Problemi aperti nella Bio-fluodinamica del contagio da COVID19
Documento della Commissione per l’Ambiente e le grandi calamità naturali
This Report reviews the state of knowledge on the bio-fluid dynamic mechanisms involved in the transmission of the infection from SARS-CoV-2. The relevance of the subject stems from the key role of airborne virus transmission by viral particles released by an infected person via coughing, sneezing, speaking or simply breathing as well as, most likely, by speech droplets generated by asymptomatic carriers.
Proper understanding of the mechanics of the complex processes whereby the two-phase flow emitted by an infected individual disperses into the environment would allow us to infer from first principles the practical rules to be imposed on social distancing, and on the use of facial and eye protections which to date have been adopted on empirical basis. These measures are in need of a broad scientific validation. A deeper understanding of the relevant bio-fluid dynamics would also allow us to evaluate the contrasting effects of natural or forced ventilation of environments on the transmission of contagion: the risk decreases as the viral load is diluted by mixing effects but contagion is potentially allowed to reach larger distances from the infected source.
To that end, this Report suggests that a formal assessment of a number of open problems is needed. The first class of open problems concerns the precise definition of the 'cloud' emitted through the varieties of respiratory emissions as a likely mode of disease transmission. Observations confirm that a sizable probability exists that normal speaking causes airborne virus transmission, especially in confined environments. The modeling of droplet generation mechanisms, that underpin potential contagion through destabilization of the mucus layer that covers the respiratory tract, is still hardly mature for generalized applications. Somewhat surprisingly, it emerges that currently experimental findings do not allow us to single out the proper probability distribution of the size of the droplets that can be associated with the various respiratory emissions. Despite a research effort that has spanned over a century and the use of progressively more refined experimental techniques, still the various studies provide results that can differ broadly, even by orders of magnitude. Recent developments based on ultra-rapid image processing techniques, suggest that, at least in the case of violent emissions (coughing, sneezing), the process of droplet formation and dispersion continues in the first phase of expulsion through the fragmentation of liquid sheets and filamentous structures, whose modeling poses a complex problem of computational fluid dynamics.
The second class of problems concerns our understanding of the transport processes through which the cloud modifies its composition on moving away from the source. These modifications affect the possible infection mechanisms. Larger droplets tend to settle in the immediate vicinity of the infected emissions, while others are advected away from the source and evaporate at rates dependent on temperature and relative humidity of the emitted clouds. Initially warmer and more humid than the external environment, clouds undergo mixing with the turbulent ambient air. Therefore, the evolution of velocity, temperature and relative humidity fields and their turbulent fluctuations ultimately determines the particle size distribution of the droplets. In the distant field, evaporation reduces the surviving droplets to their dry nuclei.
The third class of open problems originates from a simple question: does the infected droplet that undergoes evaporation, possibly shrinking to its dry core, remain infectious? This is tantamount to addressing the general problem of the prediction of the survival rates of viruses in environments characterized by different temperature and humidity conditions. Even in this domain, science does not seem to have reached an assessment to date. Our report draws together several lines of argument about the survival of SARS-CoV-2 in the environment. Given the relevance of this problem, however, it seems surprising that even the fundamental mechanisms that determine the mortality rates of viruses in air (e.g. the role of possible coating, of dissolved salts, and of pH variations to name a few) have not been conclusively assessed to date.
The Report also focuses on fluid dynamic background underlying measures of personal protection from contagion. The analysis of recent visualizations of the respiratory emissions by individuals wearing protective masks highlights limits and validity of the use of personal protection equipment like face masks and eye protections. It also emerges that the one-meter measure of social distancing recommended by the World Health Organization (WHO) is not based on direct scientific evidence. This notwithstanding, the effectiveness of social distancing for risk reduction is undisputable, especially if accompanied by suitable use of protective equipment: in their absence, the benchmark distance of 1 m provided as a standard by the WHO appears insufficient.
The report comes to a close by examining the implications of the bio-fluid dynamic underpinning of virus infections on further development of epidemiological models, strongly supporting the scientific, social and economic importance of strengthening interdisciplinary research on this topic.
5 giugno 2020
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È disponibile anche la versione inglese del documento: Biological fluid dynamics of airborne COVID-19 infection