Indoor vs. outside dining: Which restaurant tables are safer?

    (CNN) — Americans are flocking to their favorite restaurants again, especially those that have set up outside and indoor seating that follow the rules of social distancing.

Given that you can’t wear a mask while eating, is that enough to protect you from Covid-19? CNN asked infectious disease and indoor environmental quality experts to weigh in.

Is it safer to eat indoors or outdoors at a restaurant?

“Eating outside is less risky than eating inside, if everybody is six feet apart and the wait staff are all wearing masks. That keeps the risk as low as it can be,” said infectious disease expert Dr. William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University School of Medicine in Nashville.

“Studies have shown that when you’re just talking, the larger [respiratory] droplets don’t really travel farther than three to six feet,” said Linsey Marr, a professor of civil and environmental engineering at Virginia Tech, who has been studying Covid-19 transmission.

What if people walk by your table while you’re eating outside?

“We think that that’s not a danger or if it is a very, very small one,” Schaffner said. “Because in order for Covid-19 transmission to be at all efficient, what you want is at least 15 minutes of contact face-to-face in an enclosed environment.

“So that risk should be very low, but those passers-by should be masked,” he added.

It’s always going to be less dangerous outside, where the virus can dissipate into the air, say experts. To illustrate that, imagine for a moment that the virus is the smoke from a cigarette.

“If you put somebody smoking inside a room, the first puff puts a little bit of smoke out. The second puff puts a little bit more out, and it builds on the first one,” said Erin Bromage, an associate professor of biology at the University of Massachusetts Dartmouth.

“The smoke in that room builds up unless you have good filtration and good air exchange,” said Bromage, a CNN contributor who wrote a blog post in early May — “The Risks — Know Them — Avoid Them,” that got over 13 million views in one week.

However, Bromage said, when you’re outside and someone takes a puff, you get a blast of smoke but then it’s over. You get another blast and it’s over.

“It’s not an accumulating threat, like we see inside,” he said.

Do 6-foot spacing and partitions protect you indoors?

“I think the six feet indoors is not enough by itself,” said Marr, the co-author of a paper on airborne transmission of Covid-19 via very small droplets called aerosols that can float for hours.

“You have to have good ventilation in that bar or restaurant,” she added. “And since I can’t wear a mask while I’m eating, I’m just avoiding indoor restaurants until this is over.”

As should anyone in high-risk categories, experts suggested.

The US Centers for Disease Control and Prevention recently added to its growing list of those who may be hardest hit by Covid-19: pregnant women, the elderly, people with heart, lung or kidney disease, diabetes, immune disorders, people with moderate to severe asthma and those with a body mass index (BMI) of 30 or higher, which includes 42% of the US population.

“Should people who are 72 years old and have chronic lung disease or another chronic aliment go out to restaurants at the moment? My advice is no,” Schaffner said. “Keep eating at home.”

“I don’t use four letter words like safe,” he said. “Restaurants have just reduced the risk as much as they can in the circumstances.”

Will a restaurant’s HEPA filter in its air conditioning protect you?

HEPA stands for “high efficiency particulate air.” These filters are rated to remove 99.97 of dust, pollen, mold, bacteria and other airborne particles with a size of 0.3 microns. Covid-19 is thought to be between 0.06 to 1.4 microns.

“And filters are rated at their worst performance, so 99.97 is the worst it will do. And that’s rated for a particle size of 0.3 microns, but smaller and larger it actually does better. So the point is that it’s capturing nearly all particles,” said environmental health researcher Joseph Gardner Allen, who directs the Healthy Buildings program at the Harvard T.H. Chan School of Public Health.

However, few, if any, restaurants are likely to own such filters, as they are more difficult to run, maintain or retrofit. They may use other filters that may — or may not — capture such particles.

Even a HEPA filter wouldn’t make environmental engineer Marr feel comfortable eating inside a restaurant.

“I would go outside even with HEPA filtration indoors,” Marr said. “There’s just a lot more opportunity when you’re outdoors for any virus that’s released into air to become rapidly diluted.”

In order for the filter to protect you, the virus-laden air would have to pass through the air handling system and be filtered first, before it gets to you, said Marr.

“HEPA filters reduce the amount of virus in the background air, but most transmission takes place in these close contact type of scenarios where you’re near someone that’s infected and they’re spreading lots of virus. The HEPA filter doesn’t help with that,” she said.

So what should we do?

Follow public health recommendations: Wear masks when you go outside, always stay 6 feet from others who are not in your “quarantine bubble,” wash your hands with soap frequently, and try to go to outdoor places where there is excellent air circulation and filtration, experts say.

“We have an opportunity here to reopen the economy closer to normal, if more people are more willing to wear masks and pay attention to social distancing and ventilation,” Marr said.

“I want to emphasize there’s no one solution here,” she added. “Distancing by itself lowers risk, masks by themselves help lower risk and ventilation by itself helps as well. But we really need all three of these to make a difference, make a dent, in the risk of trying to to really slow transmission noticeably.”

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