Air conditioning spread the coronavirus to 10 people sitting near an infected person in a restaurant. It has huge implications for the service industry.

Three seemingly-healthy families were struck by COVID-19 in mid-January after dining on neighboring tables in a windowless restaurant in Guangzhou, China.

None of them had symptoms, but researchers studying the case believe one person had an asymptomatic case, and the restaurant's air conditioner blew viral droplets further than they might have normally gone, infecting another nine people.

It's a frightening prospect for people who are trying to keep a healthy distance from others. However, in a potentially hopeful finding for the locked-down restaurant industry, nobody else in the restaurant got sick, despite the fact that it was a full house, with 83 diners across five floors, and eight servers working around the person with coronavirus, the researchers described in a research letter published in the journal Emerging Infectious Diseases.

"To prevent the spread of the virus in restaurants, we recommend increasing the distance between tables and improving ventilation," the authors wrote.

For the struggling eateries desperate to re-open for business in the coming months, these findings are evidence that restaurant work will not just return to normal after the pandemic, but there might be ways to limit the spread. There will likely be caps on the amount of time patrons can spend eating, restaurants will have to be at half-capacity, air conditioning or heating may have to stay off, and waiters might be advised to wear masks.

Nightbird Restaurant chef and owner Kim Alter prepares meals that were delivered to hospital workers in San Francisco on March 27. Jeff Chiu/AP

The source of the infection was a 63-year-old woman in a family-of-five (Family A), who did not show symptoms (a fever and a cough) until later in the day, when she went to a hospital and tested positive for COVID-19.

Within two weeks, four of the women's relatives got sick, which was unsurprising given they lived together. But so did five other diners (in Family B and Family C), who seemed to have no other connection to the virus except for their time in the restaurant.

It surprised researchers since the novel coronavirus transmits in droplets (heavy particles that tend not to float further than six feet), rather than aerosols (as is the case for measles, which can float far in the air). The team who studied the case said that, as a result, it seems air conditioning could well blow droplets further than six feet.

The restaurant industry has been hard-hit by the pandemic

Since March 1, over 3 million people have lost restaurant jobs, and one-in-five restaurants could close permanently because of the pandemic, according to a UBS estimate. The National Restaurant Association asked Congress for $240 billion to help the struggling industry.

While most Americans want to wait before resuming their daily routines, opening up restaurants means reopening a supply chain for restauranteurs, farmers, produce salesmen, cooks and waiters, all of whom have been out of work for at least a month.

Restaurants will need new rules when they re-open, but it could vary from state to state

The research team who studied the Guangzhou restaurant did not try to replicate the phenomenon in a lab, and they don't have other cases to compare it to, so their findings have to be taken with a grain of salt.

But William Schaffner, professor of infectious diseases at Vanderbilt University Medical Center, said the research letter is a good resource to help us understand what restaurant openings would look like.

"We are going to open back up," Schaffner said. "But the trick will be to open, slowly, do it in a phased fashion. And of the phases includes opening restaurants and doing so at half capacity, spacing out the seats."

It's unclear whether spacing and capacity rules could do the trick, though it's highly likely they will be employed anyway — and we will probably start to see rural areas, which haven't been as hard-hit by the virus as large cities, try it out first.

Schaffner, who lives in Tennessee, which has not been heavily impacted by the virus yet, said he's seen the mounting pressure for restaurants to open up.

Jennifer Horney, founding director of the University of Delaware's epidemiology program, told Business Insider that she forsees a slow relaxation of emergency orders on a state-by-state or regional basis, allowing restaurants to reopen with some tweaks to traditional service. Eating out in a state with relaxed guidelines might include the use of touchless payments, disposable menus, and staff wearing face masks and gloves.

Scientists say eateries should stick to half-capacity when they re-open, but it may be a hard rule to enforce

Rather than creating new restrictions, such as a ban on air conditioning or outdoor dining, Horney said it might be easier and just as effective for restaurants to adapt existing rules, such as room capacity. "Existing regulations, like fire code occupancy numbers, could be used to set a maximum number, like 25% of usual occupancy, that could be safely served at any time," Horney said.

But Schaffner is skeptical about how we could ensure restaurants adhere to half-capacity.

"Some restaurants will say, listen, we're getting a lot of business let's just open up a few more tables," he said. "The Covid police are not going to catch us tonight."