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Micro-droplets could be the most significant form of transmission

For a century now, scientists have believed that the main means of respiratory virus transmission is through droplets released by coughing or sneezing, which are deposited on the mucus membranes (eyes, nose or mouth) of a potential new host, or on a surface via which they can infect. These droplets can travel more than 1 or 2 metres before falling to the ground.  

As well as this mode of transmission, there is also transmission by aerosol particles, micro-droplets that evaporate into the air via the simple act of breathing. These particles, less than 100 µm in size, remain in suspension and can therefore travel distances greater than 2 metres. Scientists believed, however, that transmission by this means was insignificant.

A recent international study summarized the progress made on understanding transmission by aerosol particles in the light of the COVID-19 pandemic.

At the beginning of the SARS-CoV-2 epidemic, scientists claimed that the virus was transmitted by droplets, taking into account its relatively low levels of transmission compared to measles, for example. Then the notion that the virus’ main mode of transmission could be via micro-droplets appeared. Transmission by aerosol particles is predominant in certain poorly-ventilated environments, where people breathe the same air. Aerosol particles are produced simply by breathing, from different sites in the respiratory tract (pulmonary alveoli, bronchi, larynx or mouth) and by different mechanisms, producing micro-droplets of different sizes. The majority have a size of less than 5µm, with many being less than 1µm. Depending on their size, they have differing levels of virus load, remain in suspension for a longer or shorter period of time, and can be deposited in different parts of the respiratory tract of the new host. The researchers were able to calculate that speaking produces 100 to 1000 times more micro-droplets (<100µm) than droplets (>100µm).

In addition, it would seem that the virus is more present in micro-droplets than in droplets. The life span of virions in micro-droplets is variable and depends on several parameters such as temperature, humidity and UV radiation: it generally varies from a few minutes to several hours. 

Micro-droplets can therefore be more contagious. In a new host, they can be inhaled, unlike droplets. Their size determines the site where they will be deposited in the respiratory tract. Micro-droplets of less than 5µm are able to penetrate deep into the lungs, as far as the alveoli. As a result, infection can take place if the virions present in the micro-droplets are still infectious and if the cells of the individual in this area have appropriate receptors so the virus can enter.

In conclusion, transmission of respiratory viruses by aerosol particles has been underestimated for a long time. However, epidemiological evidence of their involvement in the propagation of SARS-CoV-2 has meant that this mode of transmission could be studied close-up. And transmission by aerosol particles must also play a significant role in the transmission of numerous respiratory viruses. Understanding a illness’ mode of transmission is essential if appropriate sanitary measures are to be applied, such as mask-wearing or social distancing, as well as ventilation, air filtering or UV disinfection. However, complementary studies are still necessary, to establish a link, for example, between the viral load delivered in micor-droplets and the severity of the illness.

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