Scientists Suggest Adding Efforts to Prevent Covid Airborne Transmission

Although already well established, the route of transmission of the coronavirus through the air still raises many questions, such as what is the necessary amount of inhaled air to become infected or the safe distance between people in closed spaces.

In August 2020, the American National Academy of Sciences, Engineering and Medicine tried to answer some of these questions, which are summarized in an article published on the last 9th in the scientific journal PNAS (Proceedings of the National Academy of Sciences).

The text is by Jonathan Samet, epidemiologist and dean of the Colorado School of Public Health, Linsey Marr, an aerosol scientist and engineer at Virginia Tech University, and Thomas Burke, professor of health risk and society at Johns Hopkins University, among others.

How big are the particles emitted by people when talking and how do they dissipate into the air? Scientific evidence existing before the pandemic —especially for the flu virus— already recognized the participation of saliva droplets as transmission routes for respiratory infectious diseases, given that larger droplets, due to their greater weight, would fall quickly to the ground while the of lower density would remain in the air longer.

At the beginning of the pandemic, the WHO (World Health Organization) recognized the transmission of Covid by droplets, but focused mainly on direct person-to-person or indirect contagion by so-called fomites — droplets present on surfaces that, when touched, cause infection.

Experts in the so-called science of aerosols warned, even in the first months of 2020, that particles present in the air smaller than droplets of saliva would play a greater role in the transmission of the virus than previously thought.

Subsequent studies tried to answer some crucial questions such as how long these aerosols remain in the air, how effective different types of masks are in filtering these particles, and how fans or air conditioners can act on their dispersion.

How big are these potentially infectious particles and how long can they infect others? According to the researchers, the delay in recognizing aerosols as the main transmission route for Sars-CoV-2 has undermined the prevention measures adopted by health authorities and governments.

According to Virginia Tech’s definition of Marr, droplets are particles over 100 µm in diameter, while aerosols are smaller than that size. As a result, the aerodynamics of the droplets is affected by their size, with rapid dispersion and decay up to two meters away from the emission focus, which can be when talking, sneezing or singing, for example.

Aerosols are produced in different ways, including exhaling air, and can remain in suspension for minutes to hours, traveling and accumulating in closed spaces for a long time.

How much virus is carried in each of these particles, however, is difficult to define and is associated with the type of activity: singing and talking loudly emits more potentially infected particles, while speaking softly will produce fewer of these droplets.

For Samet, research on how much virus is carried in the air has advanced a lot, but it is still difficult to know its behavior in the real world. The masks, then, serve in two different moments: first as a physical barrier that holds the larger particles both from the mask wearer and from the people around them and, secondly, reducing the amount of inhaled aerosols.

What are the individual behaviors and external factors that determine the greatest risk of exposure to Sars-CoV-2? For specialists, in addition to care with ventilation and air filtration, Covid’s risk of contagion may vary according to individual behavior. For this reason, it is difficult to measure exactly what the potential risk of exposure to an environment is.

According to Samet, the individual risk is associated, first, with the type of space. “Thinking of the primary principle of airborne transmission, the more people [fonte primária] are indoors the greater the risk. The level of ventilation in spaces and masks are some of the elements that can reduce this cycle,” he said.

Maintaining physical distance is also critical, but in a closed room with no air circulation, even if people are more than two meters away, the risk is potentially greater, especially if the masks are removed. “We also learned a lot about the importance of air filters and CO2 level control in closed spaces,” he says.

Thus, scientists advise joining efforts: increase air circulation from the outside to the inside; proper air filtration; limiting the number of people in closed spaces and increasing the distance between them reduces the chance of an infected person infecting others and the intensity of exposure.

What do we know about the amount of infectious particles and their relationship to Covid? According to Samet, this is a question for which we still don’t have the answer. What science does know, it says, is that an infected person can produce millions of aerosol particles that are infectious, but how much of those inhaled particles are needed to cause contagion is unknown.

The viral load required for an infection can vary enormously between individuals due to a variety of factors, including that person’s environment. Genetic factors can also play a role, such as people who are apparently immune to Covid — and who are still a mystery to science.

The type of virus variant also, since it has been shown that some strains of the coronavirus, such as delta, have a viral load up to 1,260 times higher than the original strain.

Finally, experts conclude that adding layers of protection are the most effective ways to decrease transmission – even in a comfortable epidemiological time, such as we are living in today – and that these same protections must be tailored to each social or population context.