Microclimate and turbulent wind can influence COVID-19 spread over long distances

By Dr. Tomislav Meštrović, MD, Ph.D.Apr 26 2020

Respiratory particles exhaled after a sneeze can be transported by the turbulent wind more than three times further than current social or physical distancing measures recommend, reports a new study from Chile available on the preprint server medRxiv.

Study: COVID-19. Transport of respiratory droplets in a microclimatologic urban scenario. Image Credit: Elizaveta Galitckaia / Shutterstock

The global coronavirus disease (COVID-19) pandemic is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). With no vaccine or treatments available, interventions have concentrated on quarantine, contact tracing, and social distancing.

Albeit recently, a myriad of studies aimed to address the spread of respiratory droplets through the air, these were mostly tackling indoor or outdoor situations – without taking into account representative real-life scenarios.

Consequently, inadequate attention has been given to the spread of respiratory droplets in outdoor conditions under microclimatologic turbulent wind, which is of considerable importance given the current COVID-19 pandemic.

This is why Chilean researchers from the MSET Chile SpA, Mechanical Engineering Department of the Faculty of Engineering, University of Concepción, as well as the Interdisciplinary Center for the Aquaculture Research (INCAR) in Concepción decided to conduct a realistic simulation delineated in their new paper available at medRxiv.

The lack of sturdy evidence

Dr. David L. Heymann, a professor of infectious disease epidemiology from the London School of Hygiene & Tropical Medicine (and one of the prominent figures during the SARS outbreak), cautioned in his recent TED Talk that the propagation of COVID-19 in the open was one of the important unknowns yet to be realized.

The World Health Organization (WHO) recently communicated that there is no strong evidence for adopting measures against the aero-transported contagion for COVID-19. Still, it emphasized the need for taking one-meter distance from coughing or sneezing individuals.

However, this was frequently misinterpreted by the media; the lack of documented aero-transported transmission actually does not mean that the disease cannot be transmitted by air in a microclimatologic urban scenario.

Still, the absence of consensus on this topic is a burning issue in the scientific community. Although viral RNA material has been detected much further than previously thought, there has been a severe lack of necessary research to estimate infectious potential with sufficient precision.

A realistic simulation of the outdoor environment

As a research team specialized in the modeling of fluid flow, the authors performed their predictive research by simulating the dispersion of multi-dimensional polydisperse droplets exhaled during a sneeze in a microclimatologic urban setting.

Their model was based on Computational Fluid Dynamics (CFD), and the microclimatologic scenario utilized a medium intensity wind simulated by employing a wall-modeled Large Eddy Simulation (i.e., a well-described technique for simulating turbulent flows).

Finally, the dispersion of droplets and their subsequent interaction with the velocity field was described using a Lagrangian approach (also known as a particle-based approach). All these methodological steps were pivotal in achieving realistic outdoor conditions.

Long-distance spread

"Our results indicate that the effect of microclimate is very relevant over the propagation of droplets, where dispersion is enhanced by the turbulent wind," explains study authors.

Wind of moderate-intensity can transport respiratory droplets of bigger size (between 400 and 900 micrometers) up to a distance of five meters in an urban setting – and that happens within a couple of seconds.

Even more importantly, smaller droplets (between 100 and 200 micrometers) can be transported up to 11 meters within 14 seconds, which is several times over consistently cited safety recommendations.

"Although these allow one to get an idea of the dynamics, they are different from a realistic outdoor condition, where human circulation tends to be more unconcerned or unaware concerning its adverse effects, and might cause a faulty sense of safety," emphasize study authors.

Towards stricter distancing measures?

Based on the simulated realistic scenario, these results show that the particles exhaled after a sneeze can be transported by the turbulent wind more than three times further than recommended safety distances suggested during the actual COVID-19 pandemic.

"Given the uncertainty of potential contagion over this way and with this reach, these efforts are an intent to contribute to shining a light on the possibility of adopting stricter self-care and distancing measures," caution the study authors.

Nonetheless, to determine the real contagion risk at the distances mentioned above (and especially considering the diameter of those particles), further studies have to be conducted. The final conclusion needs to wait for other approaches, even in vivo tests on volunteers.

Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.