UvA (University of Amsterdam) physicists and medical researchers have discovered with the help of laser light techniques that small cough droplets remain in the air for minutes, especially in spaces that are not or poorly ventilated. Good ventilation in public areas, for example in public transport and in care homes, is therefore crucial to prevent the further spread of the covid-19 or coronavirus. The research results were published on Thursday May 28 in the scientific journal 'The Lancet Respiratory Medicine'.
The research was conducted by physicists Daniel Bonn, Stefan Kooij and Cees van Rijn from the UvA's Institute of Physics in collaboration with medical researchers Aernout Somsen (Cardiology Centers Netherlands) and Reinout Bem (Amsterdam UMC). The researchers asked healthy subjects to speak and cough, and then analyzed the droplets released by laser light. Large amounts of small droplets, roughly between 1 and 10 micrometers in size, were observed both during speech and during coughing. In addition, larger droplets were released when coughing, up to a millimeter in diameter. However, those last drops fall to the ground within a second, and therefore have a relatively small chance of transmitting viruses. Drops from coughing can be photographed using laser techniques. Large drops fall to the ground in a second, but small droplets can linger in the air for several minutes.
The small droplets, on the other hand, float very slowly towards the ground - due to the large air resistance they experience. The researchers discovered that such droplets can linger in the air for several minutes. After someone coughs once, it takes about five minutes for the number of small droplets in the air to halve. The mini-droplets are therefore much more dangerous when it comes to possible transmission of the coronavirus.
The importance of ventilation
When the experiment was repeated in a well-ventilated area, the results improved dramatically. After turning on a mechanical ventilation system, half of the droplets disappeared from the air in two and a half minutes. And in a room that also had a window and a door open, the number of droplets was even halved after thirty seconds - ten times faster than in the room without ventilation.
The results are important to take better precautions against the further spread of the coronavirus. Keeping sufficient distance from one another is not enough in public transport, for example, to limit the risks; ventilation is at least as important. Because droplets remain present for a long time, the use of apps that only measure the proximity of others is insufficient. The researchers recommend that health authorities come up with measures that guarantee good ventilation in public areas where possible.
G. Aernout Somsen, Cees van Rijn, Stefan Kooij, Reinout A. Bem and Daniel Bonn: "Small droplet aerosols in poorly ventilated spaces and SARS-CoV-2 transmission" in: The Lancet Respiratory Medicine (2020). https://doi.org/10.1016/S2213-2600(20)30245-9