Short-range exposure to airborne virus transmission and current guidelines

Abstract

After the Spanish flu pandemic, it was apparent that airborne transmission was crucial to spreading virus contagion, and research responded by producing several fundamental works like the experiments of Duguid [J. P. Duguid, J. Hyg. 44, 6 (1946)] and the model of Wells [W. F. Wells, Am. J. Hyg. 20, 611–618 (1934)]. These seminal works have been pillars of past and current guidelines published by health organizations. However, in about one century, understanding of turbulent aerosol transport by jets and plumes has enormously progressed, and it is now time to use this body of developed knowledge. In this work, we use detailed experiments and accurate computationally intensive numerical simulations of droplet-laden turbulent puffs emitted during sneezes in a wide range of environmental conditions. We consider the same emission—number of drops, drop size distribution, and initial velocity—and we change environmental parameters such as temperature and humidity, and we observe strong variation in droplets’ evaporation or condensation in accordance with their local temperature and humidity microenvironment. We assume that 3% of the initial droplet volume is made of nonvolatile matter. Our systematic analysis confirms that droplets’ lifetime is always about one order of magnitude larger compared to previous predictions, in some cases up to 200 times. Finally, we have been able to produce original virus exposure maps, which can be a useful instrument for health scientists and practitioners to calibrate new guidelines to prevent short-range airborne disease transmission.

An upper bound on one-to-one exposure to infectious human respiratory particles

“…The distance traveled and residence time of even >50- m particles in the air have been shown to be much larger than those usually considered ( 9 , 29 , 69 ). Thus, the mean infection risks shown in Fig.…”

“…However, it should be noted that there is an ongoing debate regarding the advection distance of exhaled particles in different respiratory activities and room conditions (e.g., see refs. 9 and 29 , and references therein, for more details). Particles exhaled by the infectious are moist and, depending on the RH, may decrease considerably in size by evaporation until they reach the breathing zone of the susceptible.…”

“…Even the most detailed simulations to date are carried out by assuming the exhale flow behaves similarly to a turbulent jet in a room with quiescent air (e.g., see refs. 9 and 29 , and references therein). Therefore, for situations where the infectious is not wearing a face mask, we use a simplified theoretical formulation recently proposed for particle-laden jet flows ( 26 , 27 ), that is, , where x is the distance between the source and the receptor, a is the radius of the mouth (assuming a circular shape), and α is the exhale jet half-angle.…”

“…They used multiple aerosol spectrometers and in-line holography to cover a particle size range of 50 nm to 1 mm. Here, particles of <50 m are considered for the calculation of infection risk, unless otherwise stated, as we consider this to be a conservative and realistic limit based on recent numerical simulations ( 9 , 29 ). Particles larger than 50 m are unlikely to travel the distances of 1.5 m investigated in this study or to escape from the infectious face mask, as will be shown later.…”

Modelling the direct virus exposure risk associated with respiratory events

“…This framework is so concise, readable and expressive that, since reported, it has been largely used in most public health guidelines [5,8]. The Wells model, however, presents a major weakness [3,5,8,[19][20][21]: the evaporation time of the droplets is estimated using the classical D 2 -law [22] (or constant temperature model), which states that the droplet surface reduces linearly over time at a rate determined by ambient conditions. This evaporation model ignores the presence of a turbulent cloud of moist air, which, as demonstrated in recent studies, plays a crucial role in the fate of respiratory droplets [1,23], as well as the presence of the surrounding droplets.…”