The SARS-COV2 pandemic is striking unequally according to continents, countries, seasons and topography. In France, the Brittany region seemed to be rather safe whereas the epidemic was intense in Alps valleys during the end of autumn 2020 and early winter 2020.
Correlations have been established between meteorological aspects (wind, temperature, fine particles) and contaminations episodes. However, correlation is not causality and therefore political decisions, impacting the daily life of everyone, must be taken after a correct scientific understanding of observations, even under simplified analysis, has been made. This is a key issue to take appropriate and rational action that eventually will be recognised as such and hence accepted by population.
For a long time, national and international authorities (including WHO) have left apart the possible airborne transmission route, also known as "aerosol route" which involve very small infectious particles suspended in the air. This route is now recognized as the major one to spread the virus among human beings.
Physics involved in this route is far beyond the exclusive frame of epidemiology or infectiology. It is a largely multidisciplinary field encompassing as well, physico-chemist experts in atmospheres, aerosols, air quality and contamination.
It is in this context that an investigation developing a simple model allowing one to evaluate quantitatively the risk ratio of contracting COVID-19 via the airborne route indoors and outdoors has recently been published in "Environmental Research" 198, 111189 (2021):
B.R. Rowe, A. Canosa, J.M. Drouffe et J.B.A. Mitchell
Results clearly show that outdoor risk is usually several orders of magnitude smaller than indoor risk. The risk ratio is dependent on several parameters including wind strength, people density per square metre, indoor adopted ventilation norm and also atmospheric dispersion of outdoor viral particles. The latter is very dependent on meteorological conditions.
This work highlights that the present norm in terms of indoor air renewal or cleaning is significantly underestimated. An increase by at least one order of magnitude would be necessary to equal the indoor risk to the outdoor one.
These results shed new lights and explain, at least partially, various observations. In addition, the quantitative risk assessment of the outdoor risk compared to indoors is essential to propose rational decisions as for example concerning mask wearing.