COVID-19 spreads through droplet transmission when a person coughs, sneezes, talks or sings. The bigger droplets fall on the surface of objects, which then act as fomites: high-touch surfaces that can spread the disease. Some smaller droplets stay in the air; it has previously been indicated that SARS-CoV-2 can stay for up to three hours in the air. However, scientists have not yet reached consensus over the occurrence of airborne transmission of the disease.
Recently, the Centers for Disease Control (CDC), USA said that COVID-19 can spread through airborne transmission and affect people even farther than six feet away. Several studies have reached similar conclusions in terms of physical distance.
A recent study done in the US suggested that the risk of airborne transmission decreases by half when you double the physical distance and that even simple cloth masks can reduce the risk of transmission.
However, physical activity increases the breathing rate and when more people are present in an area, it proportionally increases the risk of disease spread.
Now, a study done at the American Institute of Physics indicates that aerosol droplets are not as efficient at spreading the COVID-19 causing virus as we assumed.
The study is published in the journal Physics of Fluid.
Super-spreader events and closed spaces
Various super-spreader events or explosive outbreaks in healthcare or public settings have raised concerns about aerosol or airborne transmission.
Case studies have shown how SARS-CoV-2 spread happens in the direction of ventilation in closed spaces and can infect people sitting or standing farther away from the infected person. Experts suggest that airborne transmission of infections depends on the behaviour and properties of microdroplets.
Since infected people can spread the virus before they start showing symptoms, aerosol transmission (if it happens) is suggested to play a huge role in the spread of the disease.
The latest study
To measure the distribution of respiratory droplets, the researchers asked seven test subjects to cough or talk into a laser beam and used a tiny jet nozzle to create microscopic droplets. Here is what was found:
The size of the droplets did not change with age, sex, height or weight.
While coughing, most people produced equal numbers of cough droplets, apart from one person who produced 17 times more volume of liquid than others. The authors indicated that this type of person may be responsible for a superspreading event.
The volume of saliva produced after coughing was way more (0.07 to 0.05 g) than what was produced during talking (0.003 to 0.001 g).
Adjusting for the viral particles released during a severe infection (since there is no set limit so far, this was a rough estimate to avoid underestimation of the virus released), it was found that only one in 2,000 microdroplets contain enough virus to spread the disease.
The speed at which the droplets settle on surfaces would play an important role in airborne transmission.
Risk of infection
The risk of a person getting infected with airborne droplets would depend on their susceptibility and the number of viral droplets inhaled. S
ince they don't yet know how many viral droplets would lead to SARS-CoV-2 infection in a person, the authors of the study considered it to be the same as the other coronavirus SARS-CoV-1, which is between 10 to 100 PFU (plaque-forming units).
Taking the higher end of this number, the researchers found that an unventilated room with a dimension of 2 X 2 X 2 m3 contaminated with a cough of a single person is safe to be in for about 12 minutes due to low viral particles.
However, in a more ventilated room, the authors noted, the air would get quickly diluted and the risk of transmission would be even less.
For more information, read our article on How COVID-19 spreads.
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