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)