When someone coughs, speaks, or even breathes, they release tiny respiratory droplets into the surrounding air. The smallest of these droplets can float for hours, and there is strong evidence that they can carry live coronavirus if the person is infected.
Until these last days, however, the risk of these aerosols was not incorporated into the formal guidance of the World Health Organization (WHO) for nations. Instead, he suggested that the coronavirus is spread primarily by coughing or sneezing large droplets on someone’s face, rather than being a long-term threat that can float in the air.
It took pressure from scientists to start changing that.
More than 200 scientists published an open letter to the WHO on July 6 warning of the airborne transmission of COVID-19 via aerosols and urging the organization to recognize the risks.
The WHO responded with an update acknowledging the growing evidence of airborne spread of the disease, but did so with misgivings.
As teachers studying fluid and aerosol dynamics, we believe it is important for people to understand the risks and what they can do to protect themselves.
What is an aerosol and how is it spread?
Aerosols are particles that are suspended in the air. When humans breathe, speak, sing, cough, or sneeze, the emitted respiratory droplets mix with the surrounding air and form an aerosol. Because the largest droplets quickly fall to the ground, respiratory aerosols are often described as being made up of smaller droplets less than 5 microns, or about one-tenth the width of a human hair.
In general, droplets are formed when a sheet of liquid breaks. You have probably experienced this phenomenon when blowing soap bubbles. Sometimes the bubble does not form completely, but breaks into many drops.
Similarly, in humans, small sheets and strands of fluid, mucus, often spread through parts of the airways. This occurs most often in places where the airway opens and closes over and over again. This happens deep in the lungs when the bronchioles and alveolar sacs expand and contract during breathing, inside the larynx as the vocal cords vibrate during speech or in the mouth, while the tongue and lips move. while talking. The airflow produced by breathing, speaking and coughing separates these layers of mucus, just like blowing the soap bubble.
The size of the droplets varies depending on how and where they are produced within the airways. While a cough generates the most drops, research has shown that just two to three minutes of conversation can produce as many drops as a cough.
Drops that are smaller than 5 microns can remain suspended in the air for many minutes to hours because the effect of air entrainment in relation to gravity is great. In addition, the water content of the virus-carrying droplets evaporates while they are in the air, decreasing their size. Even if most of the liquid evaporates from a virus-laden droplet, the droplet does not disappear; it just gets smaller, and the smaller the droplet, the longer it will stay suspended in the air. Because the smaller diameter droplets are more efficient at penetrating deep into the pulmonary system, they also pose a much higher risk of infection.
WHO guidelines suggest that virus RNA found in droplets was not viable under most circumstances. However, early research on the SARS-COV-2 virus has shown that it is viable as an aerosol for up to three hours.
Do masks protect against the transmission of aerosols?
Face coverings and masks are absolutely necessary for protection against the transmission of aerosols. They serve a dual purpose.
First, they filter the air expelled by one individual, capture respiratory droplets, and thus reduce the risk of exposure for others. This is particularly important as they are more effective at capturing larger droplets that are more likely to have large amounts of virus encapsulated within them. This prevents larger droplets from directly hitting someone or evaporating to a smaller size and circulating in the air.
They also slow down the puff of air that occurs when sneezing, coughing, or talking. Decreasing the velocity of the expelled air reduces the distance that the droplets are initially transported to the surroundings of the person.
However, it is important to realize that the protection provided by face masks and coverings varies depending on the material they are constructed of and how well they fit. However, wearing face covers to decrease the risk of airborne exposure is critical.
Is staying at a distance safe enough?
The recommendation to maintain a separation of two meters is based on a study by WF Wells in 1934, which showed that a drop of expelled water falls to the ground or evaporates, at a distance of about 2 meters. However, the study did not take into account the fact that after evaporation of the water in a virus-laden droplet, the nucleus of the droplet remains, thus still posing a risk of infection in the air.
Consequently, while staying away from others reduces exposure, it may not be sufficient in all situations, such as in confined and poorly ventilated rooms.
How can I protect myself from indoor sprays?
Strategies for mitigating airborne exposure are similar to strategies for staying dry when it rains. The longer you stay in the rain, and the more it rains, the wetter you will be. Similarly, the more drops you are exposed to and the longer you stay in that environment, the greater the risk of exposure. Therefore, mitigation risk is based on decreasing aerosol concentration levels and exposure time.
Aerosol concentrations can be reduced with increased ventilation, although recirculation of the same air should be avoided unless the air can be effectively filtered before reuse. When possible, open doors and windows to increase the flow of fresh air.
Decreasing the number of emission sources (people) within a space and ensuring that face covers are worn at all times can further lower concentration levels.
Methods to inactivate the virus, such as germicidal ultraviolet light, can also be used.
Finally, reducing the amount of time you spend in crowded, poorly ventilated areas is a good way to reduce the risk of airborne exposure.
This article was originally published on The Conversation. Read the original.
Byron Erath, Associate Professor of Mechanical Engineering, Clarkson University; Andrea Ferro, Professor of Civil & Environmental Engineering, Clarkson University y Goodarz Ahmadi, Professor of Mechanical Engineering, Clarkson University
