In April, a team of scientists released an alarming video showing how spittle flies through the air when we speak. It suggested that the simple act of speaking could spread the novel coronavirus — if you happen to be within someone's splash zone.
That same team of scientists has returned to the study of spit to answer two big questions about how people get coronavirus. How long does airborne spittle say suspended, and can these tiny droplets really infect someone with coronavirus?
Led by Philip Anfinrud, a senior investigator at the National Institutes of Health, the team estimates that one minute of loud talking can generate over 1,000 potentially coronavirus-containing droplets. Those droplets remain airborne for longer than 8 minutes, and in some cases, as long as 14 minutes.
This study was published Wednesday in Proceedings of the National Academy of Sciences.
They conclude that there is a "substantial probability" that "normal speaking" could transmit the coronavirus, especially in certain enclosed environments where airflow is minimal.
That said, this study isn't definitive proof that a conversation with a loud talker is a one-way ticket to coronavirus infection. While small droplets have the power to linger the air, smaller droplets are also less likely to contain SARS-CoV-2, the virus that causes Covid-19.
The aerodynamics of spit – To uncover how long coronavirus droplets linger, the team focused on droplets produced during speech that are around 10 micrometers to 100 micrometers in size. The team calls these "medium-sized droplets" (a piece of paper is about 100 micrometers thick).
How long those droplets linger depends on how big they are to start with. For a respiratory droplet to stay airborne for a long time, it has to begin life as a small droplet, around 10 to 20 micrometers in size.
Those small speech generated droplets then follow a trajectory that allows them to linger in the air.
First, the droplet is sprayed from your mouth. Then it begins to dehydrate, shrinking to about 20 or 34 percent of its original size. This, as the authors note, makes it fall to the ground more slowly than a larger particle would.
That process allows the droplets to linger airborne for between 8 to 14 minutes, depending on their diameter, the study estimates.
This study was done in a closed, stagnant air environment. However, the research team speculates that in a real-life environment — where there is plenty of air movement — the droplets might the air even longer, stretching and potentially covering large spaces.
Anecdotal evidence has shown that the virus can spread through the air in real-life environments. A CDC report released this week investigated a choir practice where 87 percent of the group became infected. The agency found that "the act of singing" could have helped help spread the virus among the group during their rehearsals.
That said, outdoor spaces don't seem to fit this bill. When we're outside, speech droplets rapidly disappear into the open space around us, a spokesperson for the National Institute of Diabetes and Digestive and Kidney Diseases tells Inverse.
As long as we are socially distanced, the probability of becoming infected drops.
Can you actually get coronavirus from these small droplets? – Now that we know that droplets can remain suspended, the question is whether they can cause a coronavirus infection.
The team suggests that the probability that each one of these speech-generated droplets contains coronavirus is low, though the exact answer still depends on how much virus is actually present in the speaker.
The estimate used in this study comes from a case study in China, which found that during the first week of infection a coronavirus patient contained 711,000,000 copies of coronavirus RNA. That informed this study's guess at how much viral load someone has at the peak of the illness.
At the end of the day, the probability of just one virion — a particle that acts as the package that houses viral RNA — ending up on a 50-micrometer coronavirus droplet is 37 percent. If that droplet is 10 micrometers, the chance it houses coronavirus plummets to 0.37 percent.
Finally, a droplet that began at about three micrometers in size (but shrinks to one micrometer) can linger in the air "indefinitely" but it only has a 0.1 percent chance of having the coronavirus on it.
Importantly the probability that these tiny droplets contain more than one coronavirus virion is "negligible," the authors write. But when it comes to other diseases, in theory, one virion is actually enough to trigger an infection.
This idea is called the independent action hypothesis, which means that every single individual particle of virus has an equal, non-zero chance of causing an infection.
When it comes to coronavirus, we don't know for sure if the independent action hypothesis holds up. But scientists suggest that this is likely the case.
Warner Greene, an immunologist and director of the Gladstone Center for HIV Cure Research, previously told Inverse that the evidence pointed towards "any amount" of the virus being potentially infectious, especially if it comes into contact with a mucosal surface.
Even then, there is no guarantee that each virion that enters the body will cause an infection.
Taken together, if you happen to be walking into a room with stagnant air, and are surrounded by some especially loud talkers with cases of coronavirus, that could be bad news.
Abstract: Speech droplets generated by asymptomatic carriers of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) are increasingly considered to be a likely mode of disease transmission. Highly sensitive laser light scattering observations have revealed that loud speech can emit thousands of oral fluid droplets per second. In a closed, stagnant air environment, they disappear from the window of view with time constants in the range of 8 to 14 min, which corresponds to droplet nuclei of ca. 4 μm diameter, or 12- to 21-μm droplets prior to dehydration. These observations confirm that there is a substantial probability that normal speaking causes airborne virus transmission in confined environments.