To avoid Covid-19 infection in poorly ventilated indoor environments, people must space themselves a greater distance from each other than the widely recommended 1.5 meters, according to researchers.

Recent studies have revealed that droplets from a person’s mouth can travel several meters, potentially infecting people on the other side of a room.

Research on spittle, particles and air flow jointly conducted by the University of Hong Kong and mainland China’s Sun Yat-sen University and Southeast University may reset the parameters of social distancing being enforced across the globe. At present, there is no consensus among researchers and policymakers on social distancing.

In one confirmed case, droplets and aerosol particles traveled 9.5 meters.

Professor Li Yuguo, a leading authority on air flow and circulation in buildings and their environs with HKU’s Department of Mechanical Engineering, conducted a series of epidemiological and environmental investigations into two typical clusters of Covid-19 infections in the southern Chinese megacity of Guangzhou as well as the central province of Hunan.


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In a nutshell, Li found that the more airy a place is, the lower the risk of the pathogen jumping from one person to another.

In collaboration with Guangzhou’s Municipal Centers for Disease Control and Prevention (CDC), the HKU team examined pathological evidence involving a cluster of infections traced to a popular restaurant in the city in January. They found that the virus from an asymptomatic patient infected people at different tables in opposite corners. Nine members of three non-associated families dining at three separate tables in one hall became infected.

The three families had no contact with one another, and the distance between the source patient and the farthest away infected diner was more than four meters.

Li and his colleagues found that the outbreaks among bus commuters in Changsha in Hunan this January had also shown the extensive “hit range” of the virus: a patient took two buses in the city in one day and infected 10 passengers. Judging from surveillance camera footage and reports from the local CDC, Li estimated that the distance between the patient and the farthest away infected passenger may have been more than nine meters.

“In the two cases in Guangzhou and Hunan, it is apparent that airborne transmission rather than physical contacts was the most probable way of the spread of the virus, given the spatial pattern of the infections and the fact that those infected were sitting apart from each other… In both cases we found that poor ventilation could be a major contributing factor to the virus’ reach and hit range,” said Li.

The widely-adopted 1.5-meter recommendation for social distancing is based on the internationally recognized observation that, in short-range airborne transmission of diseases, droplet concentration in the exhaled jet of air from an infected person decreases at a distance of 1.5 meters.

But Li pointed out that if air ventilation was poor, then airborne transmission routes could be significantly extended and the virus could infect more people beyond those in close proximity to a patient.

During the SARS outbreak in 2003, Li and a team of mechanical engineering experts developed a mechanistic airflow model with computer-aided fluid dynamics simulation for thermo-fluid analyses. They managed to track and explain the main infection pattern and characteristics of the outbreaks at several locations across Hong Kong that were among the hardest hit.

In mid-March, Li’s team visited that Guangzhou restaurant to inspect its air-conditioning system and assess the venue’s ventilation rates. They dispersed tracer gas to see how far it could travel and also conducted computer simulations.

“The air ventilation rate was only one liter per second per person in the restaurant, compared with the international standard of at least 5L/s per person, and this can explain why people at three different tables got infected, despite having no contacts with one another.” Li said.

The tracking of airflow paths at the venue also revealed that the three tables were roughly on the same air circulation loop.

Correspondingly, the average ventilation rates in the two buses in Hunan were 1.7L/s and 3.2L/s per person, also falling short of the standard.

In conclusion, Li’s team found that risks of Covid-19 airborne transmission in an indoor environment could be heightened if the place’s ventilation rate was less than 3L/s per person. To minimize airborne infections, Li has recommenced a ventilation rate of 8L/s per person or higher.

“Our findings do not rule out large droplets or other possible sources of Covid-19 transmission such as aerosols generated during medical procedures or fomites via contaminated surfaces. The findings instead provide evidence that short-range airborne spread can be a major avenue for disease transmission in an indoor place,” he said.

Li said the most pertinent prevention measure could be effective ventilation and filtration, particularly in restaurants, stations, bars, gyms, etc, on top of mandating all people to wear masks and staying as far as possible from other people while indoors.

He also said that installing carbon dioxide sensors in a room could serve as a gauge of air flow and ventilation, as a concentration of over 1,000 ppm may indicate that the room may lack fresh air.

It is reported that Hong Kong, now grappling with an alarming spike in local, untraceable cases, may issue an outright ban on dine-in service at the city’s restaurants and shut all bars, karaoke lounges, movie theaters and other indoor venues as part of new measures being mulled to address the new wave of infections.