Airborne spread of infectious agents in the indoor environment

Abstract

Background: Since the 2003 severe acute respiratory syndrome epidemic, scientific exploration of infection control is no longer restricted to microbiologists or medical scientists. Many studies have reported on the release, transport, and exposure of expiratory droplets because of respiratory activities. This review focuses on the airborne spread of infectious agents from mucus to mucus in the indoor environment and their spread as governed by airflows in the respiratory system, around people, and in buildings at different transport stages.

Methods: We critically review the literature on the release of respiratory droplets, their transport and dispersion in the indoor environment, and the ultimate exposure of a susceptible host, as influenced by airflows.

Results: These droplets or droplet nuclei are transported by expired airflows, which are sometimes affected by the human body plume and use of a face mask, as well as room airflow. Room airflow is affected by human activities such as walking and door opening, and some droplets are eventually captured by a susceptible individual because of his or her inspired flows; such exposure can eventually lead to long-range spread of airborne pathogens. Direct exposure to the expired fine droplets or droplet nuclei results in short-range airborne transmission. Deposition of droplets and direct personal exposure to expired large droplets can lead to the fomite route and the droplet-borne route, respectively.

Conclusions: We have shown the opportunities for infection control at different stages of the spread. We propose that the short-range airborne route may be important in close contact, and its control may be achieved by face masks for the source patients and use of personalized ventilation. Our discussion of the effect of thermal stratification and expiratory delivery of droplets leads to the suggestion that displacement ventilation may not be applicable to hospital rooms where respiratory infection is a concern.

Keywords: Respiratory droplet; coughing; droplet dispersion; environmental ventilation; indoor air; infection control.

Fig 1

Schematic diagram revealing the origin and generation mechanism of respiratory droplets. (A) Instability of the airway lining fluid, (B) oral cavity model (drawn based on the atomization mechanism described in Morawska⁵), and (C) film rupture.

Fig 2

Escaped microbial aerosols of an infected individual with a mask (A) and without a mask (B) as affected by the body plume and inhalation of the airborne infectious agent(s) of a nearby individual without a mask (C) and with a mask (D).

Fig 3

Droplet transport in an isolation room by expired airflow, thermal plume, door vortices (adapted with permission from Elsever⁵⁷), human walking, 2-way buoyancy airflow, and ventilation airflow.

Fig 4

Illustration of different transmission routes. Small droplets (<5 µm), sometimes called aerosols, are responsible for the short-range airborne route, long-range airborne route, and indirect contact route; large droplets are responsible for the direct spray route and indirect contact route.