COVID-19 is in the air: Clearing the smoke on aerosol transmission

On Thursday Sept-26, during a conversation with Gov. Chris Murphy of New Jersey, Dr. Fauci said “There is good enough data that aerosol transmission does occur. Aerosol means that the droplets don’t drop immediately; they hang around for a period of time.” Dr. Fauci added that the droplets can travel farther than 6 feet.

While several members of the scientific community have been emphasizing the importance of aerosol transmission of COVID-19 for months, the messaging from health organizations has been unclear and contradictory. For instance, the CDC reversed course last week on aerosol transmission. Unfortunately, due to this mixed messaging, there are still a lot of misconceptions about the spread of COVID-19. Therefore, in this blog, I will briefly discuss the meaning of aerosol spread and more importantly, I will summarize a few studies on aerosol spread of COVID-19.

Disclaimer-1: I am not an expert on this topic and thus the discussion is based as per my understanding of the literature.

Disclaimer-2: Please keep following all the health guidelines that have been recommended: wear a mask, avoid indoor and crowded spaces, socially distance, wash hands etc. The main aim of this blog is to introduce aerosol transmission and summarize some recent studies in an easy to understand language.

What is aerosol transmission?

Aerosols are simply small particles and droplets in the air, typically 1 micrometer and smaller. To put this in perspective, human hair is about 80 - 100 micrometers thick, and thus these particles and droplets are very small.

So why does the size of the particle/droplet matter? The size matters because the duration for which these particles/droplets linger in air depends significantly on their size. The smaller the size, the longer the particles/droplets would hang around in the air.

The smoke analogy

To better appreciate the mode of aerosol transmission, I turn to the analogy provided in an article by Dr. Jimenez. When a person infected with COVID-19 speaks, sings, breathes or coughs, they produce a “smoke” of virus in the air as if they were smoking a cigar. If someone interacting with the infected person inhales this smoke, they may also get infected. However, if both of them wear masks, the chances of virus transmission will decrease. In addition, if the interaction occurs outdoors, the virus transmission will (likely) further decrease. On the contrary, if the interaction occurs in a poorly ventilated indoor space, 6 feet may not be enough to avoid transmission since the smoke may travel farther and linger around for a while.

There are many important implications of aerosol transmission. First, it highlights the universal importance of wearing masks. Second, it emphasizes that the spread is more likely to occur indoors. And third, it highlights that 6 feet may not be enough in a poorly ventilated space.



The two modes of virus transmission. The figure has taken from Zhang et al. PNAS 2020


The above analysis is in contrast with the guidelines suggested in early March. Back then, it was assumed that the virus is spread through surfaces, i.e., shaking hands or by touching infected doorknobs (oh! we all remember wiping groceries incessantly), or through directly inhaling big droplets from an infected person. Overall, these two modes suggest that the droplets do not linger in the air for long. Therefore, back in March, the importance of wearing a mask was not recognized universally, at least in the US. In addition, the 6 feet guideline was suggested to avoid these large droplets.

The case for aerosol transmission

In their investigation, Zhang et al. PNAS 2020 make the case for aerosol transmission and universal mask mandates. The authors compared the outbreaks between China, Italy and the US, and argued that a universal mask mandate helped decrease the spread of COVID-19 in China and Italy. In addition, the authors showed that when the mask/face-covering mandate was implemented in NYC, the spread started decreasing significantly. Simply put, social distancing without the usage of masks, i.e., the policy which assumes that the transmission occurs through large droplets, is not going to be sufficient in containing the spread of COVID-19.

If the major mode of transmission is through aerosols, the droplets can linger in air for long and they can travel farther than 6 feet in poorly ventilated spaces. Therefore, the usage of face masks is crucial in containing the spread.

To further support this conclusion, I now summarize a few additional studies that highlight the aerosol transmission of COVID-19.

The curious case of the bus ride

The study by Shen et al. was published in JAMA Internal Medicine. The study describes two 100-minute bus rides on January-19 2020 in Zhejiang province of China that took 128 passengers to a 150-minute workshop event.

The two bus rides had 60 and 68 passengers, respectively. In both the buses, no passenger was wearing masks and the air conditioners were circulating air internally. In the first bus, the study reports that no one contracted COVID-19 because there was no passenger who was infected with the virus before the bus ride began. In contrast, in the second bus, a passenger who had the virus before the bus ride began, referred to here has the index patient, transmitted the virus to 23 other individuals during the bus ride.


Schematic of second bus: The figure has been adapted from Fig.2 of Shen et al.


The findings are even more consequential if one focuses on the details of the sitting arrangement inside the second bus. The image above shows that the index patient was sitting in Row-8. However, the transmission was not limited to passengers closest to the index patient. In fact, the difference in infectivity wasn’t necessarily dependent on the proximity to the index patient, and several passengers who contracted coronavirus were sitting farther than 6 feet from the infected individual.

In summary, the paper describes a scenario of a poorly ventilated space where no one was wearing masks. If the transmission occurred through large droplets that were directly inhaled or through large droplets that fell on surfaces, it is unlikely that the disease would have transmitted to 23 other individuals. Therefore, the aerosol transmission appears to be the dominant mode.

The 2.5-hour choir practice

The study by Miller et al. was published in Indoor Air. The report describes the super spreading event of a choir practice in Washington, USA on March-10 2020. As detailed in the report, the choir practice was attended by 61 members and one of them had cold-like symptoms. It is crucial to note that the choir practice started at 6:30 pm and ended around 9:00 pm.

Ultimately, COVID-19 was contracted by 52 other individuals (i.e., 53 in total), where 33 cases were confirmed through COVID-19 tests and 20 other cases had symptoms consistent with COVID-19. Unfortunately, 2 members passed away.

Overall, the report concludes that in addition to being indoors, the transmission was widespread because of the activity of singing that can produce large amounts of aerosols. In addition, the report rules out other modes of disease transmission and also concludes that aerosol transmission was likely the dominant mode.

Looking forward: Beyond the 6 feet guideline

In their study (not yet peer-reviewed), Bazant and Bush describe the guidelines to limit indoor transmission of COVID-19. They categorize the aerosol volume fractions between different activities such as singing, speaking, whispering and breathing. In fact, they also compared the rates to the scenario of the above choir practice (see Fig. 2 in Bazant and Bush). The authors then quantify the importance of masks, density of occupancy inside a room and ventilation, among other factors.

Going forward, it is possible that more guidelines will appear for indoor activities so as to limit the spread of COVID-19. For instance, there may be greater emphasis on improving the ventilation inside indoor public spaces while limiting the indoor occupancy. But first, it is crucial for health organizations to recognize that aerosol transmission is important and cannot be overlooked, or as Dr. Fauci described in his interview with Gov. Murphy, “This (aerosol transmission) becomes very relevant when you are indoors and when there is not good ventilation.”


Acknowledgement: I thank Upasana Raval, Akshi Singla and Charu Mehta for their valuable inputs.

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