To what extent do indoor and outdoor settings differ for the short-ranged transmission of virus-laden droplets? How come the odds of Covid-19 infection in outdoor settings are weaker than in enclosed spaces?
A large part of the answer is naturally believed to lie with the risks of air-borne transmission via aerosols, which may remain aloft for tens of minutes or hours in poorly ventilated enclosed spaces, while they are swiftly dispersed outdoors. But are there other factors that may result in lower risks of direct viral transmission in the short range?
Apart from the possibility of humidity and temperature effects, we have numerically observed that even modest air flows have a paramount importance on the short-range propagation of (potentially virus-laden) respiratory droplets in generic settings. By simulating the propagation of the respiratory droplets emitted while breathing with computational fluid dynamics and coarse-graining the numerical output to roughly assess the risks of viral transmission between standing adults, we have come to the conclusion that even very modest air flows (which are not even worthy of the denomination of 'light breeze') strongly affect and disperse the cloud of droplets in the short range. Therefore, it is critical to mind these air flows, especially in non-confined settings, if one aims for a quantitative account of the risks of viral transmission.
These findings, which generalise previous reports in specific settings (e.g., in a restaurant, a bus, etc.), are reported in a very succinct preliminary note (Simon MENDEZ, Alexandre NICOLAS). A longer report will be submitted for publication in the next couple of months.