Direct or indirect exposure of exhaled contaminants in stratified environments using an integral model of an expiratory jet

Abstract

The risk of cross‐infection is high when the susceptible persons are exposed to the pathogen‐laden droplets or droplet nuclei exhaled by infectors. This study proposes a jet integral model to predict the dispersion of exhaled contaminants, evaluating the exposure risk and determining a threshold distance to identify the direct and indirect exposures in both thermally uniform and stratified environments. The results show that the maximum concentration of contaminants exhaled by a bed‐lying infector clearly decreases in a short distance (<1.8 m) in a uniform environment, while it maintains high values in a long distance in a stratified environment. The lock‐up phenomenon largely weakens the decay of the concentration. The direct exposure of the receiver is determined primarily by the impact scope of the exhaled airflow, while the indirect exposure is mainly related to the ventilation rate and air distribution in the room. In particular, the distance of direct exposure is the longest (approximately 2 m) when the receiver's breathing height is at the lock‐up layer in a stratified environment. Our study could be useful for developing effective prevention measures to control cross‐infection in the initial stage of design of indoor layouts and ventilation systems.

The impact of indoor thermal stratification on the dispersion of human speech droplets

“…However, in some indoor environments, such as buoyancy‐driven ventilated spaces with DV, 31,32 UFAD, 33,34 and/or NV, 35,36 the air temperature is vertically stratified because the low‐momentum air supply is attached to the floor, which is more likely to be seen in public places like shopping centers, railway waiting rooms, and the rebuilt Fangcang hospital. In such environments, the exhalation airflow and airborne pollutants were found to be easily trapped at a certain height and were able to penetrate a very long distance along the exhalation direction, which was referred to as the lock‐up phenomenon, as reported in previous studies 21,27,30,37,38 . Bjørn and Nielsen 17 observed that if the temperature gradient was sufficiently large, the lock‐up layer of the exhaled airborne pollutants could settle at breathing height, leading to a high inhalation risk for exposed people.…”

“…Bjørn and Nielsen 17 observed that if the temperature gradient was sufficiently large, the lock‐up layer of the exhaled airborne pollutants could settle at breathing height, leading to a high inhalation risk for exposed people. Liu et al 30 proposed a theoretical model to predict the exposure risk of susceptible people and found that the thermal stratification greatly weakened the decay of the exhaled pollutant concentration in the lock‐up layer, thereby increasing the cross‐infection range. This ventilation‐induced airborne transmission may be one important reason for the reported cluster outbreak cases in public places 27,39,40 .…”

“…From these studies, it is noted that the dynamic characteristics of respiratory droplets are likely to be a function of the generation process, size distribution, and the environmental conditions. To be specific, the size variation and the dispersion of droplets are significantly affected by respiratory activities, 8,22 thermal plumes, [27][28][29] indoor airflow patterns, [25][26][27]30 etc Among these factors, the indoor airflow patterns, for example, natural ventilation (NV), mixing ventilation (MV), displacement ventilation (DV), downward ventilation (DnV), and under-floor air distribution (UFAD), have been recognized as an important element that distinctively affects the transmission risk via exhaled droplets or droplet nuclei due to different air distribution patterns and supply flow rates. In the experimental work of Qian et al, 27 it was found that in a room with MV, the air temperature and airborne pollutants were generally well mixed, in which case the exposure risk to exhaled pollutants was close to that in exhaust.…”

“…In such environments, the exhalation airflow and airborne pollutants were found to be easily trapped at a certain height and were able to penetrate a very long distance along the exhalation direction, which was referred to as the lock-up phenomenon, as reported in previous studies. 21,27,30,37,38 Bjørn and Nielsen 17 observed that if the temperature gradient was sufficiently large, the lock-up layer of the exhaled airborne pollutants could settle at breathing height, leading to a high inhalation risk for exposed people. Liu et al 30 proposed a theoretical model to predict the exposure risk of susceptible people and found that the thermal stratification greatly weakened the decay of the exhaled pollutant concentration in the lock-up layer, thereby increasing the cross-infection range.…”

Characterization and mitigation of aerosols and splatters from ultrasonic scalers

“…Without a commonly recognized size threshold, splatters are generally considered to be large droplets and/or debris that is heavy enough to settle rapidly without spreading a long distance. Aerosols, on the other hand, are small droplets that can become smaller through evaporation and result in smaller residual aerosols (10 microns in size or smaller) that may stay suspended in air indefinitely since they are sufficiently small to overcome gravitational settling, especially in a thermally stratified indoor environment [32][33][34] . Although most of these residual aerosols are the result of the supplied cooling water, they can include patient saliva, dental plaque, calculus, and blood, which can be infectious to dental practitioners or other dental patients if a patient carries SARS-CoV-2 or other infectious agents.…”