That covid-19 is transmitted mainly through the air is an indisputable reality by now.
It does so through the now famous aerosols, which are nothing more than small particles of saliva or respiratory fluid emitted by people when they breathe, speak, scream or cough.
Although it is clear that aerosols emitted by healthy people are not a problem, those emitted by infected people can contain viruses. The problem is that they remain floating in the air for minutes or hours and, in that time, they can move several meters.
In poorly ventilated indoor environments, aerosols from an infectious person are distributed throughout the space with the risk of other people being infected by inhaling them. The air of a closed room works like a swimming pool: if there is a fountain in the pool that draws water with dye (our virus), after a while the entire pool (our air) will have changed color. It does not matter if I am near or far from the spout: the water will be colored.
How do we know all this? In addition to the prepandemic knowledge of fluid and aerosol dynamics, in the last year multiple studies have been carried out. Some of them have detected infectious SARS-CoV-2 in indoor air. Animal experiments have shown that contagion exists with no contact at all.
Multiple supercontagion events in which a single person infects many that can only be explained by aerosol transmission have also been studied.
It has even been determined that being infected indoors is 20 times more likely than outdoors, which again can only be explained by aerosol transmission. The health journal The Lancet has recently published an article that leaves no doubt as to the importance of the route of transmission by aerosols.
It is known that infected people are mainly contagious before they have symptoms (presymptomatic) or without developing them (asymptomatic). Thus, it is impossible, in the absence of immediate, reliable and abundant diagnostic tests, to know who is infectious and who is not. Therefore, you have to act as if all people were. We have to protect ourselves continuously.
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Does surface disinfection make sense?
For a variety of reasons, surface and droplet transmission have long been considered the main ones, despite the lack of evidence.
Surface transmission is when a person touches a surface that contains viruses and then touches their eyes, nose, or mouth. Drops are large particles emitted when talking, coughing or sneezing that would hit the eyes, inside of the nose or inside of the mouth of another person. For this reason, the measures adopted focused mainly on disinfection of surfaces and protection against drops (distance, physical barriers).
But the reality is that the main transmission is by aerosols, also at short distances. Even the Ministries of Science and Health in Spain published reports on aerosol transmission at the end of 2020, although the conclusions have not been greatly reflected in the measures applied since then.
You have to change the strategy. The finding of aerosol transmission of SARS-CoV-2 is not bad news. SARS-CoV-2 has been transmitted this way since the beginning of the pandemic. Ignoring it has led us to misdirect efforts. Knowing what is the main form of transmission of covid-19 is our best tool to prevent it.
Multiple scientific articles reflect this. Nature magazine itself in its February 2021 editorial called for change: “The coronavirus is in the air: there is too much emphasis on surfaces“.
We have to do it now. How? More than 100 Spanish scientists have identified eight key points to end the pandemic. It is a consensus reached between various areas of knowledge such as virology, engineering, environmental science or medicine. They are explained in a letter, promoted by the Aireamos group, addressed to the competent authorities in Spain, central and regional.
1. General purpose masks must be effective. It is necessary to identify and withdraw from the market those that are not and emphasize the need for a good fit. A poorly fitting mask (with gaps between the edge of the mask and the face) can see its effectiveness cut in half. In shared interiors, including of course workplaces, it must always be carried, regardless of the distance between people.
2. The activities abroad. This implies facilitating the use of parks and gardens and keeping an eye on ‘false exteriors’ such as enclosed terraces.
3. Indoor spaces must be ventilated with outside air continuously and sufficiently, by natural or mechanical ventilation. It is about adding clean water to our pool continuously and gradually drawing colored water. How much? Enough so that the pool is never too dark, despite the fact that the spout with coloring does not stop.
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The criteria have to be clearly defined. Even the WHO has published recommendations on ventilation, although they do not yet explain clearly how infections occur.
4. It has to measure CO₂ indoors to verify adequate ventilation. CO₂ is emitted along with aerosols when we breathe, so it is a very good indicator of how much used air is in a place. It is the best solution currently available to indicate the risk of contagion.
5. The efficacy and possible risks of the various air cleaning technologies. The preferred technology is filtering (the known HEPA filters) by effectively removing respiratory aerosols.
6. It has to pay special attention to educational centers. They are spaces with the characteristics that propitiate super contagion events: many people, many hours a day and sometimes poor ventilation.
7. Clear and effective criteria, procedures and regulations must be developed and applied to reduce the risk of contagion. The first may be a 700-800 ppm CO₂ limit in shared interiors (up to 1000 ppm if there is sufficient filtration).
8. Quality information is the best defense. Clear messages are needed about how the virus is transmitted and how to protect ourselves. It is essential that the population understand the logic of the rules to adopt the ideal behavior in each situation.
Is not difficult. Let’s do it.
* This article was originally published on The Conversation. You can read the version original and see links to scientific studies here.
María Cruz Minguillón She is a tenured scientist at the Institute for Environmental Diagnosis and Water Studies (IDAEA), of the Higher Council for Scientific Research (CSIC) of España.
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