April 7, 2021 -- Poor ventilation could have led to airborne transmission of COVID-19 in a restaurant in Guangzhou, China, that infected 10 people in three families, according to a new study published in Building and Environment.
To prevent the spread of the coronavirus, the researchers wrote that indoor spaces should prevent overcrowding, open windows or doors, and ensure adequate air distribution.
“Leading health authorities have recognized the importance of airborne transmission in special settings since October 2020,” they wrote. “However, the effective minimum ventilation rate for avoiding airborne transmission remains unknown.”
The research team analyzed an outbreak that occurred in Guangzhou which was linked to three unrelated families. Local health officials learned that the families ate lunch at the same restaurant on Chinese New Year’s Eve on Jan. 24, 2020. The three families sat at adjacent tables in a crowded section of the restaurant, and a person at the middle table was considered the index COVID-19 case who began experiencing symptoms later that day. During the following two weeks, 9 other people in the three families tested positive.
The three families hadn’t met previously and didn’t have close contact during the meal other than sitting near each other in the restaurant, with some sitting back-to-back. None of the restaurant workers or other 68 patrons at 15 other tables contracted the virus.
The research team obtained a video recording and seating arrangement from the restaurant and looked at the restaurant’s air conditioning system across five zones of the establishment. Using the original table setup, they tested the dispersion of a warm tracer gas, which acts like exhaled virus droplets, to simulate the spread of droplets in the restaurant. They found that the measured ventilation rate was .9 L/s per person, which is lower than recommended standards.
The seating area where the three families sat was found to be covered by one air conditioning unit. The simulation showed that the droplets exhaled from the index person rose into the air and were carried to the other tables by the air conditioning. The droplets also reached other nearby tables but were dispersed by the other four air conditioners in the room. The three affected tables were in a recirculation zone, or bubble, that had a higher concentration of droplets, the researchers wrote.
Patrons had a higher risk of contracting COVID-19 if they were exposed to a higher concentration of droplets and had longer exposure, the researchers wrote. The three families shared the same space for 53-75 minutes. The restaurant workers had short exposure times in the recirculation zone while the infectious person was present, which could explain why none of them tested positive, the authors wrote. In addition, the patrons at other nearby tables had short overlap times with the infectious person.
Ultimately, the researchers concluded that the contaminated recirculation bubble can’t solely explain the outbreak. The low ventilation, lack of outdoor air supply and close distance between the tables appeared to contribute to the droplet dispersion among those who were infected. The restaurant was crowded due to Chinese New Year’s Eve and had added extra tables to accommodate the higher number of customers.
“It is important to note that our results do not indicate that long-range airborne transmission of SARS-CoV-2 can occur in any indoor space, but rather that transmission may occur in a crowded and poorly ventilated space,” the study authors wrote.
“A sufficiently high ventilation flow-rate reduces the contribution of airborne transmission to a very low level, whereas a low ventilation flow-rate leads to a relatively high contribution of aerosols to transmission,” they added.