In a time of standing six feet apart and avoiding confined spaces, there are naturally concerns about the health safety of air travel where passengers are crowded together on an aircraft.
Some may see a parallel in a limited study of how a restaurant in southern China’s Guangzhou was attended by a diner from Wuhan unknowingly infected with the coronavirus. Nine others around the diner became infected. The shared meal was on January 24 when the virus’ spread was limited and the other diners had no other known exposure.
Airlines are planning to block the middle seat on flights so passengers are spaced out, yet the restaurant study would appear to show social distancing has limitations.
Above image: Index case-patient denoted by a yellow-filled red circle. Red circles indicate seating of future case-patients. Dates are when the diagnosis was confirmed.
Of the nine other diners who later became infected, two were on the diner’s immediate right side at the same table – an easy argument for social distancing. Yet two other infections occurred to diners 10 feet away at a separate table.
“The key factor for infection was the direction of the airflow,” researchers at the Guangzhou Center for Disease Control and Prevention concluded in a research letter to be published in the July Emerging Infectious Diseases journal published by the U.S. Centers for Disease Control and Prevention.
The role of airflow and ventilation is relevant to aircraft. The case study’s restaurant had no windows and was dependent on air conditioners that ingested air from the room and then put it back out.
“Virus transmission in this outbreak cannot be explained by droplet transmission alone. Larger respiratory droplets remain in the air for only a short time and travel only short distances,” they said. “However, strong airflow from the air conditioner could have propagated droplets.”
Aircraft are commonly thought to be like the restaurant: stale from using recycled air. Yet the direction of airflow is different.
Whereas the restaurant’s air conditioner made the air move horizontally across many tables and diners, aircraft cabin airflow is vertical.
Air is dispersed at the top of the cabin and on side walls. This is most prominent when the air is cold but the outside is warm and it appears fog is drifting in from the top of the cabin. The air is then pulled out of the cabin through grilles at the bottom.
“This design feature makes the direction of airflow in the cabin basically vertical, and the airflow in the horizontal direction of the fuselage is very small,” the Civil Aviation Administration of China said in February. “Under normal circumstances, diseases will not pass through the aircraft air conditioning system.”
An aircraft cabin’s air is replaced every two to three minutes. Aircraft engines take in fresh air and mix it with cabin air that passes through a HEPA filter that can capture over 99.97% of micron particles, the CAAC said.
The researchers studying the restaurant case do not believe the virus spread by the air conditioner taking in contaminated air from the room and recycling it out. Six smear tests of the air conditioner returned negative for SARS-Cov-2.
The researchers did not say if the restaurant’s air conditioner had a HEPA filter, but usually only high-end or industrial air conditioners have one.
“HEPA filters have demonstrated good performance with particles of the SARS-Cov-2 virus size,” the European Union Aviation Safety Agency advised aircraft operators in April.
While most aircraft use HEPA filters, EASA advised that aircraft without the filters should “avoid the use of cabin air recirculation completely” by turning off recirculation fans.
The CAAC told airports to increase terminal ventilation.
EASA suggests, but does not mandate, passengers have an empty seat between them. Carriers from Air New Zealand to Delta Air Lines DAL to Wizz Air are giving empty middle seats. But it is not indefinitely: Alaska Airlines ALK is giving empty middle seats only through May 31.
EASA also advises what would have been impractical at the restaurant: covering your face with a mask.
The Guangzhou case study traced three separate families who sat at neighboring tables at the restaurant. In the middle was the person who had arrived from Wuhan the day before.
Incubation periods make the researchers suggest three diners at one table were infected by the primary diner. The two families overlapped at the restaurant for 53 minutes. It is also possible, the researchers note, that the primary diner infected just one diner at the other table and then later passed the infection to two family members.
For the table with two infections, which overlapped with the primary table for 73 minutes, one or both could have been from the primary infected diner.
Each table had at least one person infected from the primary diner, who was “the only known source of exposure for the affected persons.”
Later after going to the restaurant, the primary diner went to the hospital. The four family members associated with the primary diner could have been infected outside of the restaurant, but the researchers did not offer an opinion.
The round tables were located 3.3 feet away from each other. The researchers traced 91 people – 83 customers and 8 staff members – who were in the restaurant. Of the 83 customers, 10 developed COVID-19 while the other 73 quarantined for 14 days and did not develop systems. None of the staff became infected.
The researchers made three recommendations for restaurants: screening for temperatures, increasing the distance between tables and improving ventilation.