Over the last few months, there has been controversy over the way SARS-CoV-2, the virus that causes, travels from an infected person to others. While official guidance has often been unclear, some aerosol scientists and public health experts have maintained that the spread of the virus in aerosols moving through the air at distances of less than and over 6 feet has played a more important role than appreciated.
In July, 239 researchers from 32 countries called on the World Health Organization (WHO) to recognize the possible role that airborne transmission plays in the spread of SARS-CoV-2.
Three days later, the WHO did so, saying that “short-range aerosol transmission, especially in certain indoor locations, such as overcrowded and inadequately ventilated spaces over an extended period of infected persons, cannot be ruled out under certain conditions.”
Many researchers welcomed social media when the CDC seemed to agree and recognized for the first time in a September 18 update that aerosols play a meaningful role in spreading the virus. The update said that COVID-19 can spread “through respiratory droplets or small particles, such as such as those in aerosols produced when an infected person coughs, sneezes, sings, talking or breathing. These particles can be inhaled into the nose, mouth, airways and lungs and cause infection. This is thought to be the main way in which the virus spreads. ”
However, controversy arose again when the CDC three days later took down this guide and said it had been sent by mistake without proper review.
Right now, the CDC website does not recognize that aerosols typically spread SARS-CoV-2 beyond 6 feet, instead saying: “COVID-19 spreads mainly among people in close contact (within about 6 feet) for a long period of time. Dispersion occurs when an infected person coughs, sneezes or speaks, and drops from the mouth or nose are thrown into the air and land in the mouth or nose of people nearby. The drops can also be inhaled into the lungs. “
The website says that respiratory droplets can land on different surfaces and people can become infected by touching these surfaces and then touching their eyes, nose or mouth. It goes on to say, “Current data do not support long-range aerosol transmission of SARS-CoV-2, as seen with measles or tuberculosis. Short-range aerosol inhalation is an option for COVID-19, as with many respiratory pathogens. not easily distinguished from “drop” transmission based on epidemiological patterns.Short distance transmission is an option, especially in overcrowded medical wards and inadequate.. “
Confusion has surrounded the use of words like “aerosols” and “drops” because they are not defined consistently. And the word “airborne” is of particular importance to infectious experts and public health officials because of the question of whether infection can be easily spread by “airborne transmission.” If SARS-CoV-2 is easily spread by airborne transmission, stricter infection control measures must be adopted, as is the case with airborne diseases such as measles and tuberculosis. But the CDC has told CBS News medical correspondent Dr. Jonathan LaPook that although airborne spread plays a role with SARS-CoV-2, the role does not seem to be nearly as important as with airborne infections such as measles and tuberculosis.
All of this may sound like crazy scientific discussion deep down in the weeds – and it is – but it has major 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, outdoors is safer than indoors. But what about the “6 foot” rule for maintaining social distance? If the virus can move indoors at distances greater than 6 feet, is it not logical to wear a mask indoors when you are with people who are not part of your “pod” or “bubble?”
Understanding the basic science behind how SARS-CoV-2 moves through the air should help provide us with strategies for safety. Unfortunately, there are still many open questions. For example, even though aerosols produced by an infected person can float across a space, and even though the aerosols contain some viable virus, how do we know how important a role the possible form of transmission plays in the pandemic?
As we await answers from ongoing research, Dr. LaPook turned to three leading scientists to try to purify the air. Recognizing that science is still not set in stone, they have generously agreed to give us their best advice on how to think about protecting ourselves, based on their current understanding of how SARS-CoV-2 can be spread. Below, atmospheric chemist Kimberly Prather, airborne virus expert Linsey Marr, and professor of environmental hygiene Donald Milton discuss the best precautions you can take to reduce your risk of infection.
Clears the air
Contrary to early thinking about the importance of transmission when in contact with large droplets of respiration, it turns out that an important way in which people become infected is by breathing into 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 humans at both close and long distances. Aerosols can be considered as cigarette smoke. While they are most concentrated close to a person who has the infection, they can travel longer than 6 feet, linger, build up in the air, and remain contagious for hours. As a consequence, it is important to take all of the following steps to reduce the chance of inhaling this virus:
Practice physical distance – the longer the better.
Wear a face mask when you are with others, even when you can maintain physical distance. Face masks not only reduce the amount of virus coming from people who have the infection, but also reduce the chance of you inhaling the virus.
Improve ventilation by opening windows. Learn how to clean the air effectively with methods like filtration.
Wear a face mask if you cannot physically distance yourself by at least 6 feet or ideally more.
Whenever possible you should move group activities outside.
Whether you are indoors or outdoors, keep in mind that your risk increases with the duration of your exposure to others.
With the issue of transmission, it is not only the public that is confused. There has also been confusion among researchers, medical professionals and public health officials, partly because they have often used the words “drops” and “aerosols” differently. To tackle the confusion, participants in an August workshop on airborne transmission of SARS-CoV-2 at the National Academies of Sciences, Engineering and Medicine unanimously agreed on these definitions of respiratory droplets and aerosols:
Drops is larger than 100 microns and falls to the ground within 6 feet and travels like small cannonballs.
Aerosols is less than 100 microns, is very concentrated close to a person, can travel longer than 6 feet and can linger and build up in the air, especially in rooms with poor ventilation.
All breathing activities, including breathing, speaking, and singing, produce far more aerosols than drops. A person is far more likely to inhale aerosols than to be sprayed by a drop, even at short range. The exact percentage of transmission versus droplets versus aerosols remains to be determined. But we know from epidemiological and other data, especially super-spreading events, that infection occurs by inhalation of aerosols.
In short, how do we get infected with SARS-CoV-2? The answer is: In the air. Once we have recognized this, we can use 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.