most respiratory tract viral infections are spread by droplets or aerosols from coughing or sneezing
smaller sized droplets less than 5 microns in diameter can become aerosols and hang in the air and circulate through the room or into other rooms for a number of hours vastly increasing the range of infective transmissibility, and in addition, these sized droplets, unlike larger droplets, can diffuse down to the alveoli hence are termed respirable aerosols
The infectious dose required for inoculation by the aerosol route relative to contact or droplet transmission is unclear, but it seems that the infectious dose by the aerosol route is likely considerably lower than the infectious dose by intranasal inoculation and that aerosol inoculation results in more severe symptoms
use of surgical masks and N95 respirators reduce forward dispersion but increase lateral dispersion of the cough aerosol
patients with higher viral loads in their nasopharyngeal region generally shed more viral RNA during coughing and thus there tends to be large variations in viral loads between patients and some individuals shed much greater quantities of aerosol particles during breathing and coughing than do others and perhaps allows for the concept of “super-spreaders”
influenza virus shedding peaks early in the course of the illness (typically about 2 days after the onset of symptoms)
Droplet size and physics
greater than 50 micron diameter droplets
these are large ballistic droplets which are gravity affected and generally only travel about 1m and fall to the ground within 1 minute, although coughing, and even more so, sneezing, can increase this range to a number of metres
most of the exhaled viral load is contained in these larger droplets
intermediate sized droplets
10 - 50 microns in diameter
subject to impaction with objects
can travel further with the air flow and hence reach a distance of about 2m before settling
less than 10 micron in diameter
are much less prone to impaction or settling; they can remain airborne for an extended time and be spread throughout a room by air currents, especially after drying
very small droplets
less than 2 micron in diameter
are unaffected by gravity and form aerosols as per small droplets
Physiology of airway droplet creation
alveolar aerosol formation
The passage of inhaled and exhaled air over the lung lining fluid entrains pathogens, resident in the lungs, in the form of droplets of airway lining fluid (ALF) that contain lung mucus and surfactant material
Momentum transfer from flowing air in inhalation and to a lesser extent, exhalation results in wave-like disturbances that can lead to droplet creation – similar to aerosol formation from the surface of a wind-stirred sea
a critical airspeed that initiates wave disturbances in the lungs is required and these level of minimum speed will vary according to several parameters, such as film thickness and the surface and bulk physical properties of the mucus layer
droplet creation will be strongly influenced by the surface and bulk rheological properties of the lung surfactant fluid
a potential mechanism for droplet creation during calm breathing relates to the reopening of closed small airways upon deep exhalations
deep exhalation resulted in a four- to sixfold increase in concentration of exhaled droplets, and rapid inhalation produced a further two- to threefold increase in concentration. In contrast, rapid exhalation had little effect on the measured concentration.1)
Mouth breathing and droplet size
compared to coughing and talking, mouth breathing produced the highest number of droplets <1μm in size, whereas coughing produced approximately five-times as many total droplets per maneuver.2)
it appears that people may be low droplet producers with < 100 droplets per liter over a six hour or “super-producers” with 500-3000 droplets per liter over a six hour during normal breathing
coughing, talking and sneezing produce more droplets than normal breathing, however normal breathing, being continuous, probably accounts for the majority of expired bioaerosols over the course of a day.3)
Talking and droplet size
Fabian et al. 4) and Stelzer-Braid et al. 5) detected influenza viral RNA produced by influenza patients during breathing and talking. Fabian et al. showed that 60% of patients with influenza A and 14% of patients with influenza B had detectable levels of viral RNA in their exhaled breath; they also reported that over 87% of the exhaled particles were less than 1 µm in diameter.
75% of droplets observed are moving at velocities less than 0.5 m/s and the motion is equally distributed in all the directions, which implies that they do not settle rapidly and may follow the ambient airflow pattern. These results points toward high aerosol generation, as the behaviour of these droplets is like airborne particles.
A cough is typically characterized by inhalation to a high lung volume, closure of the glottis, an increase in intrathoracic pressure, a sudden opening of the glottis, and a high initial peak in outward air flow followed by a gradual decrease.
After exiting the mouth, the cough air flow forms a jet that gradually widens and then dissipates
Large amounts of aerosol particles from the respiratory tract can be carried into the environment.
this is 5-1000 times that of normal mouth breathing 6)
The sizes of cough-generated aerosol particles have a substantial impact on their behavior.
voluntary coughs generate droplets ranging from 0.1 - 900 microns in size with 97% being smaller than 1 micron
some droplets are larger than 1mm in diameter and are visible
in one study, 81% of the influenza-positive patients had detectable levels of influenza viral RNA in their cough aerosols. 65% of the influenza viral RNA was contained in particles in the respirable size fraction (<4 µm).7)
Effect of nebulizers
nebulizer therapy is traditionally regarded as an AGP
however, studies have shown that delivering ∼1 g of isotonic saline (orally via nebulized aerosols, 5.6 μm in diameter) over a short period of time reduces the total amount of expired aerosols (among the super-producing individuals) by ∼72% over a six hour period and markedly diminishes total expired bioaerosol production for the entire group, and increases the size of the aerosol particles 8)
Room ventilation and aerosol
With perfect mixing, 63% of airborne droplets can be removed by each air exchange, however, because perfect mixing is rarely achieved, only 20-60% of droplets are typically removed from the circulation in ventilated buildings.
Masks and filters
HEPA or N95 filters generally provide higher protection against airborne pathogens than dust filters.
Studies have shown that a ‘fit test’ is crucial before the performance of all face masks can be guaranteed