GUANGZHOU, China – Researchers found that the air conditioning system at a restaurant in China likely helped pass COVID-19 from a single diner to nine other people.

The field study was performed by Chinese researchers and published on the website of the Centers for Disease Control and Prevention. It may provide insight into how cities and states can safely open restaurants in the future – as well as the perils of doing so.

The researchers added that their study did have limitations – for instance, they weren’t able to simulate the airborne transmission route.

The study involved three families, one of which, family A, had just traveled from Wuhan to Guangzhou Jan. 23. On January 24, the infected member of family A sat with three other family members around a table at the windowless Guangzhou eatery.

The two other families, B and C, sat at adjacent tables. An air conditioning unit was blowing air across the restaurant directly at all three families.

By Feb. 5, all four members of family A, three members of family B and two members of family C had COVID-19.

Researchers found no other known source of exposure besides the trip to the restaurant. At least one person in families B and C caught the virus at the restaurant, but it’s not known whether or not the other cases were from family transmission.

The study’s authors point out that the 73 other diners who were at the restaurant that day didn’t get sick, but those sitting in the path of the AC unit’s airflow did.

“To prevent spread of COVID-19 in restaurants, we recommend strengthening temperature-monitoring surveillance, increasing the distance between tables, and improving ventilation,” they wrote.