A new study by researchers at the University of Hong Kong (HKU) has documented how poor building ventilation plays a role in indoor transmission of Covid-19. The study looked at the air flow and infection dynamics from three important Covid-19 outbreaks in China and Japan, concluding that spread of the virus was “definitely favored by poor air ventilation”.
“It can be understood that a normally non-long-range-airborne infection turns to long-range airborne in poorly ventilated spaces. Opportunistic airborne transmission can lead to super-spreading events”, said Professor Yuguo Li, Chair Professor of Building Environment at HKU.
“The findings provide evidence that short-range airborne is also a major transmission channel, which in turn suggests that other precautionary measures have to be taken in order to better contain the pandemic.
Why The Hong Kong Study Is Important
Recently 239 international experts signed a an open letter to WHO asking it to recognize that the SARS-CoV-2 virus transmission is occuring not only via large virus droplets at very close ranges of 1 meter, but via smaller viral particles that become “airborne” and travel a few meters away, particularly indoors. While experts debate ‘droplets’ versus ‘airborne’ particles, the HKU study points to ventilation as the “elephant in the room” that no one is examining closely.
Even WHO admits that some droplets may evaporate into tiny “droplet nuclei” that can become airborne. The HKU study says that in poorly ventilated rooms, viral droplets or particles will remain suspended in the air for longer – and travel further with a greater potential to infect. The HKU study calls this “opportunistic airborne transmission”:
“In short-range airborne transmission of diseases, droplet concentrations in the exhaled jet of air from an infected person continually decrease away from the mouth and the exhaled jet becomes sufficiently weakened to be indistinguishable from the background room air at a distance of approximately 1.5 m. However, if air ventilation is insufficient, the short-range airborne transmission route can be extended to result in a long-range airborne route to infect more people beyond the proximity.”
A Study Of Three Major Outbreaks
In collaboration with the Guangdong and Hunan Centers for Disease Control and Prevention, Sun Yat-Sen University and Southeast University, Professor Li and his team at the Department of Mechanical Engineering of the University of Hong Kong (HKU) studied three major outbreaks that occurred between January and March 2020: These included outbreaks on Hunan’s bus system (January 22, 2020), where 10 bus passengers sitting as far as 9.5 meters from the infected passenger came down with Covid-19; at the Guangzhou Restaurant (January 24, 2020) where 9 people sitting as far away as five meters were infected by one person; and on the Diamond Princess Cruise ship where some 696 people became ill (January to March 2020).
SARS-CoV-2 transmission dynamics in the Guangzhou restaurant
While the team didn’t find evidence of long distance viral transmission via the cruise ship’s air conditioning system, it found clear evidence of transmission beyond the range of 1.5 meters within the Hunan buses and the Guangzhou restaurant. At the restaurant, the ventilation flow rate was only 1 L/s per person. The two Hunan buses studied had a time-averaged ventilation rate of 1.7 L/s. Professor Li:
“In both the Guangzhou Restaurant case and the Hunan buses case, there is evidence that airborne is the probable way of transmission of the virus, given that those infected were sitting at a distance, and quite a number of them more than 2m apart from the index patients, and no other transmission route can explain the spatial pattern of the infection. We found that a poor ventilation rate is a major contributing factor to the virus’ spreading in the environment.”
Air Flow & Ventilation Rate
They concluded that transmission of the SARS-CoV-2 virus in the indoor environment is more likely when the ventilation rate is less than 3 litres/second per person, with a sufficient exposure period. The international standard ASHRAE 62.1 – requires at least 5 liters of air per second per person (or per cubic meter of space). In fact, a 2008 WHO guideline refers to a desired 10 l/s per person for office spaces, for effective infection prevention. That works out to about 36 air exchanges per hour for 1 cubic meter of space. Professor Li:
“Airborne transmission of Covid-19 outbreak in the indoor environment is likely when the ventilation rate is less than 3 L/s per person, with a sufficient exposure period.”
Poor ventilation increases the infective range of SARS-CoV-2 virus particles
Mechanistic Airflow Models
During the SARS epidemic of 2003, Professor Li and a group of mechanical engineering experts at HKU developed an advanced mechanistic airflow model with computational fluid dynamics simulations and detailed thermo-fluid analyses to track and explain the main infection patterns and characteristics of the SARS outbreak at Amoy Gardens and at Ward 8A of Princess of Wales Hospital, where poor ventilation was widely attributed to broader airborne transmission of the virus. Based on these models, they studied the actual data and air flow dynamics from the ventilation sytems in the more recent Covid-19 cases, to reach their conclusions.
Solutions To Airborne Infections
As the major culprit is opportunistic airborne that occur in poorly ventilated areas, the most effective interventions prevent the opportunistic transmission of the virus through effective ventilation and filtration control, the authors conclude:
- Enhance air ventilation in indoor environments, particularly in restaurants, public transport, bars, gyms, etc.
- Ensure even higher ventilation rates for more strenuous indoor activities. Avoid social gatherings in an indoor environment where sufficient ventilation is not provided.
- Install Carbon Dioxide (CO2) sensors as a “proxy” measure of ventilation quality. While not a perfect indicator, a concentration of over 1,000 parts per million (ppm) may indicate that the room is not sufficiently ventilated.
- Hand hygiene and social distancing remain critical, with the use of masks in indoor spaces as essential barrier measures (handled with clean hands when putting it on and taking it off).
Improved Filtering of Recirculated Air
To ensure effective ventilation, air conditioning systems must include high quality HEPA filters or equivalent portable filtering technologies to capture and prevent the recirculation of viral particles indoors, added Harvard University Professor Joseph Allen, at a university webinar on the ventilation issue on Tuesday.
“Air conditioning in itself is not the [only] problem”, said Allen, founder of the Healthy Buildings Program at the Harvard Chan School of Public Health. “You [also] want to…filter out any recirculated air.”
Another oft-ignored factor in virus travel behaviour is humidity, Allen observed. He noted that viral respiratory droplets are more likely to evaporate into smaller aerosol particles that travel further in the air when the humidity is below 40%. At humidity levels above above 40%, virus particles are more likely to drop to the ground because the air is already saturated with water.
“Less evaporation of droplets [means they will] settle out of the air quicker. It’ll be a heavier, bigger aerosol”, he added. “If it’s drier, you get a higher likelihood of droplet nuclei which can stay airborne for longer periods of time.”
In 2008, WHO published formal guidelines on “Natural ventilation in health care settings” which discussed the fundamental relationship between ventilation and infection transmission in indoor spaces. Annex C, based on a WHO systematic review, talks about the potential of respiratory droplets to become suspended in the air as “respiratory nuclei” in poorly ventilated spaces. Annex D of these guidelines refers to a desired ventilation standard for office buildings, stating:
“In an office, we need a 10 l/s per person ventilation rate.”
Current WHO guidance about Covid-19 management in community settings does not, however, relate to ventilation issues. A WHO spokesperson said WHO had no comment about the Hong Kong study, but conceded that the ways in which poor ventilation may exacerbate virus transmission needs more systematic review:
“Outside of healthcare settings, some outbreak reports suggest the possibility of airborne transmission in indoor crowded spaces with poor ventilation. WHO calls for more systematic research of these kind of settings/outbreaks so we can have more definitive answers, and is activating the R&D Blueprint to accelerate research in this area.”