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What you need to know about how COVID-19 spreads through air

  • What you need to know about how COVID-19 spreads through air

    Since the first cases of COVID-19 in Wuhan, China, in December 2019, scientists and others have been playing a constant game of catch up, trying to figure out in real time how the virus spreads and what are the best ways to prevent transmission. Now, more than 10 months later, as a global milestone of over 40 million documented infections is reached, scientists have much more information about how the virus moves through the air and infects others.

    Scientists now believe that the three main forms of transmission of SARS-CoV-2, the virus that causes COVID-19, are droplets; aerosols; and fomites, or surfaces. This increased understanding has led to a scientific consensus about the importance of wearing masks and also reinforces recommendations that have been made since the beginning of the year of consistent hand-washing and keeping distance from one another.

    In order to better understand what concrete steps should be taken in light of these most likely transmission pathways, a group of scientists and engineers with research experience in aerosols and air quality compiled a report titled “FAQs on Protecting Yourself From COVID-19 Aerosol Transmission.” A full list of authors is available on the fourth page of the report, which explains the best types of masks to wear, the safest activities to partake in and how aerosols spread, among other important information.

    Stacker consulted this report and compiled the 50 most important things to know about how COVID-19 spreads through the air. It has used version 1.78 of this report, which was published on Oct. 1, 2020. As information about COVID-19 is complicated and ongoing, it is important to always look for the latest information from reliable sources. Given the information that is currently agreed on by scientists, the following can offer some helpful information to help you make the safest choices during this pandemic.

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  • COVID-19 spread through large droplets

    One of the main methods of transmission of Sars-COV-2, the virus that causes COVID-19, is through large droplets, which are particles of saliva or other respiratory fluid that are expelled from individuals when they cough or sneeze or occasionally just through talking. When these droplets are expelled by an infected person, they fly like a projectile through the air, hitting other people on their mouths, nostrils, or eyes. When there is no one for these droplets to hit, they fall to the ground within a 3- to 6-foot radius.

  • COVID-19 spread through aerosols

    Along with larger droplets, COVID-19 also spreads through aerosols. These are particles of saliva or respiratory fluid that are smaller than roughly 100 micrometers and can linger longer in the air, sometimes for hours. They can also travel longer distances due to their small size. They infect people by being inhaled through the nose or mouth or in rare cases, by deposition on the eyes.

  • Size distinction between droplets and aerosols

    The difference between droplets and aerosols comes down to size. Particles smaller than around 100 micrometers are aerosols and are able to travel further and linger longer in the air. Larger particles are considered droplets. Droplets larger than around 300 micrometers have enough force to reach someone else at 0.5-1 meter, or approximately 3 feet, roughly the distance people stand from one another when having a conversation. Between 100 and 300 micrometers, particles typically are unable to infect others as they are too large to linger like aerosols, but too small to move as ballistic droplets and reach others.

  • Secondhand smoke as an analogy for aerosols

    When thinking about the spread of aerosols, it can be helpful to use secondhand smoke as an analogy. Like secondhand smoke, aerosols do not fall to the ground quickly and are most concentrated in front of the person who expelled them, then mixing in the air currents and building up in rooms with low ventilation. In most indoor environments, less than 10% of the aerosols will deposit onto indoor materials and the rest will stay airborne until released to the outdoor air.

  • Most common form of COVID-19 spread

    Scientists are still debating the most common form of COVID-19 spread. The most recent CDC guidance says that the droplet is the principle mode, that aerosols also play a role, and that fomites play a more minor role. However, this information could change.


  • Different people have different contagious levels

    Different people have different levels of contagion at different times. Peak contagiousness happens just before and at the onset of symptoms, and then the contagiousness decreases. By the time people are sick enough to go to the hospital, many are no longer contagious. And virus emission, or shedding, is not necessarily a constant event but rather sporadic. In addition, some individuals are likely more contagious than others due to higher viral load at the onset of symptoms, higher emissions of particles, or both. Some people exhale up to 10 times more aerosols when speaking.

  • Superspreader events

    The very high rate of spread of SARS-CoV-2 at certain large events can only be explained by airborne aerosols. The likelihood of an event becoming a superspreader depends on the activities attendees are participating in, such as loud talking or singing for example, and the ventilation in the space. This leads to a skewed distribution of R0 values, the rate of spread, because there can be very low or high values depending on specific situations.

  • Size of infectious aerosols

    The size of the SARS-CoV-2 virus itself is very small, around 0.12 microns. However, the aerosols in which these viruses are contained are larger, which has implications for the ability of masks and filters to remove it from the air, for how deeply it will enter into the lungs and how long they will stay in the air, making them more likely to be inhaled by others.

  • How infectious aerosols are exhaled

    The size of aerosols and amount of virus exhaled vary from person to person and is based on an individual’s stage of infection. They also vary based on the activity being undertaken, for example, talking, singing or shouting can lead to more aerosol exhalation. Aerosols can stay floating in the air for minutes and sometimes up to hours. If another person enters the space and inhales them, they can become infected and go on to infect others.

  • Where aerosols enter the respiratory system

    Coming back to the smoke analogy, aerosols operate in a similar fashion. When aerosols are inhaled, some deposit in the respiratory tract and some are exhaled back out, like smoke. The aerosols that are around the size of 0.3 micrometers are more likely to be exhaled back out, so even though masks are not very efficient at blocking such small aerosols, those same aerosols are less likely to stay in our respiratory tracts.