Droplet size, humidity … We now know more about the conditions at risk of transmitting the virus

Droplet size, humidity … We now know more about the conditions at risk of transmitting the virus

Droplet size, humidity … We now know more about the conditions at risk of transmitting the virus

How long do virus-containing drops stay in the air? Do we risk being contaminated if we spend five minutes in the same room as a positive person? Does the virus stay on the surface of objects for more than a few hours? In the first days of the health crisis, there was talk of everything, and often everything, about the risks of transmitting SARS-CoV 2, due to the lack of precise scientific studies. “There has been a lot of debate on the subject. Is it humid air, dry air, what size of the droplet, do they store for hours or minutes, is it really airborne or by other means? remembers Kevin Roger, CNRS researcher at the Chemical Engineering Laboratory in Toulouse.

To shed some light on the subject, this specialist in “drying complicated fluids” decided to investigate the matter taking into account all these elements at the same time. The results of his study, published in the prestigious American scientific journal PNAS, challenge a number of ideas received. They also provide answers on the most risky situations and on the effective or not effective measures to be taken in the event of a new wave.

The importance of size and origin

To achieve this, the team conducted various experiments in the laboratory, sieving the famous drops using optical, spectroscopic and microbiological techniques in order to discover their composition or even the drying time. All kinds of drops. Because there are many different dimensions and these data are decisive on their role in the contamination. Just like their origin, because they are different whether they come from the depths of the lungs or the mouth.

“There are drops of all sizes in an expired aerosol! They can be grouped into two main populations: a population of small drops of a few micrometers, formed in the lower and numerous respiratory tracts, and a population of large drops of several tens of micrometers, formed in the upper respiratory tract and which represent much greater volume and therefore capable of carrying more viruses, ”explains the researcher.

The small drops, those we emit when we breathe normally, and which remain the longest in suspension in the air, are therefore not the most dangerous. In fact, they carry fewer viruses and the latter will degrade quite spontaneously in environmental conditions. “After an hour, there isn’t much active virus left. The threat of saying “you will have aerosols that will hang in a room for hours and when you enter you are contaminated” is a bit of a myth, “points out Kevin Roger.

The large drops, those emitted from the upper tract, when coughing or talking loudly, are therefore more problematic. And even more so since the virus colonizes our upper respiratory tract, as is even more the case with the Omicron variant.

The law of gravity is involved

But the drops are also less resistant to certain environmental conditions. Thus, humidity tends to accelerate their neutralization, through a fairly simple phenomenon: gravity. And the virus, contained in the drop, does not support it better. “When we increase the humidity, it is not conducive to transmission. For the drops we slow down their evaporation, so we prevent them from losing mass. And the heavier they are, the faster they fall and the more we limit their accumulation. The virus is found in a dried saliva matrix, but as soon as we have some water, there are processes that deactivate it, “continues the researcher.

If it is necessary to prevent the drop from evaporating so that it is heavier and falls, logically the hotter it is, the greater the risk of contagion. In reality this effect subsides because the virus does not like heat and deactivates more quickly when the thermometer is high.

One of the best ways to fight effectively is ventilation, which allows you to approach external conditions, unfavorable to contamination. But continuously, not just once an hour or when the CO2 sensor alarm starts ringing. “We said that we had to measure the quantity of air, therefore of CO2, because it is a measure of the quantity of drops we emit when we breathe. But the sensors track the small drops, but after an hour their viral load is almost zero. This gives an impression of safety, but does not track the large drops at all, which remain in suspension for only a few minutes and have a shorter action time but are more “effective” for transmission, notes the scientist.

Group work and the canteen, moments at risk

However, this hourly ventilation has been one of the creeds of health protocol in schools. “The class situation is not the most risky as long as not everyone speaks at the same time. A closed amphitheater where people don’t talk is the same. The very problematic situations are those of teamwork, toilets, canteens, when you have no protection, when you are in a closed environment, when you speak loudly and open your mouth. The solution would be to have drafts, eat outside and filter the air in the room as much as possible. But that has its limits, especially in winter, ”continues Kevin Roger.

A lot of information that can, however, allow public authorities to make more adequate decisions if a new virus of this type should emerge or if a new variant of the coronavirus develops in the coming months. They could also use these results to accurately calculate the risk for each situation taking into account airflow, room size, ventilation mode, number of guests … and determine if, at Christmas, grandfather and grandmother will they’ll have to eat their share of the log in the kitchen.

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