Each orange dot represents a dose of respiratory particles capable
of infecting someone if inhaled by breathing, speaking, and shouting
In the worst-case scenario (lower right corner) – shouting or singing in a closed space for an hour – a person with Covid-19 releases.
Risk of coronavirus infection changes depending on the number of contagious particles you breathe in. El Pais illustrated the differences when you take certain measures, namely wearing masks, ventilation, and decreased exposure time. The suggestions are based on statistical models, so there is more uncertainty than I think the explanations provide, but the sequence of illustrations provides a clear picture of what we can do — if you must do things indoors.
In the spring, health authorities failed to focus on aerosol transmission, but recent scientific publications have forced the World Health Organization (WHO) and the CDC to acknowledge it. An article in the prestigious Science magazine found that there is “overwhelming evidence” that airborne transmission is a “major transmission route” for the coronavirus, and the CDC now notes that, “under certain conditions, they seem to have infected others who were more than six feet [two meters] away. These transmissions occurred within enclosed spaces that had inadequate ventilation. Sometimes the infected person was breathing heavily, for example, while singing or exercising.”
At present, health authorities recognize three vehicles of coronavirus transmission: the small droplets from speaking or coughing, which can end up in the eyes, mouth, or nose of people standing nearby; contaminated surfaces (fomites), although the US Centers for Disease Control and Prevention (CDC) indicates that this is the least likely way to catch the virus, a conclusion backed by the European Center for Disease Control and Prevention’s (ECDC) observation that not a single case of fomite-caused Covid-19 has been observed; then finally, there is transmission by aerosols – the inhalation of invisible infectious particles exhaled by an infected person that, once leaving the mouth, behave in a similar way to smoke. Without ventilation, aerosols remain suspended in the air and become increasingly dense as time passes.
At the beginning of the pandemic, it was believed that the large droplets we expel when we cough or sneeze was the main vehicle of transmission. But we now know that shouting and singing in indoor, poorly ventilated spaces over a prolonged period of time also increases the risk of contagion. This is because speaking in a loud voice releases 50 times more virus-laden particles than when we don’t speak at all. These aerosols, if not diffused through ventilation, become increasingly concentrated, which increases the risk of infection. Scientists have shown that these particles – which we also release into the atmosphere when simply breathing and which can escape from improperly worn face masks – can infect people who spend more than a few minutes within a five-meter radius of an infected person, depending on the length of time and the nature of the interaction. In the following example, we outlined what conditions increase the risk of contagion in this situation.If the buildings are properly ventilated, with good air conditioning, there is less risk. University of Colorado Boulder atmospheric chemist Jose-Luis Jimenez has released an airborne transmission pilot tool that may help us answer some of these questions, or at least provide some informed guidance.
