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For years, doctors have known viruses spread more rapidly in cold dry air. That’s why winter is so devastating for flu cases and deaths and now the same is holding true for SARS-CoV-2, the virus that causes COVID-19. A new Yale-Columbia University study outlines the problem and how it may impact outdoor dining.

When cities and states shut the doors for indoor dining, they forced patrons outside. It was warm then. Now as cold, dry air engulfs the Chicago area, things are changing.

Columbia University environmental sciences professor Dr Jeffrey Shaman has been studying virus transmission for years.

“Colder temperatures and lower relative humidity conditions seem to favor the survival and transmissibility of the virus,” he said.

So, unless you are in an individual bubble, if you are eating outside, in a large tent Shaman says there is some concern.

“And the question is what kind of ventilation is really in there?” he said.

Ventilation can become a question especially as colder temperatures prompt people to put up walls on the tents to keep diners warm.

“If they are really cramming people together in there, they are packing them in tightly and they are providing opportunities for transmission that wouldn’t exist if people weren’t doing that,” Shaman said. “When you breathe, when you speak, when you cough, when you sneeze, you expel droplets of that oral and nasal mucosa stuff, saliva, mucus.”

The expelled droplets from infected people contain virus particles.

“Those viral particles go out and they can be droplets that can hit somebody in the face,” he said. “They can be evaporated in the sub-saturated air that is around us all the time.”

Unsuspecting people inhale the virus and become infected. It’s a process that happens more easily in the cold.

“We can see that there is this very strong seasonal cycle for influenza. We all know it. We all experience it. We know flu occurs in the wintertime,” Shaman said. “But coronavirus patterns match that of flu. And now to be in a tent, in particular when there are lots of people, it’s something that I would actually recommend avoiding. I don’t think it’s worth the risk.”

Indoors, if air flow is good, may be a better option.

“It would be better to have better ventilation or to have barriers,” he said.

And unlike outside, indoors, we can alter humidity levels — pumping in more moist air which does not favor virus transmission.

“In a place like Chicago, indoors if you were to measure the humidity in many buildings, you would find it bone dry. You have a lot of room to start humidifying it,” Shaman said.

It’s good advice for your home and office space as well. More humidity is better in the cold, dry, winter months.