With many states still imposing restrictions on indoor dining, restaurants throughout the United States have had to get creative as temperatures dropped, investing heavily in outdoor dining setups such as yurts, igloos, and greenhouses. But experts say some of these techniques might not be as safe as people think.

Restaurants set up outdoors in the winter

The restaurant industry is expected to lose over $230 billion in 2020, NPR reports. To mitigate some of those losses, many restaurants in states that have imposed restrictions on indoor dining have invested in tents, yurts, igloos, and a variety of other structures to make sure customers can dine "outdoors" while staying warm.

For instance, Seattle-based restaurant Canlis installed a "yurt village" in its parking lot, where customers are brought to semi-enclosed structures featuring a single table after undergoing temperature checks and are attended to by servers wearing N95 masks. The structures are sanitized after each party finishes their meal. Similarly, San Fermo, also in Seattle, has erected transparent igloos that are limited to two patrons at a time and ventilated with industrial hot air cannons after each party departs.

But other restaurants have opted to take a less individualized approach, Charlottesville Tomorrow reports. For example, Petit Pois in Charlottesville, Virginia, has three tables in its indoor dining room, and then several spaced tables on its outdoor patio, some of which are housed under a tent that has one wall rolled up for ventilation.

"We're trying to do everything we can to expand the outdoor dining season for as long as possible," Mike Whatley, from the National Restaurant Association, said.

But even that might not be enough—according to Whatley, more than 100,000 restaurants are either "completely closed or not open for business in any capacity," and a survey from the National Restaurant Association found that outdoor dining dropped from 74% in September to 52% in November.

"It's going to be a hard and tough winter," Whatley said. "As you see outdoor dining not being feasible from a cold-weather perspective, or, unfortunately, from a government regulations perspective, you are going to see more operators going out of business."

How safe is outdoor dining?

However, the safety of outdoor dining amid the new coronavirus epidemic is a "wide spectrum," according to Richard Corsi, an air quality expert and dean of engineering and computer science at Portland State University in Oregon.

"The safest that we're talking about is no walls—a roof," he said. "And then the worst is fully enclosed—which is essentially an indoor tent—especially if it doesn't have really good ventilation and good physical distancing."

Ventilation is key, health experts say. "Airflow is needed to disperse potential virus carrying droplets," Sara Dayley, from the St. Louis County Department of Public Health, said. "If those walls are down, you're going to trap everything inside."

According to Corsi, outdoor dining with no shelter at all is safest because there are "higher air speeds, more dispersion, and more mixing than indoors," meaning respiratory droplets with the new coronavirus are less concentrated in well-ventilated areas.

And if an outdoor area has heaters, "then you're going to actually have pretty good ventilation," Corsi said. "The air will rise up when it's heated, and then cool air will come in."

Separately, William Bahnfleth, a professor of architectural engineering at Pennsylvania State University, said restaurants should focus primarily on increasing what he calls the "indoor air change rate," or how frequently air in a given space is recycled. Bahnfleth said plexiglass partitions or vinyl flaps could obstruct the natural airflow of an area.

"My feeling is that it's very hard to tell if a space is well-ventilated," Bahnfleth said. "You might be able to tell if it's poorly ventilated, but even that could be misleading. I would recommend that you dine entirely outside where there's good air movement."

However, ventilation is only one part of keeping people safe, Dayley said. "You have social distancing between tables 6 feet or more, people wearing masks when they're not eating or drinking," she said.

And some cities consider even outdoor dining too much of a risk, "NPR" reports. For instance, Baltimore and Los Angeles currently permit only carryout—banning all indoor and outdoor dining. And other cities, such as Charlottesville, have opted to categorize tented seating as indoor seating.

Ultimately, according to Dayley, "We can't stress enough to be 100% safe in what you're doing, wear a mask, drive-thru, carry-out, wash your hands—all things we've been doing since early in the pandemic" (Stone, "Shots," NPR, 12/31/20; Woods, Charlottesville Tomorrow, 12/14/20; Louie, KSDK, 12/10/20; Nguyen, "The Goods," Vox, 12/8/20).