The COVID-DROPLETS project, led by Dr Gaetano Sardina from Chalmers University of Technology, Sweden, in collaboration with Dr Francesco Picano from University of Padua in Italy, is investigating the lifetime of expiratory droplets released by individuals infected with SARS-CoV-2 (the coronavirus strain causing COVID-19). Surprisingly, the current recommendations to hinder the transmission of respiratory infectious diseases are based on a simple model developed 90 years ago.
The widely adopted two-metre social distancing rule assumes that the dominant routes of transmission of SARS-CoV-2 are via large respiratory droplets (>10 µm) falling on others or surfaces. However, this one-size-fits-all rule is not consistent with the underlying science of exhalations and indoor air. Such rules are based on an over-simplistic picture of viral transfer, which assume a clear dichotomy between large droplets and small airborne droplets emitted in isolation without accounting for the exhaled air. The reality probably involves a continuum of droplet sizes and an important role of the exhaled air that carries them.
Research has shown that smaller airborne droplets laden with SARS-CoV-2 may spread up to eight metres concentrated in exhaled air from infected individuals, even without background ventilation or airflow. Whilst there is limited direct evidence that live SARS-CoV-2 is significantly spread via this route, there is no direct evidence that it is not spread this way.
“Our numerical simulations provide a more accurate estimation of what happens when someone sneezes than the classical model … we can see that the lifetime of droplets in a sneeze are much longer compared to isolated droplets in the same environment.”
Consequently, Sardina’s research assumes that the scenario is more complicated than has previously been assumed, and so he and his colleagues are modelling sneezes and coughs as turbulent jets using high-performance computers. Additionally, they are modelling speaking and breathing via homogeneous isotropic turbulence. They will simulate different levels of atmospheric temperature and humidity to work out what weather conditions can enhance or hinder evaporation and transmission of the disease. PRACE awarded the project with 20 000 000 core hours on Joliot-Curie Rome, hosted by GENCI at CEA, France.
Investigating sneezes is somewhat of a departure from Gaetano’s normal line of work. “My research normally involves investigating droplet evaporation in clouds,” he says. “What we are doing here is using the same equations to look at sneezes and coughs. Before the project, I had no experience at all of investigating respiratory droplets, but in the end the physics is very similar. All we had to do was make a few small modifications to our code and then change the parameters.”
With each simulated sneeze, the lifetime of every droplet is calculated from the moment of emission to the point that they have completely evaporated. “Our numerical simulations provide a more accurate estimation of what happens when someone sneezes than the classical model,” says Sardina. “Evaporation time of a specific droplet depends on many parameters such as temperature, relative humidity, and even the presence of other droplets around, and so we can see that the lifetime of droplets in a sneeze are much longer compared to isolated droplets in the same environment.”
Visualization of the instantaneous relative humidity field with droplet positions after 0.04 seconds of a sneeze event. The ejection velocity of the sneeze is 20 m/s while the relative humidity and temperature of the environment are 50% and 5°C. Droplet diameters are proportional to their sizes (not in scale) while their contours correspond to their velocities.
Underlying turbulence in a room may also play a factor in transmission. Large particles of the order of 50-100µm, which usually land quickly on the floor due to gravity, can remain suspended for much longer and increase risk of viral infection if there is a certain level of turbulence in a room. These size droplets can be filtered out by face masks of almost any quality, such as those made of cloth, and the results of the simulations show that stopping these could well be helpful for reducing potential infections.
Visualization of the instantaneous relative humidity field with droplet positions after 0.08 seconds of a sneeze event. The ejection velocity of the sneeze is 20 m/s while the relative humidity and temperature of the environment are 50% and 5°C. Droplet diameters are proportional to their sizes (not in scale) while their contours correspond to their velocities.
The researchers also want to test whether there is a link between air pollution and the spread of the virus. To this end, the scientists will check different levels of PM concentration and calculate the collisions with the pathogenic droplets to assess the hypothesis of a potential connection with the growth of COVID-19 transmission.
The final goal of the project is to provide guidelines for policymakers to slow down the spread of the pandemic. These guidelines will help them to set a more realistic safe distance between individuals, regulate temperature and humidity of internal public environments to accelerate the evaporation of pathogen-bearing droplets, and order targeted lockdowns when particular weather or high-pollution events occur.
However, turning the results of this work into concrete recommendations for policymakers to follow is not straightforward. There is an ongoing debate in the epidemiological field about which of two types of transmission is the most significant. The first of these is via direct host-to-host transmission, which relies on large droplets from coughs or sneezes entering the body of another. The second type of transmission, however, involves smaller droplets that haven’t evaporated completely that remain suspended in air and can travel much further distances.
Sardina and his colleagues are not directly contributing to this debate through their research, but viewing the results of their work through the lenses of the two different types of transmission gives two different pictures and can lead to two different sets of recommendations. “For example, in our simulations we see that host-to-host transmission via large droplets is increased at higher humidity levels, with droplet lifetimes sometimes being 200 times longer. However, we also see that transmission via the smaller airborne droplets is increased in very dry environments. As you can imagine, these results make it difficult for us to provide guidelines for environmental conditions unless we know the dominant form of transmission.”
Sardina believes that the most important in the coming months is that people from different fields of research strike up strong dialogues with each other. “With the pandemic, we are not going to be able to solve problems if we remain within our areas of knowledge and don’t communicate with each other,” he says. “For instance, there is a lot of contradictory data about initial droplet distribution in a sneeze. This provides the starting point for our simulations and plays a large part in what happens in them, and so we are working closely with people working on that problem so that we can answer the larger questions about how to proceed as a society together.”
This article was also published in PRACE Digest 2020.
20 million core hours on Joliot-Curie Rome hosted by GENCI at CEA in France