical ventilation system.

Using nontoxic colored candles and steam, they simulated how air flowed in a variety of buses. They found that HVAC systems are extremely efficient and hold cool or the warm air inside a bus a lot longer than some may expect.

When the team introduced smoke, they saw that it spread in seconds and filled the whole cabin. Even when they opened the doors and introduced fresh air into the HVAC systems, the smoke lingered for minutes. The researchers think the virus behaves like the smoke did and could linger even after an infected person has left a bus.

“These were unpleasant findings, because it means that opening the doors and windows doesn’t help much,” Tawfik said. “The systems were designed to treat air fast and keep it inside the cabin for a long period of time.”

With another experiment, the team tried to see if they could treat the air to make it safer. They tested the buses with three viruses that were similar to the coronavirus.

Cooling the air mitigated an average of about 80% of the viruses, and heating was at about 90%.

“That does not necessarily mean that…