Annex CRespiratory droplets

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According to Wells (1955), the vehicle for airborne respiratory disease transmission is the droplet nuclei, which are the dried-out residual of droplets possibly containing infectious pathogens.

C.1. Droplet generation and sizes

The term “droplet”, as used in this context, consists mostly of water with various inclusions, depending on how it is generated.

Naturally produced droplets from humans (e.g. droplets produced by breathing, talking, sneezing, coughing) include various cells types (e.g. epithelial cells and cells of the immune system), physiological electrolytes contained in mucous and saliva (e.g. Na+, K+, Cl-), as well as, potentially, various infectious agents (e.g. bacteria, fungi and viruses).

With artificially generated droplets in a health-care setting (e.g. suction of respiratory tract), the main constituent will also be sterile water, with various electrolytes (e.g. “normal” or physiological saline, including Na+, Cl-) and often the molecules of a drug (e.g. salbutamol for asthmatics).

Both these naturally and artificially generated droplets are likely to vary in both size and content. Droplets >5 μm tend to remain trapped in the upper respiratory tract (oropharynx — nose and throat areas), whereas droplets ≤5 μm have the potential to be inhaled into the lower respiratory tract (the bronchi and alveoli in the lungs).

Currently, the term droplet is often taken to refer to droplets >5 μm in diameter that fall rapidly to the ground under gravity, and therefore are transmitted only over a limited distance (e.g. ≤1 m). In contrast, the term droplet nuclei refers to droplets ≤5 μm in diameter that can remain suspended in air for significant periods of time, allowing them to be transmitted over distances >1 m (Stetzenbach, Buttner & Cruz, 2004; Wong & Leung, 2004). Other studies suggest slightly different definitions, with ranges for “large” droplets, “small” droplets and droplet nuclei being >60 μm in diameter, ≤60 μm in diameter and <10 μm in diameter, respectively (Tang et al., 2006; Xie et al., 2007). The concept is that the naturally and artificially produced aerosols will contain a range of droplet sizes, whose motion will depend significantly on various environmental factors, such as gravity, the direction and strength of local airflows, temperature and relative humidity (which will affect both the size and mass of the droplet due to evaporation).

There have been several studies on the number and size of droplets of saliva and other secretions from respiratory activities (Jennison, 1942; Duguid, 1945; Hamburger & Roberston, 1946; Loudon & Roberts, 1967; Papineni & Rosenthal, 1997; Fennelly et al., 2004) and excellent reviews have been written (Nicas, Nazaroff & Hubbard, 2005; Morawska, 2006). These studies and reviews note that the size of droplet nuclei due to sneezing, coughing and talking is likely to be a function of the generation process and the environmental conditions. The actual size distribution of droplets also depends on parameters, such as the exhaled air velocity, the viscosity of the fluid and the flow path (i.e. through the nose, the mouth or both) (Barker, Stevens & Bloomfield, 2001). There is also a great individual variability (Papineni & Rosenthal, 1997; Fennelly et al., 2004).

Humans can produce respiratory aerosols (droplets) by several means, including breathing, talking, coughing (Figure C.1, A), sneezing (Figure C.1, B) and even singing (Wong, 2003; Toth et al., 2004).

Figure C.1. (A) Schlieren image (visualization using light refraction caused by differences in air density) of a human cough, and (B) flash photo of a human sneeze.

Figure C.1

(A) Schlieren image (visualization using light refraction caused by differences in air density) of a human cough, and (B) flash photo of a human sneeze. Source: Photographs reproduced with the kind permissions of (A) Prof Gary S Settles, Department of (more...)

There is a natural physiological variation in the volume and composition of such aerosols generated between individuals and even within the same individual during any of these activities. An infection is likely to increase this variability, which itself may vary as the host immune system starts responding to the infection over time. For example, a patient with chickenpox will have no specific antibodies to the virus at the beginning of the infection, making the viral load much higher and thus potentially more transmissible during the acute, febrile, coughing, prodromal phase of the infection than later, when the specific antibody response starts to develop.

