Diners walking into a restaurant might be able to tell a lot about how the establishment is mitigating the risk of spreading the novel coronavirus. They can see the servers in masks, the touchless systems many are offering for payment, the frequent disinfection by staff of commonly touched surfaces.

But there’s one thing they can’t see that could play a part: The air around them.

By now, we’re used to staying six feet from others, per social-distancing recommendations. For months, public health experts have described the virus as being primarily transmitted through droplets from an infected person’s cough or sneeze to nearby people or surfaces.

Lately, research and discussion has focused on airborne transmission over longer distances. Some scientists say covid-19 can spread by traveling in small particles called aerosols.

“That evidence is building right now,” says Chad Roy, director of infectious-disease aerobiology at the Tulane National Primate Research Center. “It’s not as prominent a pathway [as droplets or infected surfaces], but it’s one we need to pay attention to.”

How the virus is transmitted might be more important in restaurants than in many other venues, notes L. James Lo, an assistant professor at Drexel University in Philadelphia who studies airflow and how viruses circulate, because people linger there far longer than they do in, say, a grocery store. Exposure to the virus can come from encountering a high dose for a short time or a low dose over a longer period, he says.

“In a restaurant, you’re enjoying your dinner and spending more time, which means you are stuck with the same people for a long time.”

As restaurants reopen their dining rooms, here are experts’ answers to some of the questions would-be diners might have about the air around them.

Should you sit inside or out?

Public health experts are recommending alfresco activities, including dining, over indoor ones. The virus is quickly diluted in fresh air, Roy notes. “It’s much easier to socially distance outdoors,” which is the most effective preventive measure (along with washing your hands), says Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security. “You can space tables farther,” he says.

Cities around the world are moving to accommodate more people engaging in open-air activities. Some are closing off streets to cars to allow more room for pedestrians and additional patio seating for restaurants. The Lithuanian capital of Vilnius is allowing bars and restaurants to take over public spaces, essentially turning the city into a massive outdoor cafe across its plazas and squares. And San Francisco’s mayor just approved a plan to allow eateries to apply for permits to use sidewalks and parking lanes once dine-in service is allowed.

Completely open-air dining is safest; covered patios are better than indoors, Lo says. “The more obstruction for natural air movement, the less flushed-out the air is going to be,” he says.

Can air-conditioning systems spread the virus?

The short answer is that it’s possible but unlikely, according to experts.

A recent study of an incident that took place at a restaurant in China, where the virus originated, found nine people were infected with covid-19 by a diner sitting near an air-conditioning vent. A study of the transmission, published in a Centers for Disease Control and Prevention journal, showed how one diner infected diners at adjoining tables, as droplets containing the virus were apparently carried by the air conditioning.

Four people at the person’s table later tested positive for covid-19, as well as five people at neighboring tables, some as far as 14 feet away.

Scientists caution that the study documents a single incident and note that the restaurant’s air conditioning system was very different from those used by U.S. restaurants. “The ventilation was one-tenth of what it should be if you use standards that apply to most U.S. restaurants,” says William Bahnfleth, a professor of architectural engineering at Pennsylvania State University and chairman of the epidemic task force convened by the American Society of Heating, Refrigerating and Air-Conditioning Engineers. He pointed to a later analysis and simulation of the incident indicating poor ventilation was the culprit.

Without fresh air from outside, “the infected diner was putting out infectious material, and there was nowhere for it to go,” he says. There is no known instance of a coronavirus transmission through an HVAC system in the United States, he notes.  

So what systems are best?

According to experts, two functions of air-conditioning systems can help prevent the spread of a virus: Ventilation — fresh air coming in to the building from outside — and filtration, or removing small particles from the air.

Standard systems used in most commercial and public buildings, including restaurants, do both and “limit risk of aerosol transmission of covid-19,” Bahnfleth says.

The engineering society, which develops standards used in building codes around the country, is still recommending building owners take steps to further reduce risks. Those include increasing the amount of outside air being brought in, Bahnfleth says.

Under most current codes, a restaurant should replace all of its air with outside air about once every hour, or what’s called an “air exchange rate” of one. The society is recommending upping that to three times an hour, Lo notes.

The problem for diners is that it’s almost impossible to assess the technical specs of a restaurant’s HVAC system to understand the risk. “They shouldn’t have to do that,” Roy says. Instead, he suggests using “a reasonable-person standard.”

“If a place feels stuffy, maybe it’s not a great idea to stay there,” he says.

As they reopen dining rooms, some restaurants have touted their filtration or air “scrubbing” systems along with other measures meant to stop the virus’s spread. But some experts say such claims might be more about marketing than meaningful risk-reduction. “What you’re going to see is restaurants trying to distinguish themselves from each other, and a lot are going to go above and beyond what the science says is necessary,” Adalja says. “They are trying to entice people to come to their place.”

Where should diners sit inside?

The coronavirus might redefine what makes a table the best seat in the house. If you do decide to dine inside a restaurant, Lo suggests nabbing a seat by an open window if there is one; that’s the next-best thing to being outdoors. Bahnfleth, however, cautions that open windows create unpredictable flow directions — that is, while fresh air is coming into the room, air from inside has to go out somewhere, and that could be the window you’re sitting by.

Barring that, seek out a table that is near the AC register or vent where air is being pumped in and diffused around the space — that’s where you’ll find the most fresh air, Lo advises.

Though it seems counterintuitive to dine at a restaurant and avoid your fellow man, that’s exactly what you should aim for, whether you’re inside or out, experts agree. Sneezing and coughing can launch viral particles beyond the six feet being recommended for distancing, they say. Airborne transmission could mean they travel even farther. Most restaurants that are reopening are doing so at reduced capacity to prevent crowding.

The best seat, then, is the one that puts as much space as possible between you and other diners (at least those you’re not quarantining with). As Lo puts it, “Just try to stay away from other people — farther is likely safer.”

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