Dine-outs not safe; study says COVID-19 spreads through central ACs

The researchers studied a case where nine people from three families had contracted COVID-19 while having lunch at a restaurant in Guangzhou, a metropolis in China

The ventilation systems can create complex patterns of airflow and keep the virus aloft, the experts say

Eating out may not be the same again. The centrally air-conditioned restaurants pose an increased risk of the spread of coronavirus, a Chinese study has said.The centrally air-conditioned restaurants pose an increased risk of the spread of coronavirus, a Chinese study has said.

The transmission of the virus-laden droplets was found to be prompted by air-conditioned ventilation. It has emerged that the direction of the airflow determines the infection spread.

The ventilation systems can create complex patterns of airflow and keep the virus aloft, the experts from the Centre for Disease Control (CDC) in China, said.

Curious case

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The researchers studied a case where nine people from three families had contracted COVID-19 while having lunch at a restaurant in Guangzhou, a sprawling metropolis in southern China, on January 24.

What is important in this case was the finding that the infections took place because the three unrelated families were in the draught of the same air conditioner as the index patient, although they were on different tables.

The virus travelled through the air conditioner duct and infected three families sitting in the vicinity of each other who never engaged with the other.

The study, to be published in the July issue of CDC’s journal “Emerging Infectious Diseases”, stresses the role played by air currents in spreading the illness in enclosed spaces.

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

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“On January 23, family A travelled from Wuhan and arrived in Guangzhou. On January 24, the index case-patient who was asymptomatic (patient A1) ate lunch with 3 other family members (A2–A4) at the restaurant. Two other families, B and C, sat at other tables. Later that day, patient A1 experienced fever and cough and went to the hospital. By February 5, a total of nine others (four members of family A, three members of family B, and two members of family C) had become ill with COVID-19,” the study noted.

The researchers said the infections show that the virus was spread through droplets carried in the air-conditioning draught.

“We conclude that the cause of this outbreak was droplet transmission. However, virus transmission cannot be explained by droplet transmission alone. Larger respiratory droplets remain in the air for only a short time and travel only short distances, generally less than a meter. The distances between patient A1 and persons at other tables, especially those at table C, were all more than one meter. However, strong airflow from the air conditioner could have propagated droplets from one table to the other,” the researchers said.

The lack of infection in others at two tables close by, but not in the line of the air flow, and in other floors of the restaurant appeared to suggest the virus did not spread through “aerosol transmission”.

If a virus spreads as an aerosol, it is harder to contain since it lingers in the air for longer and disperses more easily over longer distances.

The findings offer new insights into three aspects:

1) How the virus travels in closed space. In this case, it has emerged that only those in the air current were infected while others were safe. 2) Challenges in detecting infected persons. The index patient did not have any symptoms at that time. 3) Consequences for closed, air-conditioned spaces like restaurants, offices and even public transport.

Sketch showing arrangement of restaurant tables and air conditioning airflow at the site of outbreak of 2019 novel coronavirus disease, Guangzhou, China, 2020.

Home ACs safe

The experts say that the home air-conditioners are safe. They do not pose any additional risk of COVID-19 infection.

However, coronavirus can spread within spaces that are centrally air conditioned — such as shopping malls, theatres and restaurants, especially if an infected person is inside such spaces.

“This virus is not airborne in the same way as viruses that cause common cold and flu. It is just that if a person sneezes, a thick spray gets created — that is when a virus is in the air. But it does not float around in the air — it settles on surfaces and can stay there for a very long time,” says Sambuddha Chaudhuri, an epidemiologist at the Indian Institute of Technology, Bombay.

Passenger cruise

The researchers at Purdue University in the United States studied the air conditioning systems on the Diamond Princess — a 3,700-passenger cruise that was placed on lockdown off the coast of Yokohama after a passenger disembarked in Hong Kong and tested positive for the novel virus in early March.

According to a Centre for Disease Control and Prevention (CDC) report, 46 per cent of the passengers on board the Diamond Princess had infections at the time of testing. Traces of the SARS-CoV-2 was identified on a variety of surfaces in cabins of both symptomatic and asymptomatic passengers up to 17 days after cabins were vacated on the Diamond Princess.

According to Qingyan Chen, a researcher at Purdue University, the cruise ship air conditioning systems are not designed to filter out particles as small as the coronavirus, allowing the disease to rapidly circulate to other cabins.

“It is a standard practice for the air conditioning systems of cruise ships to mix outside air with inside air to save energy. The problem is that these systems can’t filter out particles smaller than 5,000 nanometers. If the coronavirus is about the same size as SARS, which is 120 nanometers in diameter, then the air conditioning system would be carrying the virus to every cabin,” Qingyan was quoted in the media as having said.

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