There is no author summary for this article yet. Authors can add summaries to their articles on ScienceOpen to make them more accessible to a non-specialist audience.
Abstract
A large number of infectious diseases are believed to be transmitted between people
via large droplets and by airborne routes. An understanding of evaporation and dispersion
of droplets and droplet nuclei is not only significant for developing effective engineering
control methods for infectious diseases but also for exploring the basic transmission
mechanisms of the infectious diseases. How far droplets can move is related to how
far droplet-borne diseases can transmit. A simple physical model is developed and
used here to investigate the evaporation and movement of droplets expelled during
respiratory activities; in particular, the well-known Wells evaporation-falling curve
of droplets is revisited considering the effect of relative humidity, air speed, and
respiratory jets. Our simple model considers the movement of exhaled air, as well
as the evaporation and movement of a single droplet. Exhaled air is treated as a steady-state
non-isothermal (warm) jet horizontally issuing into stagnant surrounding air. A droplet
is assumed to evaporate and move in this non-isothermal jet. Calculations are performed
for both pure water droplets and droplets of sodium chloride (physiological saline)
solution (0.9% w/v). We calculate the droplet lifetimes and how droplet size changes,
as well as how far the droplets travel in different relative humidities. Our results
indicate that a droplet's size predominately dictates its evaporation and movement
after being expelled. The sizes of the largest droplets that would totally evaporate
before falling 2 m away are determined under different conditions. The maximum horizontal
distances that droplets can reach during different respiratory activities are also
obtained. Our study is useful for developing effective prevention measures for controlling
infectious diseases in hospitals and in the community at large.
Our study reveals that for respiratory exhalation flows, the sizes of the largest
droplets that would totally evaporate before falling 2 m away are between 60 and 100
microm, and these expelled large droplets are carried more than 6 m away by exhaled
air at a velocity of 50 m/s (sneezing), more than 2 m away at a velocity of 10 m/s
(coughing) and less than 1 m away at a velocity of 1 m/s (breathing). These findings
are useful for developing effective engineering control methods for infectious diseases,
and also for exploring the basic transmission mechanisms of the infectious diseases.
There is a need to examine the air distribution systems in hospital wards for controlling
both airborne and droplet-borne transmitted diseases.