The risk of infection is highest in indoor spaces, but it can be reduced by applying all known measures to combat aerosol-based transmission. Here is an overview of the likelihood of infection in three everyday scenarios, based on the safety measures in place and the duration of exposure.
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Coronavirus spreads through the air, especially in indoor spaces. Although it is not as contagious as measles, scientists now openly acknowledge the role that aerosol transmission plays — small infectious particles exhaled by an infected person that remain airborne indoors. How does transmission work? And, more importantly, how can we stop it?
| Aerosols These are respiratory droplets smaller than 100 micrometers in size, which can remain airborne for hours. |
Droplets These are particles larger than 300 micrometers that fall to the ground within seconds due to air currents. |
Each droplet releases 1,200 aerosols.
Currently, health authorities recognize three modes of coronavirus transmission :
- Small droplets from talking or coughing that can enter the eyes, mouth, or nose of people nearby;
- Contaminated surfaces, although the U.S. Centers for Disease Control and Prevention (CDC) indicate that this is the least likely way to contract the virus—a conclusion supported by observations from the European Centre for Disease Prevention and Control (ECDC), which notes that multiple cases of COVID-19 caused by contaminated surfaces have been observed.;
- The last way is through aerosol transmission — inhaling invisible infectious particles exhaled by an infected person, which, once they leave the mouth, behave similarly to smoke. Without ventilation, aerosols remain suspended in the air and over time become increasingly concentrated.
Breathing, talking, and shouting
At the beginning of the pandemic, it was believed that the large droplets expelled when coughing or sneezing were the primary means of virus transmission. We now know that shouting and singing indoors, poorly ventilated spaces, and spending extended periods in enclosed areas also increase the risk of infection. This is because speaking loudly releases 50 times more virus-laden particles than not speaking at all. These aerosols, if not removed by ventilation, become increasingly concentrated, raising the risk of infection. Scientists have proven that these infectious particles released into the air can infect people who spend more than a few minutes within a five-meter radius of an infected person. Depending on the duration and nature of the interaction, the likelihood of infection increases. In the following example, we outlined conditions that increase the risk of infection in such a situation.
Each orange dot represents a dose of respiratory particles capable of infecting someone upon inhalation.
In the worst-case scenario—shouting or singing indoors for one hour—a person with Covid-19 exhales 1,500 infectious doses.
In the spring, health authorities did not focus on studying virus transmission through aerosols, but recent scientific publications have forced the World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC) to acknowledge it. An article in the prestigious journal Science found that there is "strong evidence" that airborne transmission is a "major driver" of coronavirus infection. The CDC now notes that "under certain conditions, one infected person has transmitted the virus to people more than two meters away. These transmissions occurred in enclosed spaces with inadequate ventilation. Sometimes the infected person was breathing heavily.
Bar or restaurant
Coronavirus outbreaks during events, as well as those in venues like bars and restaurants, account for a large number of infections. Moreover, they tend to be the most explosive: each outbreak in a nightclub infects an average of 27 people, compared to only six during family gatherings—as explained in the first chart. One such super-spreader event occurred in a club in the Spanish city of Córdoba in southern Spain, where 73 people tested positive after one night. Scientists have also recently analyzed an outbreak in a bar in Vietnam, where the results were similar.
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School
Schools represent only 6% of the coronavirus outbreaks recorded by the Spanish health authorities. The dynamics of aerosol transmission in the classroom change completely depending on whether the infected person — or patient zero — is a student or a teacher. Teachers speak much more than students and tend to raise their voices to be heard, which multiplies the release of potentially infectious particles. In comparison, an infected student will only speak occasionally. According to guidelines from the Spanish National Research Council (CSIC), the Spanish government recommends ventilating classrooms — even though this may cause discomfort during the colder months — or using ventilation systems to air out the classrooms.
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To calculate the probability of transmission between people in “high-risk” situations, we used the airbone droplet transmission estimator for Covid, developed by a team of scientists led by Professor José Luis Jiménez from the University of Colorado. This tool is intended to highlight the importance of measures that prevent aerosol transmission. The calculation is not exhaustive, nor does it cover all the countless variables that may influence transmission, but it serves to illustrate how the risk of infection can be reduced by changing the conditions we can control.
During the simulations, the subjects maintained the recommended safe distance, eliminating the risk of droplet transmission. However, they could still become infected if all possible preventive measures are not applied simultaneously: proper ventilation, reducing time spent indoors, limiting the number of participants, and wearing protective face masks. The ideal scenario—regardless of the context—would be outdoors, where infectious particles quickly disperse. If a safe distance from an infected person is not maintained, the likelihood of transmission increases significantly, as there is a risk of both droplet and aerosol infection. To make matters worse, even if ventilation is present, it may not be enough to disperse aerosols effectively if two people are in close proximity to each other.
The calculations shown in the three different scenarios are based on studies of how aerosol transmission occurs, using real outbreaks that have been analyzed in detail. A particularly illustrative case for understanding the dynamics of indoor transmission is the choir rehearsal in Washington State, USA, in March 2020. Only 61 of the choir’s 120 members attended the rehearsal, and efforts were made to maintain safe distancing and hygiene measures. Unfortunately, they were in a scenario of maximum risk: no masks, no ventilation, singing, and sharing a space for an extended period of time. Just one infected person transmitted the virus to 53 others within two and a half hours. Some of those infected were as far as 14 meters (46 feet) away, meaning only aerosols could explain the transmission. Two of the people who contracted the virus later died.
One infected person, seated in the front rows of a hall, ends up infecting all the others.
After carefully studying this outbreak, scientists were able to estimate the extent to which the risk could have been reduced if airborne transmission measures had been taken. If masks had been worn, the risk would have been halved, and only about 44% of attendees would have been affected, compared to 87% if no masks were worn. If the rehearsal had been shorter and the room had been equipped with and had the ventilation system turned on, only two singers would have been infected.
These increasingly common scenarios prove to be critical for the development and spread of the pandemic, which means that having measures to prevent mass transmission at such events is crucial for controlling the pandemic: shorter duration, protective masks, and mandatory ventilation.
Methodology: We calculated the risk of Covid-19 infection using a tool developed by José Luis Jiménez, an atmospheric chemist at the University of Colorado and an expert in the chemistry and dynamics of airborne particles. Scientists worldwide have reviewed this estimator, which is based on published methods and data, to assess the importance of various measurable factors involved in an infection scenario. However, the accuracy of the estimator is limited because it relies on figures that are still uncertain—numbers that describe, for example, how many infectious viruses are emitted by an infected person. The estimator assumes that people follow the two-meter social distancing rule and that no one is vaccinated. Our calculation is based on a default value for the general population, which includes a wide range of masks (surgical and cloth) and loud speaking, which increases the amount of aerosols emitted.
The original article from EL PAIS: link
Authors of the article: Mariano Zafra & Javier Salas
Video by Luis Almodóvar.
English version by Heather Galloway.
For any questions regarding this article, please contact its co-author Javier Salas at javier@esmateria.com
Bulgarian version by Ivan Armianov, Manager of the Sales Department at TANGRA – AV Ltd.
