In recent months, there has been controversy over how SARS-CoV-2, the virus it causes COVID-19, travels from one infected person to others. Although the official guidance has often been unclear, some aerosol scientists and public health experts have argued that the spread of the virus in aerosols traveling through the air at distances both less than and greater than 6 feet played a more significant role than appreciated. .
In July, 239 scientists from 32 countries urged the World Health Organization (WHO) to recognize the possible role of airborne transmission in the spread of SARS-CoV-2.
Three days later, WHO did so, stating that under certain conditions, “short-range aerosol transmission, particularly in specific indoor locations, such as crowded and inadequately ventilated spaces for an extended period of time with uninfected people can be excluded “.
Many scientists cheered on social media when the CDC seemed to agree, first acknowledging in a website update on Sept. 18 that aerosols play a significant role in the spread of the virus. The update said COVID-19 can spread “through respiratory droplets or small particles, such as those in aerosols, produced when an infected person coughs, sneezes, sings, talks or breathes. These particles can be inhaled into the nose, mouth, in the airways. lungs and cause infections. This is believed to be the primary way the virus spreads. ”
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However, the controversy arose again when, three days later, the CDC withdrew that guide, saying it had been published in error, without proper revision.
At this time, the CDC website does not acknowledge that aerosols typically spread SARS-CoV-2 beyond 6 feet, instead saying, “COVID-19 mainly spreads among people who are in close contact (within approximately 6 feet ) for an extended period. Occurs when an infected person coughs, sneezes or talks and droplets from the mouth or nose are thrown into the air and land in the mouth or nose of nearby people. The droplets can also be inhaled into the lungs. ” .
The site claims that respiratory droplets can land on various surfaces and people can become infected by touching those surfaces and then touching their eyes, nose or mouth. He goes on to say, “Current data do not support long-range aerosol transmission of SARS-CoV-2, such as that seen with measles or tuberculosis. Short-range aerosol inhalation is a possibility for COVID-19, as for many respiratory pathogens However, this cannot be easily distinguished from “droplet” transmission on the basis of epidemiological models. Short-range transmission is a possibility especially in crowded medical wards and so ventilated spaces. “
Confusion has surrounded the use of words like “aerosol” and “droplets” because they have not been defined consistently. And the word “airborne” takes on special meaning for infectious disease experts and public health officials because of the question of whether the infection can be readily spread by “airborne transmission.” If SARS-CoV-2 spreads readily by airborne transmission, stricter infection control measures should be adopted, as is done with airborne diseases such as measles and tuberculosis. But the CDC told CBS News chief medical correspondent Dr. Jonathan LaPook that although airborne spread plays a role with SARS-CoV-2, the role does not appear to be as important as for airborne infections such as the measles and tuberculosis.
This may all sound like a shaky scientific discussion that’s deep in the weeds – and it is – but it has big implications when people try to figure out how to stay safe during the pandemic. Some tips are intuitively obvious: wear a mask, wash your hands, avoid crowds, keep your distance from others, it’s safer outdoors than indoors. But what about that “6 foot” rule to keep social distance? If the virus can travel indoors for distances greater than 6 feet, isn’t it logical to wear a mask indoors whenever you’re with people who aren’t part of your “pod” or “bubble”?
Understanding the basic science behind how SARS-CoV-2 travels through the air should help us provide strategies for staying safe. Unfortunately, there are still a lot of open questions. For example, even if aerosols produced by an infected person can float in a room, and even if aerosols contain some viable virus, how do we know how significant the role the possible mode of transmission is playing in the pandemic?
While awaiting answers from ongoing research, Dr. LaPook approached three leading scientists to try to clean up the air. Recognizing that the science is not yet set in stone, they generously agreed to give us their best advice on how to think about protecting us, based on their current understanding of how SARS-CoV-2 can spread. Below, atmospheric chemist Kimberly Prather, airborne virus expert Linsey Marr, and environmental health professor Donald Milton discuss the best precautions you can take to reduce your risk of infection.
Lighten the air
In contrast to the initial thinking about the importance of transmission by contact with large respiratory droplets, it turns out that one of the main ways people get infected is by breathing the virus. This is most common when someone is within 6 feet of a person who has COVID-19 (with or without symptoms), but it can also happen more than 6 feet away.
Viruses in small airborne particles called aerosols can infect people in both short and long range. Aerosols can be seen as cigarette smoke. Although they are most concentrated near someone who has the infection, they can travel over 6 feet, linger, accumulate in the air, and remain contagious for hours. Consequently, to reduce the possibility of inhaling this virus, it is essential to perform all the following steps:
Practice physical distance – the further away, the better.
Wear a face mask when you are with others, even when you can keep physical distance. Face masks not only reduce the amount of virus coming from people who have the infection, they also reduce the chance of inhaling the virus.
Improve ventilation by opening windows. Learn how to clean the air effectively with methods such as filtration.
Wear a mask if you can’t physically distance yourself at least 6 feet or, ideally, more.
Whenever possible, move group activities outside.
Whether you are indoors or outdoors, remember that your risk increases with the duration of your exposure to others.
With the issue of broadcasting, it’s not just the audience that’s confused. There has also been confusion among scientists, medical professionals and public health officials, in part because they often used the words “droplets” and “aerosols” differently. To address the confusion, participants in an August seminar on SARS-CoV-2 airborne transmission at the National Academies of Sciences, Engineering, and Medicine unanimously agreed these definitions for respiratory and aerosol droplets:
Droplets they are larger than 100 microns and fall to the ground within 6 feet, traveling like tiny cannonballs.
Aerosol they are smaller than 100 microns, are highly concentrated near a person, can travel over 6 feet, and can remain and accumulate in the air, especially in rooms with poor ventilation.
All respiratory activities, including breathing, speaking and singing, produce far more aerosols than droplets. A person is much more likely to inhale aerosols than to be sprayed by a drop, even at a short distance. The exact rate of droplet transmission relative to aerosols has yet to be determined. But we know from epidemiological and other data, particularly super spread events, that infection occurs through aerosol inhalation.
In short, how do we get infected with SARS-CoV-2? The answer is: in the air. Once we recognize this, we can use the tools we already have to help end this pandemic.
Kimberly A. Prather, PhD, Distinguished Chair in Atmospheric Chemistry, Scripps Institution of Oceanography, UC San Diego.
Linsey C Marr, PhD, Charles P. Lunsford Professor of Civil and Environmental Engineering, Virginia Tech.
Donald K Milton, MD, DrPH, Professor of Environmental Health at the University of Maryland School of Public Health.
Video: CDC director warns 90% of US population susceptible to COVID-19 (CBS News)
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