Here we are, more than six months into the worst pandemic since 1918 and scientists are still trying to figure out how people catch the coronavirus. Much of the debate centers around airborne transmission of COVID-19. Do people catch the virus by breathing in respiratory droplets or aerosols? By now, you would think there should be a clear answer so we could better protect ourselves.
Why Understanding Aerosol Transmission of COVID-19 Matters!
It is hard to design successful interventions until we truly understand how SARS-CoV-2 moves through the air. That includes traveling on buses, trains or airplanes. It also concerns schools, supermarkets, restrooms and restaurants.
Aerosols vs. Droplets?
Here is how three very smart Harvard physicians describe the difference between droplets and aerosols (JAMA, August 4, 2020).
“Droplets are classically described as larger entities (>5 μm) that rapidly drop to the ground by force of gravity, typically within 3 to 6 feet of the source person. Aerosols are smaller particles (≤5 μm) that rapidly evaporate in the air, leaving behind droplet nuclei that are small enough and light enough to remain suspended in the air for hours (analogous to pollen).”
If you would like to better understand the difference between greater (>) or less than (≤) 5 microns, here is an article we wrote to give you some sense of proportions. This link compares the thickness of a sheet of toilet paper to an E. coli bacterium to the flu virus to the coronavirus:
Can You Catch COVID-19 By Breathing Aerosolized Viral Particles?
Are you worried about catching the coronavirus? How far away from others do you have to be to stay safe? Is it possible to catch COVID-19 just by breathing?
Size Matters when It Comes to Transmission of COVID-19:
Here is why the size of aerosol particles carrying SARS-CoV-2 is so important. If droplets are the primary mode of transmission of COVID-19, a face mask will be protective, especially if people stay at least six feet away from each other.
If smaller aerosols are the transport vehicles for SARS-CoV-2, we’ve got problems according to the Harvard scientists.
They note that if the coronavirus is transported by:
“…aerosols that can remain suspended in the air for prolonged periods, medical masks would be inadequate (because aerosols can both penetrate and circumnavigate masks), face shields would provide only partial protection (because there are open gaps between the shield and the wearer’s face), and 6 feet of separation would not provide protection from aerosols that remain suspended in the air or are carried by currents.”
The authors point out that speaking can send aerosol particles distances of up to 27 feet and that SARS-CoV-2 can “…remain suspended in the air and viable for hours…” That is scary!
Don’t Freak Out Just Yet:
Just because viral particles can remain floating for hours does not mean there is aerosol transmission of COVID-19. How much virus is in the air and how long someone is exposed are critical factors.
Train Travel and Aerosol Transmission of COVID-19:
How risky is travel in the age of COVID-19? This question was addressed by Chinese and British scientists (Clinical Infectious Diseases, July 29, 2020) They collected data on Chinese passengers on high-speed trains between December, 2019 and March, 2020.
Passengers sitting next to an infected individual had a 3.5% chance of catching the coronavirus. For every additional hour of travel next to an infected person, the risk rose by 1.3%.
Sitting in the same seat that a patient had previously occupied was not particularly dangerous. The risk of transmission is related to both time and distance spent near a person with COVID.
Moving around the train to visit the restroom or obtain food or beverages also increases the chance for spread.
“When a passenger leaves a window or middle seat, the other passengers in the row need to let them pass, potentially increasing close face-to-face.”
So, what are the conclusions from this study of train passengers?
“Therefore, social distancing is an important method of reducing the risk of disease transmission on public transportation. The allocation of passenger seats on a train should be carefully considered to reduce the risk of transmission. Given the attack rates estimated for passengers on seats within the same row as the index patient, it follows that within 1 hour spent together, the safe social distance is more than 1 meter. After 2 hours of contact, a distance of less than 2.5 m can be insufficient to prevent transmission.”
We suspect that this would apply to buses, subways and airplanes as well as trains.
There are no absolute answers when it comes to the transmission of COVID-19. We find that shocking. We would have thought that the world’s best scientists would have made this issue one of their highest priorities.
The Harvard honchos who wrote in JAMA this week acknowledge that:
“All told, current understanding about SARS-CoV-2 transmission is still limited. There are no perfect experimental data proving or disproving droplet vs aerosol-based transmission of SARS-CoV-2. The balance of evidence, however, seems inconsistent with aerosol-based transmission of SARS-CoV-2 particularly in well-ventilated spaces. What this means in practice is that keeping 6-feet apart from other people and wearing medical masks, high-quality cloth masks, or face shields when it is not possible to be 6-feet apart (for both source control and respiratory protection) should be adequate to minimize the spread of SARS-CoV-2 (in addition to frequent hand hygiene, environmental cleaning, and optimizing indoor ventilation).”
Optimizing indoor ventilation may be a lot harder than most people think. There was a time when people could open windows and let fresh air in. That is uncommon these days. For decades, builders have tried to make houses and apartments as “tight” as possible to improve heating and cooling efficiency. There is even something called “sick building syndrome.” It is often attributed to poor air quality.
Restrooms, restaurants, office buildings, factories, pharmacies and lots of other public spaces have highly variable air quality. Some locations provide lots of fresh air, while others offer very little. Some have excellent air filtration systems but others do not. There is almost no way to know. Until the scientists can tell us with some degree of certainty about the transmission of COVID-19, about all we have to protect ourselves and those we love are face masks, distance and good hygiene.
So…Do Masks Make a Difference?
You would think that by now we would have great science and definitive information to answer this crucial question. We don’t! What we do have are some intriguing hints.
Kansas has inadvertently performed a mask experiment. In early July, the governor of the state was very concerned about the rising number of COVID-19 cases in her state. She issued an executive order calling for face masks to be worn when people entered public buildings. There was significant pushback. Officials in 90 counties in the state of Kansas refused to adopt the mask mandate. That was roughly 80% of the total.
Officials in 15 counties did accept the mandate. These were more urban counties with denser populations.
A natural experiment was under way. Think of the populations not wearing masks as the control group. The counties requiring masks were the experimental group. Although not a randomized, double-blind, placebo-controlled experiment, the Kansas experience has produced some fascinating results.
Dr. Lee Norman is the secretary of the Kansas Department of Health and Environment. He is a physician, former flight surgeon and former chief medical officer of the University of Kansas Health System. On August 5, 2020, Dr. Norman released data that deserve your attention.
Dr. Norman reports that (Kansas City Star, August 6, 2020):
“’Some counties have been the control group with no mask and some counties have been the experimental group where masks are worn,’ Norman said, ‘and the experimental group is winning the battle. All of the improvement in the case development comes from those counties wearing masks.’”
What Do You Think?
We would love to read your perspective in the comment section below. Many visitors to this website believe that wearing a face mask is a waste of time. You can read their comments at this link:
How COVID-19 Spreads So Easily | Scientists Disagree
There are still a lot of questions about aerosol transmission of the coronavirus. Why don’t experts know how COVID-19 spreads? Will masks help protect us?
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