Ghosts in the Air: The lesson learned from a bus trip.
Contracting COVID-19 likely requires extended breathing of air containing substantial quantities of active virus (primarily airborne suspensions of fine droplets that are ~5 microns in size). This likely occurs in a poorly ventilated or closed environment shared with an infected, even if asymptomatic, individual.
Early in the pandemic, there were substantial fears that touching contaminated surfaces was the primary risk. However, it turns out that successful infection this way rarely occurs. The skin has its own natural antiviral defenses, and most individuals taking routine hygiene precautions, e.g. frequent handwashing, masking and social distancing, are adequately protected from infection. We also now understand that the virus dies in a matter of hours on most surfaces. There is now a strong and growing consensus that the primary and most dangerous route of transmission is inhaling airborne virus particles.
An unusual and compelling example of this occurred on Jan. 19 this year when a group of 128 adults and their two drivers took a 100-minute-each-way day trip on two buses to a large outdoor communal worship event in the Zhejiang province of China. Both buses had recirculating air conditioning. Unknown to anyone in the group, one of the passengers on Bus No. 2 had an asymptomatic COVID-19 infection. The entire group was in close proximity at the event, but after the return, no passengers on Bus No.1 were infected, while 24 of the 69 people who rode Bus No. 2 with the index patient became ill with COVID-19. The infected individual was seated in the center of the bus. Given the seating pattern of those infected, it is clear that proximity to the infected person conferred no greater or lesser risk of infection. The passenger to the immediate left of the infected individual contracted COVID-19, but the person on his right did not. The resulting cases were well distributed around the bus, including those who were seven rows away. There were seven in middle seats, seven in window seats and nine in aisle seats who became infected. Inhaling virus particles in recirculated air in the bus is the most likely suspect for the super-spreading event that occurred.
With strong and growing evidence for aerosolized particle transmission of SARS CoV2, we need to be more conscious of the importance of the environment in which we live and work. Aerosolized particles will become a bigger issue as colder weather descends upon the northern hemisphere over the next few months, forcing more people indoors. Maximum ventilation is critical to reducing spread.
According to The Wall Street Journal on September.1, 2020, “Ideally,… public spaces like a standard classroom should aim to have air replaced with clean air between four to six times an hour to dilute Covid-19 particles that might accumulate. That can be done, aerosol scientists and building engineers say, through strategies that introduce outdoor air and filter indoor contaminants. Those include opening windows and doors, installing window fans, using portable air purifiers with high efficiency particulate air, or HEPA, filters, and upgrading heating, ventilation and air-conditioning systems to meet certain standards.”
Implication? If the person with the virus is in the same closed ecosystem as you, it does not matter if they are sitting right next to you or three rows back. Aggressive airflow, wearing a tight-fitting mask and not touching your eyes, nose and mouth is your best defense.
October 5th update: CDC acknowledges the primary role of respiratory particles in COVID transmission. https://www.washingtonpost.com/health/2020/10/05/cdc-coronavirus-airborne-transmission/
Reference: Shen et al; Jama Internal Medicine; 9/1/2020; doi:10.1001/jamainternmed.2020.5225