Airborne spread of expiratory droplet nuclei between the occupants of indoor environments: A review

Abstract

This article reviews past studies of airborne transmission between occupants in indoor environments, focusing on the spread of expiratory droplet nuclei from mouth/nose to mouth/nose for non-specific diseases. Special attention is paid to summarizing what is known about the influential factors, the inappropriate simplifications of the thermofluid boundary conditions of thermal manikins, the challenges facing the available experimental techniques, and the limitations of available evaluation methods. Secondary issues are highlighted, and some new ways to improve our understanding of airborne transmission indoors are provided. The characteristics of airborne spread of expiratory droplet nuclei between occupants, which are influenced correlatively by both environmental and personal factors, were widely revealed under steady-state conditions. Owing to the different boundary conditions used, some inconsistent findings on specific influential factors have been published. The available instrumentation was too slow to provide accurate concentration profiles for time-dependent evaluations of events with obvious time characteristics, while computational fluid dynamics (CFD) studies were mainly performed in the framework of inherently steady Reynolds-averaged Navier-Stokes modeling. Future research needs in 3 areas are identified: the importance of the direction of indoor airflow patterns, the dynamics of airborne transmission, and the application of CFD simulations.

Airborne transmission between room occupants during short‐term events: Measurement and evaluation

“…It is therefore obvious that the four samples obtained by the INNOVA cannot indicate both the evolution of the exposure index over time and the time‐averaged exposure index during this period of time. The limitation of the slow instrument to measure accurately is more significant at D = 0.35 m than at D = 1.0 m, basically because the flow interaction in the breathing zone is stronger at D = 0.35 m than it is at D = 1.0 m . In general, the comparison made here suggests that fast concentration measurements using the FCM is a more reliable method for investigating dynamic airborne transmission during short‐term events.…”

“…Indirect transmission occurs when the air exhaled by the infected person disperses and mixes with the room air before it reaches the breathing zone and is inhaled by the exposed person. The risk of cross‐infection from direct transmission can be influenced by a number of different parameters, such as air distribution, distance between the persons, positions and orientations of the persons, breathing mode, activity level, and occupant movement, while the risk of cross‐infection from indirect transmission is influenced mainly by the volume of the occupied space and the supply air flow rate . Past studies have examined these parameters and some important findings have been reported.…”

“…The present study focuses only on the second type. Third, in order to evaluate the risk of cross‐infection and formulate cost‐effective intervention measures for a certain type of pathogen, it is important to compare the timescale of accumulating a dose and the survival time . This comparison can be made only on the basis of dynamic measurements.…”

Human behavior during close contact in a graduate student office

“…People naturally move their heads when they speak; this plays a direct role in the perception of conversation . Head posture directly determines exhalation flow direction, which is a key factor influencing the risk of cross‐infection . Body sway during conversation may be understood as a cross‐person coordinative structure that embodies the goals of the joint action system .…”

“…Head and body movement and orientation reflect personality, psychopathology, cultural background, social class, etc . In infection transmission, together with various breathing modes, different head and body postures and their variation lead to large variations in the risk of cross‐infection between two closely located individuals . Studies have investigated personal exposure based on relative position and posture without reference to realistic close contact data.…”