Droplet vs Aerosol
The new β-coronavirus belonging to the subgenus botulinum of
Coronaviridae is a chimeric virus between a bat coronavirus and a
coronavirus of unknown origin.(8, 9)
Respiratory droplets (5-50 µm) is the main route of transmission, which
is affected by gravity, and may cause direct transmission via close
contact (including the conjunctivae), or via surface
contamination.(10-19) SARS-nCoV-2 can remain infectious on inanimate
surfaces at room temperature for up to 9 days.(20)
It is highly contagious and can be transmitted via smaller aerosols with
a droplet nuclei ≤5µm, which can travel long distances and remain
airborne for 2 – 4 hours, depending on the ambient conditions
(temperature, humidity and ventilation).(21-23) The size and number of
droplets produced is dependent on exhalation velocity (higher velocity
results in finer and greater droplets).(21) The particle size determines
the location in the respiratory tract it is deposited when it is
inhaled.(21, 23) Large droplets (>5µm) rapidly fall to the
ground when produced by coughing or sneezing.(21) Certain events (eg
coughing or sneezing, cardiopulmonary resuscitation) and aerosol
generating procedures (AGP) (eg intubation, tracheostomy) can generate
aerosols composed of smaller virus containing particles suspended in
air.(18)
Modelling of droplet sizes of less than 60µm can travel more than 2
meters, with reports up to 8 meters.(24) Droplets produced during
coughing and sneezing travel a meter from the mouth, and some may travel
as far as 3 meters away but the amount of virus is lower the further
away.(25) During a cough, droplets of airway secretions become airborne
and is expelled from the mouth in a high velocity aerosol plume.(25) The
aerosol droplets generated during coughing span a broad size range, and
dispersion in the environment depends on the size: Smaller aerosol may
remain airborne and spread throughout the room, while larger droplets
may settle onto surfaces, impact on the face/eyes.(25) The droplet size
is dynamic and changes during its transit from the respiratory tract to
the environment with evaporation, and a large droplet can become an
airborne particle in less than a second.(24) This is dependent on the
type of aerosol generating procedure, temperature, relative humidity,
ventilation and air exchanges in the environment.(24) Aerosolised viable
SARS-CoV-2 virus has been be detected in the air up to 3 hours
after.(26)
However, infection is not neatly separated into the dichotomy of droplet
versus airborne transmission route.(27, 28) There is heterogeneity of
the viral load between patients and the quantity of aerosol particles
generated.(23, 25)