Dr
K K Aggarwal
President
Confederation of Medical Associations of Asia and Oceania
351: What is droplet
Fact: The term
“droplet” consists mostly of water with various inclusions, depending on how it
is generated.
Naturally
produced droplets from humans (droplets produced by breathing, talking,
sneezing, coughing) include various cells types (epithelial cells and cells of
the immune system), physiological electrolytes contained in mucous and saliva (Na+,
K+, Cl-), as well as, potentially, various infectious agents (bacteria, fungi
and viruses).
352: What are artificially generated droplets in a
health-care setting
Fact: During 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).
353: What are the sizes
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).
354: What are the types
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).
355: Is there any other classification
Other studies define
“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).
356: The movements of droplets depend on what
factors
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).
357: On what factors actual size distribution of droplets depends
The actual
size distribution of droplets 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).
358: How are droplets produced in the body
Humans can
produce respiratory aerosols (droplets) by several means, including breathing,
talking, coughing, sneezing and even singing (Wong,
2003; Toth et al., 2004).
359: What are the physiological variations
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.
360: Can infection change these variabilities
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.
361: How to explain super spreaders
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).
362: What is the difference between coughing and
sneezing
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.
363” What are infectious aerosols
They are
generated when they come into contact and mix with exhaled air that may carry
infectious agents from patients' respiratory tracts.
·
intubation and related procedures (e.g. manual
ventilation, suctioning)
·
cardiopulmonary resuscitation
·
bronchoscopy
364: What is the difference between droplet and air
born illness
A 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.
Wells' study
also demonstrated that droplets could transform into droplet nuclei by evaporation.
365: How are droplet nuclei floating on the air
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).
366: What about daily body movements
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.
Reference
2009, World Health Organization. Atkinson J, Chartier Y, Pessoa-Silva CL, et al., editors.
https://www.ncbi.nlm.nih.gov/books/NBK143281/
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