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Information on the transmission of the SARS-CoV-2 virus has constantly been evolving right from droplet spread to the virus being declared airborne. The acknowledgment of airborne transmission prioritizes ventilation along with masks, hand hygiene, and social distancing. This article focuses on ventilation, the most critical measure to curb the spread by intentionally introducing outdoor air into closed spaces.
SARS-CoV-2 infected individuals generate many small respiratory particles laden with the virus. When they exhale, some of these particles will be inhaled almost immediately by those within a short distance (<1 m). In contrast, many particles disperse over longer distances and can be inhaled by others that are further away (>2 m). Traditionally we refer to the larger-size short-range particles as droplets and the smaller-sized long-range particles as droplet nuclei. However, we need to understand that they are all aerosols because they can be inhaled directly from the air.
Ventilation flow rate and viral transmission1,2
Increased viral transmission in crowded and indoor places is not a novel concept and has been previously documented during influenza and the H1N1 pandemic. A study shows that a low ventilation flow rate is associated with relatively high aerosol transmission. In contrast, a sufficiently high ventilation flow rate reduces the chances of airborne transmission to very low levels.
Superspreaders of COVID-191,3
Superspreading is loosely defined as a situation wherein a single person infects many others in a short time. Poorly ventilated spaces like slaughterhouses, megachurches, fitness centers, and nightclubs have given rise to many such superspreading events during this pandemic.
There have been many instances of COVID-19 superspreading events reported in indoor spaces and have surprisingly affected not only people in close contact with the infected person but also those at a distance, thereby ruling out any chances of droplet or surface transmission.
Examples of some superspreading events:
- The Guangzhou restaurant outbreak,
- Bus in Ningbo carrying 68 passengers, out of which 23 got infected during the one-and-a-half-hour journey,
- Choir practice in Skagit Valley infected 53 out of the 61 people who attended a two-and-a-half-hour meeting.
A detailed epidemiological and environmental study of the restaurant outbreak in Guangzhou (China) showed how families that did not have close contact during lunch got infected. Many members were sitting back-to-back, which ruled out droplet and surface transmission.
This study suggested that "strong airflow from the air conditioner" could have propagated droplets from one table to another and predicted that there was recirculation of a contaminated bubble in the area where these families were sitting.
The most crucial finding from this study was the evidence of low ventilation rates due to a lack of outdoor air supply. The exhaust fans in the walls were turned off, exacerbating the ventilation deficit where these families were sitting.
Carbon-dioxide (CO2) and COVID-19 transmission3,4
CO2 concentrations have been suggested to be a valuable proxy for clean air. According to air quality experts,
- CO2 concentration below 500 ppm in a room means the ventilation is good.
- At 800 ppm, 1% of the air someone is breathing has already been exhaled recently by someone else, and
- At 4,400 ppm, it rises to 10% and would be considered dangerous. Such dangerous levels are seen only in crowded spaces with poor airflow.
An experiment showed that outdoor air contains around 400 parts per million (ppm) of the CO2, and people's exhaled breath contains about 40,000 ppm. So, exhaling in a room can gradually raise its CO2 concentration unless the ventilation is good enough to remove all the excess. Thus, to keep the risk of COVID-19 transmission low, CO2 levels should be below 700 ppm.
Measures to improve ventilation3,4,5
A work published by Martin Bazant and John Bush from the Massachusetts Institute of Technology calculated how long it could be considered safe to stay within a room that contains an infected person.
Dr. Hong, using computer models, studied how aerosols would spread in a classroom. He demonstrated how placing an air cleaner or extractor fan at the front of the room sets up an airflow that prevents aerosol movement. One can achieve an even better aerosol-cleansing effect by elevating the position of fans and filters above the people in the room, for which one can use small, cheap box fans.
Air filters, disinfection of air by placing germicidal ultraviolet lamps within air-conditioners or near the ceiling in a room, issuing ventilation certificates for buildings, CO2 level monitors and sensors, etc., are ways to minimize COVID-19 spread in less ventilated places.
We need to focus on reducing COVID-19 transmission in indoor settings as the risk for outdoor transmission seems to be minimum6. Improving ventilation will help us achieve virus-free air indoors, and further help us prevent major superspreaders.
- Li, Yuguo & Qian, Hua & Hang, Jian & Chen, Xuguang & Cheng, Pan & Ling, Hong & Wang, Shengqi & Liang, Peng & Li, Jiansen & Xiao, Shenglan & Wei, Jianjian & Liu, Li & Cowling, Benjamin & Kang, Min. (2021). Probable airborne transmission of SARS-CoV-2 in a poorly ventilated restaurant. Building and Environment. 196. 107788. 10.1016/j.buildenv.2021.107788
- X. Gao, J. Wei, P. Xu, et al., Building ventilation as an effective disease intervention strategy on a large and dense social contact network, PLos One 11 (9) (2016) e0162481, https://doi.org/10.1371/journal.pone.0162481.
- Improving ventilation will help curb SARS-CoV-2. https://www.economist.com/science-and-technology/2021/05/26/improving-ventilation-will-help-curb-sars-cov-2
- Environ. Sci. Technol. Lett. 2021, 8, 5, 392–397. Publication Date:April 5, 2021 https://doi.org/10.1021/acs.estlett.1c00183
- Bazant MZ, Bush JWM. A guideline to limit indoor airborne transmission of COVID-19. Proc Natl Acad Sci U S A. 2021;118(17):e2018995118. doi:10.1073/pnas.2018995118
- A Misleading C.D.C. Number. https://www.nytimes.com/2021/05/11/briefing/outdoor-covid-transmission-cdc-number.html