A controversial PNAS study led by a group of experts in atmospheric dynamics has shown how the finest droplets (aerosols) we exhale they can stay and travel through the air with enormous ease. And, therefore, to be an excellent way of contagion of the SARS-CoV-2.
Does this compromise the idea that optimal safety distance Is it 1.5m to 2m between people? The WHO maintains this recommendation, while it has not declared that aerosols are a main route of spread of SARS-CoV-2.
And it is one thing that the product of our breathing travels and is suspended in the air and another, that it is loaded with enough viruses as to infect people when they breathe that air.
In this video we compile the evidence we have regarding the main routes of transmission of SARS-CoV-2 and its ability to infect and make COVID-19 sick. It is summarized in:
The droplets we exhale do not usually go beyond 1.5m or 2m
The transmission of SARS-CoV-2 by aerosols “occurs in particles of sizes between nanometers to micrometers, since the virus (about 100 nm in size) does not travel alone, but does so as part of particles of various sizes that they also contain salts and proteins from the mucosa, ”says María Cruz Minguillón (IDAEA-CSIC), author of this report on the emissions of the virus.
As he adds in this work, a traditional distinction is made between “drops” (> 5 micrometers) and “aerosols” or “micro-drops” (<5 micrometers), although the reality is that the size distribution generated by breathing, coughing or sneezing it is a continuous size range. It is not that we emit only one type of droplets.
Coronavirus transmission by respiratory droplets
Several works, even before the SARS-CoV-2 eruption, have shown that when we speak and, especially, when we scream, thousands of them come out of our mouths, more than 10,000.
The heaviest ones, about 500 micrometers, soon fall to the ground or surfaces (fomites, which if we touch and take our nose, mouth or eyes, they can infect).
Others, smaller, can reach up to 2 meters away from the mouth. Up to 8 meters in a sneeze. The preferred ‘travel’ routes for the coronavirus, according to the WHO, remain these, with the current evidence in hand.
The smallest drops can be left floating in aerosols
In early research work in China, at the start of the epidemic, it was suggested that aerosols or airborne particles could be carriers of coronaviruses. However, it was only possible to measure clearly in hospital settings.
Intubating a person with COVID-19 was the typical case of risk of infectious aerosols collected by these works. And the WHO continues to maintain this path as a clear possible contagion.
However, it has been empirically observed how people who were more than those 2 meters from security have been infected in closed places: restaurants without ventilation, buses and offices. What do these sites have in common?
The places closed and poorly aerated The chances of contagion are increased by up to 18 times, according to this observational study from Japan. If we add the low temperatures and dryness, typical of air conditioning, we have an x-ray of the first outbreaks of the epidemic.
Proximity and recirculating air conditioning, fundamental
If talking without a mask means supplying the environment with droplets that can be loaded with viruses, there is a paradigmatic case in which to study the phenomenon: call centers or call centers.
In Seoul, 94 telemarketers were infected on the floor of the office building where they worked. The enclosure was divided into two wings, with common areas, but separated by automatic doors. The infections were concentrated in one. The office had no outside ventilation.
Something similar to what happened in a Guangzhou restaurant in January. Why did 10 people end up infected, if some sat more than two meters from the sick person? Air conditioning recirculating It seems to have served to get those droplets to three tables, the ones he had online.
One of the nearby tables, even possibly affected by the current from the air conditioning, did not register infections. Barely coincided 18 minutes with the infected person, compared to more than 45 minutes from other diners.
Something similar occurs with coaches with relatively long journeys, recirculating air and proximity between travelers. It is the same question that planes pose. Aircraft manufacturers, however, have been quick to remember that they are equipped with filters and the air is renewed.
Although in China it was observed (preliminarily) that a third of the initial infections occurred in public transport, in Tokyo so far there has not been a single regrowth associated with the commuter train or subway. Experts point out that wearing masks and not speaking during trips, which are not long, can help protect passengers from infection.
The longer the worse, only indoors
Dr. Lloyd-Smith, this year has dedicated part of his research to observe to what extent the air that we exhale It can spread to other people, in case we have SARS-CoV-2 in our throat.
Basically, he did an experiment that showed how someone’s respiratory sprays can get floating up to 3 hours. But every minute that passes they lose infectious capacity. Microdroplets also fall on surfaces, where they were found to hold up relatively well on plastics and steel, and far worse on copper.
Many drops and very loaded with viruses
What is the minimum amount of virus to inhale to allow SARS-CoV-2 infection? We still don’t know exactly. But it does not seem irrelevant the fact of joining with the respiratory droplets of someone particularly loaded with viruses. Someone who participates in so-called super-contagion events.
The phenomenon, described by James Lloyd-Smith (UCLA) in Nature in 2005, allows us to measure how uniformly the epidemic progresses. As Monica G. Salomone recalls in Sinc, the most extreme case identified is that of patient 31 from South Korea. He had attended several religious meetings in February and in a few days more than 5,000 contagions were recorded in the town of Daegu.
In a very precise work, but full of limitations, Giorgoo Buonanno calculated that in the smallest droplets, those capable of perhaps floating, there would be only 0.236 particles per cubic centimeter.
How much virus in the droplet does it take to get infected? It has not yet been clearly measured. This author uses the model of the first SARS, but it is not so clear that they are comparable, given the highly contagious nature of the new coronavirus.
The professor of physics at Cassino University believes that the measures imposed in most countries with improved ventilation, as well as reduced exposure times «have reduced the contagion values between 80 and 90%«.