Relatively few studies have characterized the number, size and content of droplets generated by either natural or artificial means. Also, because of individual variation, studies on naturally generated droplets may be of limited use, and will not necessarily be relevant to so-called “super-spreaders” — infected individuals who manage to infect many others, generating many more secondary cases than is expected on average. This may be due to a number of reasons, including a poor host immune response to controlling the infection, concomitant diseases or other respiratory infections that increase the degree of shedding of the infectious agent, and environmental factors favourable to the survival of such agents (Bassetti, Bischoff & Sherertz, 2005).

Published data have suggested that sneezing may produce as many as 40 000 droplets between 0.5–12 μm in diameter (Cole & Cook, 1998; Tang et al., 2006) that may be expelled at speeds up to 100 m/s (Wells, 1955; Cole & Cook, 1998), whereas coughing may produce up to 3000 droplet nuclei, about the same number as talking for five minutes (Cole & Cook, 1998; Fitzgerald & Haas, 2005; Tang et al., 2006). Despite the variety in size, large droplets comprise most of the total volume of expelled respiratory droplets. Further data on the behaviour of droplet dispersion in naturally generated aerosols are needed.

Infectious aerosols are generated when they come into contact and mix with exhaled air that may carry infectious agents from patients' respiratory tracts. Several medical procedures generate aerosols, and some of these procedures may be associated with an increased risk of pathogen transmission. However, many of the most recent studies of these procedures have significant methodological flaws that preclude the use of their conclusions to draw recommendations. Overall, the risk associated with many of the aerosol-generating procedures is not yet well defined, and understanding the aerobiology of the aerosol-generating procedures may change with further studies. For the purpose of this guideline, the term aerosol-generating procedure associated with a documented increase in risk of pathogen transmission refers to the performance of the following procedures in acute respiratory disease patients:

  • intubation and related procedures (e.g. manual ventilation, suctioning)
  • cardiopulmonary resuscitation
  • bronchoscopy
  • surgery and autopsy.

C.2. Droplet evaporation

In the classic study of airborne transmission, Wells (1934) was able to identify the difference between disease transmission via large droplets and by airborne routes. Wells found that, under normal air conditions, droplets smaller than 100 μm in diameter would completely dry out before falling approximately 2 m to the ground. This finding allowed the establishment of the theory of droplets and droplet nuclei transmission depending on the size of the infected droplet. The Wells evaporation-falling curve of droplets (see Figure C.2) is important in understanding airborne transmission and transmission by large droplets. Wells' study also demonstrated that droplets could transform into droplet nuclei by evaporation.

Figure C.2. The Wells evaporation-falling curve of droplets.

Figure C.2

The Wells evaporation-falling curve of droplets.

C.3. Movement of air

Droplet nuclei floating on the air may be carried by the movement of air. Entrainment of air into neighbouring airspaces may occur during the most innocuous daily activities; for example, as a result of people walking, or the opening of a door between a room and the adjacent corridor or space (Hayden et al., 1998; Edge, Paterson & Settles, 2005; Tang et al., 2005, 2006). In addition, the air temperature (and therefore air density) differences across an open doorway will also cause air exchange to occur between the two areas, providing a second mechanism to allow air into other areas (Tang et al., 2005, 2006) (see Figure C.3).

Figure C.3. Patterns of air exchange during daily activities.

Figure C.3

Patterns of air exchange during daily activities. (A) Demonstration of how a walking person may entrain air into their wake (Tang et al., 2006). (B) Demonstration of how opening a door may transport air from inside an isolation room to the outside, during (more...)

Even a patient simply sitting in or beside the bed will create air temperature differences from their body heat. A higher air temperature directly above the patient's head (or body, if lying down) will create convective air currents that may entrain potentially infectious air from neighbouring spaces into the higher temperature column rising air above the patient (Craven & Settles, 2006). Patients lying in bed, breathing or sleeping, may produce exhaled airflows that can reach the airspace of a patient in the neighbouring bed, and even further in the presence of certain types of ventilation systems (see below) (Qian et al., 2006). In the same way, other mechanical devices, including fans, televisions and medical equipment, may also disturb nearby airflows and disseminate air from nearby patients to the rest of the ward.