Even as 2020 heads to a close, the global coronavirus pandemic still seems far from over - with its recent resurgence revealing another mode of transmission: airborne via aerosols.

As early as March, the World Health Organization (WHO) has recognized two possible modes of COVID-19 transmission: the first being respiratory droplets and indirect contact with contaminated surfaces.

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Identifying the Third Mode of Transmission

Respiratory droplets are substances we expel from the body, mainly through exhaling, usually greater than 5 micrometers in diameter. Droplets larger than this usually travel a limited distance from the body before falling and settling on whatever surface catches these materials. In the context of COVID-19, if a person is within a meter of an infected person, and the infected starts speaking, coughing, or sneezing, the other person can inhale the droplets and risk infection too. This explains the recommended distance of at least 1 meter apart and the use of facemasks and face shields.

The second mode of transmission is in contact with surfaces where the respiratory droplets from infected persons rest. These surfaces, called fomites, transmit the disease after the person who touches them subsequently touches their eyes, nose, or mouth. Since this is a largely unconscious occurrence, it prompted WHO's handwashing recommendations.

However, after three months, scientists submitted a letter to the WHO regarding another potential transmission mode: via aerosol transmission. The health organization acknowledged in October that aerosol transmission could actually occur under specific conditions.

As a transmission medium, aerosols are considerably smaller compared to the larger respiratory droplets. WHO explains that "aerosol transmission can occur in specific settings," specifically indoor, crowded, and inadequately ventilated locations. The organization also noted that "more studies are underway" to understand better the circumstances that allow COVID-19 transmission via aerosol.

Among the recent studies that demonstrate the aerosol transmission of COVID is a November 2020 study published in the journal Environment International.

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Putting Particles in Scale

While respiratory droplets are generally defined as larger than 5 micrometers, aerosols are limited by the WHO to be less than 5 micrometers, with their average diameters going less than 1 micrometer. Since aerosols - like smoke, dust, or fog and mist - are significantly smaller, they can stay in the air for longer and travel greater distances. This poses a threat since if the aerosol particles also contain traces of infected vector from a person with COVID-19, it poses greater risks of being absorbed by others.

An article from Medium features an artwork that puts the size of aerosols and respiratory droplets in perspective. The series of concentric circles begin with a magnified 1mm diameter circle, roughly equivalent to a pinhead or a flea size. It goes smaller, with 500 micrometers (amoeba), 300 micrometers (dustmite), down to the 0.1-micrometer diameter of the SARS-CoV-2 particle - the virus that causes COVID-19. The visual presentation required magnifying the original artwork three times to display the smaller particle in the scale. It means that a SARS-CoV-2 particle is about a tenth of an average aerosol particle.


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