Modeling aerosol flows in hospital rooms

Simulation of aerosol's propagation in an hospital room.

The Covid-19 pandemic has put unprecedented pressure on hospitals in France and around the world. At the beginning of the first containment, in March 2020, a few dozen students from Ecole Polytechnique volunteered in the hospitals of the Assistance publique-Hôpitaux de Paris (APHP). This is how Maxime Lancelot and Cyril Crawford, then in their 4th year at l’X, joined the intensive care unit at the Bichat-Claude Bernard hospital. They had no idea that this would lead them to publish in the journal Scientific Report a year later. "Professors Jean-François Timsit and Lila Bouadma suggested that we use our scientific skills on medical research issues. "The risk of airborne contamination by pathogens has always been a concern, highlighted by the pandemic," adds Jean-François Timsit. We need to oxygenate patients while minimising the risk of infection to other patients and personnel. "

How can we limit the risk ? Protections already exist such as visors, gloves, masks. As for the rooms, they are equipped with two air vents that create an air flow, sometimes supplemented by a mobile ventilation system. In addition, the pressure inside these chambers is kept lower than outside to prevent pathogens from escaping when the door is opened. Nevertheless, depending on how they are oxygenated, patients still emit potentially pathogenic aerosols when they breathe, cough or sneeze. "Droplets larger than 5 micrometres fall quickly near the patient and are already well studied. It is the smaller droplets that interested us, those that remain in the air and can spread several metres," explains Cyril.

To better understand this complex phenomenon, the students sought experimental support. They were directed to the Hydrodynamics Laboratory (LadHyX*) where Baptiste Decorde was working. In his third year at l’X, his internship in the United States had just been cancelled due to the pandemic. Christophe Josserand and Camille Duprat, researchers at LadHyX, therefore suggested that he set up experiments to understand respiratory flows, in connection with Covid-19. "As aerosol droplets are difficult to observe, we decided to visualise the flows of exhaled air carrying them using a schlieren optical method," explains Baptiste. With this technique, contrasting the light that has passed through the air emitted by the mouth with the light that has passed through the ambient air allows to visualise the turbulence caused by breathing, coughing or sneezing. The velocity field and flow rate can be derived. The three students carried out the experiments on themselves with hospital equipment using two methods of oxygenation, either by nasal canulae or by CPAP breathing mask.

A schlieren optical method helps visualise the air flow.

The results of these experiments were compared with numerical simulations carried out in parallel in collaboration with Dassault Systèmes. The simulation reproduces the cough of a human being emitting an aerosol-laden fluid whose trajectory is calculated using the Boltzmann lattice method. "There are of course limits to our experiments, but they correspond to the simulations, in particular the different phases of expansion of the jet containing the aerosols, both in terms of time and morphology," emphasises Cyril.

These simulations made it possible to test different room configurations.  It is optimal when the bed is aligned with the air vents. Moreover, the placement of the mobile ventilation system is significant: if it is located under the extraction vent, the flow it emits hinders the evacuation of the air. If it is well placed, it improves the performance of the system. In the future, these simulations could help in the design of hospital rooms, a subject which Dassault Systèmes and APHP continue to investigate. Work is also continuing at LadHyX, where a thesis has begun to better characterise the flow of breathing droplets when wearing a protective mask.

*LadHyX: a joint research unit CNRS, École Polytechnique - Institut Polytechnique de Paris