What is the likelihood of contracting COVID-19

Since the beginning of 2020, the world has been shocked by the news of the new COVID-19 virus. Due to its spread, many states had to limit or completely close the message both within the country and between states. Being in an enclosed space for several hours, people transmit this virus to each other by airborne droplets quite quickly. But is it really that terrible? Let’s figure out how high the probability of infection is in different cases, which involve crowds of people.

The risk of infection on an airplane

Contrary to the belief that a closed room with many people is potentially dangerous for high morbidity, the aircraft, on the contrary, is one of the safest “rooms”. It’s all about air filtration, which is essential for passenger aircraft of any category and configuration. And even the seating density of people does not greatly affect, since 99,99% of all particles in the air are filtered in a maximum of 5 minutes. In other words, the air in the cabin is updated every 2-5 minutes with HEPA filters, so the risk of infection is very low.

These conclusions were reached thanks to a specially conducted experiment on board the Boeing 767 and 777 at the end of August 2020. The initiators were TRANSCOM and Air Force to identify the real threat of the spread of the virus on commercial flights. The bottom line was to imitate the behavior of an infected person on board the aircraft, which was played by a mannequin with an aerosol spray of liquid attached to it. Actually, this imitated a person sneezing or coughing, and the contents consisted of fluorescent particles that were easy to track under special lighting. The spraying itself was carried out both in the air and on the floor. At the same time, the mannequins playing the role of passengers were seated as usual, with some wearing a mask, some without.

During the experiment, several scenarios were played with an open door to the cabin, as well as with spraying microparticles only in the cockpit. All tests showed inspiring results, so it can be said with almost no doubt that wherever a HEPA filter unit is installed, including an airplane, the safest places ^[1].

The risk of infection on the bus

Like an airplane, a bus also has an air system. However, in the latter case, the equipment is significantly different, especially when using recirculation.

So in China, after a trip on a bus in which internal air recirculation was turned on, an outbreak of COVID-19 occurred in January 2020. Scientists have revealed infection statistics, unfortunately, after the fact – after a mass disease. People traveled to the event for 50 minutes in both directions. The total number of passengers was 128 people, so they had to be divided into 2 buses: 68 on one and 60 on the other. The first of them was a woman infected with COVID-19, but not knowing about it.

It is easy enough to predict what happened next: after a trip on the bus in which the sick woman was, 23 infected out of 67 people who traveled with her were identified. Moreover, the number of infected people increased by another 7 out of 172 people who participated in the event.

Given all these facts, Chinese scientists calculated the risk of infection and identified factors that could prevent infection of other passengers. The most important thing in these statistics is that the probability of getting infected was higher for those traveling on the bus with the carrier of the disease, and amounted to 34,3%. Moreover, the distance between her and other passengers did not affect in any way, whether they were in the nearest or a few rows from the sick woman. There is another interesting detail – the probability of getting infected on the same bus in people sitting at the window or door was much lower. All this proves that the virus spreads not only directly by droplets, but also through the air, especially in a room without ventilation. Therefore, to maintain health, there must be constant access to fresh air, and in no case recirculation without filtration. ^[2].

risk of infection indoors

Coughing and sneezing are not the only ways the disease can be transmitted through the air. Even during a conversation, microscopic droplets are produced that can carry the virus and thereby contribute to the illness of others. The virus remains in the air for quite a long time (from 8 to 14 minutes) after it leaves the human body. If people are in a closed room with poor ventilation, while communicating with each other, the likelihood of infection increases significantly. This happens very often, for example, in offices or educational institutions.

Scientists from the University of Pennsylvania conducted an experiment with a laser scanner that highlighted swirls with microdroplets when a person spoke, and their distribution in a limited space. With its help, it was possible to trace how potentially virus-carrying droplets can spread within a room and settle on surfaces, such as doorknobs or tables.

This study does not show how many microscopic droplets can contribute to the disease, and also implies a limited space with stagnant air, which does not always correspond to the real conditions of an office with working ventilation. However, it shows the importance of maintaining social distancing, as well as wearing a mask when communicating with other people, as it limits the entry of droplets with the virus into the environment, thereby reducing the risk of infection. These measures are especially important in confined spaces. ^[3].

The risk of infection on the street

In each of the above situations, air appears as a means of transporting the virus. But how are things in the case of open space, and what is the probability of getting infected on the street from a random passerby?

In fact, it’s all about the concentration of potentially dangerous droplets inhaled by one person. The exact amount required for infection is still not known, but one thing is clear: the more such microdroplets are inhaled, the higher the risk. On the street, the concentration of exhaled particles is significantly reduced, especially if the density of people is sufficiently low. That is why the World Health Organization (WHO) recommends social distancing of at least 1 meter. ^[4].

You also need to take into account the factor of frequency and strength of exhalation. For example, when coughing and sneezing, the speed of movement and the concentration of microdroplets are much higher than from simple breathing. If a passer-by breathes calmly, then the probability of picking up a virus from him is quite low.

The human nose is a natural filter that prevents many pathogenic particles from entering the lungs, settling on the mucous membranes and hairs. In addition, since the inhalation of air occurs only from the bottom up, while the particles spread in all directions, the number of microdroplets entering the respiratory system is also reduced.

So, through many medical studies and experiments, facts have been established that help determine the risk of possible infection, and the places where you are most likely to get sick with the virus. The safest are the plane and the street. At the same time, in buses and enclosed spaces, the likelihood of infection is highest, especially under certain conditions, namely in cases of stagnant air, poor ventilation or recirculation.

Nevertheless, as scientists have proven, the main danger is not the places, but the microparticles produced by the infected person with a high spread rate. However, the death of the virus in them occurs 8-14 minutes after the microdroplets leave the patient’s body. And as long as social distancing is observed, each person can reduce the likelihood of infection.

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