It has been established that drops caused by coughing and sneezing play an important role in the spread of SARS-CoV-2. It is unclear what contribution smaller moisture droplets make. Below the state of the art.
What are aerosols?
Aerosols exist according to the RIVM from a cloud of large and small drops and from droplets. Large drops have a diameter of more than 5 to 10 micrometers. Small drops have a diameter of less than 5 micrometers. Drip cores do not contain moisture. When coughing and sneezing, large drops are released that quickly fall to the ground. Fine moisture droplets and droplet cores remain in the air longer. In common parlance, aerosols often mean small drops of moisture.
Are there virus particles in fine moisture drops?
The genetic material of SARS-CoV-2 can remain in the air. Researchers from the University of Nebraska took air samples in patient rooms and ward corridors. They found viral genetic material in several samples. However, the researchers were unable to infect cells with it.
Two other research groups state that they could infect cells with virus particles from fine moisture droplets. However, those droplets did not come from covid-19 patients, but from an atomizer in a test rig. The one group states that virus particles remain contagious for 3 hours, the other group comes out at even 4pm.
Does ventilation reduce the amount of fine droplets in the air?
Amsterdam researchers showed that ventilation quickly reduces the number of fine drops in the air. In their experiment, the number in the best-ventilated room halved in thirty seconds. In less ventilated rooms this took one to four minutes. Also professor of civil engineering at Eindhoven University of Technology Bart Blocken underlines the importance of ventilation. According to him poor ventilation may explain the many infections in nursing homes and in choirs.
Are there contagious amounts of virus in fine droplets?
In very specific circumstances, such as when inserting a breathing tube into a patient’s windpipe, many fine moisture droplets are created with enough virus particles to cause infections. It is unknown whether drop cores can contain infectious amounts of SARS-CoV-2.
How do fine moisture droplets form?
Fine moisture droplets are not only created during specific medical procedures, but also, for example, when speaking, screaming, singing and playing wind instruments. The louder someone speaks, the more fine droplets. Why there are big differences between persons is still unclear.
What indications are there that aerosols play a role in the spread of SARS-CoV-2?
A Chinese study reconstructs how in a restaurant in Guangzhou, ten out of 21 guests contracted SARS-CoV-2 at three tables. No one of the 68 other guests and dozens of employees became infected. The three contaminated tables were in a corner of the restaurant with hardly any ventilation. The researchers conclude that fine moisture droplets may contaminate people with SARS-CoV-2 in a crowded, poorly ventilated area.
Studies in laboratory animals also indicate the spread of the virus. Virologist Sander Autumn of Erasmus MC showed that ferrets can contract the virus if they live in a cage with an infected ferret or if they are at a distance of at least 10 cm from the infected ferret. Due to the small distance between the ferrets, it is unclear whether the virus was transmitted through large or small drops or both, Autumn and colleagues enroll a pre-publication of their research.
Are investigations currently underway that could provide more clarity?
A follow-up experiment with ferrets will soon start in Rotterdam to see if they can also infect each other by air if they are 1 meter apart. Virologist Autumn hopes to know that in about 3 weeks. Erasmus MC also wants to collect fine moisture droplets from the air around infected patients or animals. Autumn wants to investigate whether it contains virus particles that in theory can infect other people.
Why does RIVM argue that aerosols have not been proven to play a role in the spread of SARS-CoV-2?
Several studies have shown that fine moisture droplets that humans excrete can contain virus particles. However, RIVM says, no research proves that it is enough virus particles to infect people. “When someone speaks, screams, coughs or sneezes, you get clouds of air with droplets in them,” Jaap van Dissel told NOS on Saturday.
“Those droplets have different sizes. If you have a droplet of 10, 1 or 0.1 micrometre, they differ in volume by a factor of a thousand. So you can imagine that on average large drops contain much more virus than small drops.” Those big drops fall down earlier than the very fine ones. And that happens within a meter and a half.
“And then you see at covid-19 that in all countries where 1.5 meters are held, it has led to a decrease in the number of infections.” The relatively low infection rate of SARS-CoV-2 also indicates that infection mainly occurs through large drops. Viruses that spread through fine moisture droplets have a much higher infection rate.
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