Airborne transmission


Respiratory droplets are generated within the human respiratory tract, thoracic or extrathoracic, with possibly different pathogen loads, or upon release from an infected person via lung-fluid fragmentation.

Respiratory droplets are expelled by expiratory events that include violent events such as coughing or sneezing and quiescent ones such as talking, breathing, or laughing. 

Respiratory droplets have been associated with three modes of pathogen transmission: “contact,” “droplet,” and “airborne” transmission modes.


Contact transmission, direct or indirect, occurs via contact with pathogen-laden droplets: transfer of pathogens via physical touch between a susceptible and an infected host such as 

hand contact; direct contact transmission.


Transfer mediated by fomites containing settled droplets is classified as indirect contact transmission. 


Large droplets settle faster than they evaporate, contaminating the immediate vicinity of the infected individual. 


Small droplets evaporate faster than they settle.


As small droplets transition from the warm and moist conditions of the respiratory system to a cooler and drier outside environment, they evaporate and form residual particulates made of the dried material from the original droplets.


These residual particulates are referred to is droplet nuclei or aerosols.


Droplet transmission refers to transmission by large droplets (diameter >20 microns that are transported by the turbulent air flow generated by a violent expiratory event. 


Subsequently droplets are sprayed and directly deposited upon the conjunctiva or mucus membranes of a susceptible host. 


Large droplets settle rather quickly, droplet transmission is considered important at close range.

In still air, a 50-micron droplet crosses a vertical 1.5 m distance in 20 s.


Airborne transmission, (aerosol transmission), refers to pathogen transmission via inhalation of small respiratory droplets that are typically smaller than 5 microns: 


Aerosols rapidly evaporate in the air, leaving behind droplet nuclei it is small enough and light enough to remain suspended in the air for hours.


A 10-micron droplet settles in still air within approximately 9 min.


Aerosols are relatively small and may deposit deep into the respiratory tract, including the alveolar region. 


These droplets,  are small enough to remain airborne for sufficient time to transmit the pathogen. 


Airborne transmission does not require direct face-to-face contact.


Droplets associated with droplet transmission may be transported by a turbulent jet and subsequently inhaled.

Respiratory-droplet diameters vary from 0.5 microns to 1000 microns.


Respiratory droplets are generated in a nearly 100% relative-humidity environment. 


When exhalation into a lower-humidity ambient environment the respiratory droplets shrink by evaporation on the order of seconds or less, depending on droplet size, composition, and relative humidity to reach their equilibrium diameter. 


It is  estimated that droplets may shrink to about half their original size.


Droplets classically or larger entities that typically are grounded by forces of gravity within 3-6 feet of the source person.


Pathogen transmission depends on the interplay of a number of factors, including frequency of violent droplet-generating events such as coughing, sneezing, droplet size distribution, ambient relative humidity, viral load, virus inactivation rates, deposition location of inhaled droplets in the airway, and infectious dose. 


Singing and speaking produce similar numbers of airborne particles, but mass grows with volume.


Exhalations, sneezes, and coughs consist of mucosal salivary droplets that short range semi ballistic emission trajectories indoor primarily made of a multiphase turbulent gas, or puff, cloud that entrains ambient air and traps and carries within clusters of droplets with a continuum of droplet sizes.


When air is moist and warm in the atmosphere within the turbulent gas cloud, it allows the droplets to evade evaporation for a longer period then occurs with isolated droplets. 


The lifetime of a droplet under these circumstances could be extended by a factor of up to 1000, from a fraction of a second to minutes.


Because the momentum of the gas cloud bearing droplets are propelled a greater distance than if they were emitted  in isolation without a turbulent puff cloud trapping and carrying them forward.


Given a person’s physiology and environmental conditions, such as humidity and temperature, a gas cloud and it’s payload of droplets of all sizes can travel 23-27 feet.


While speaking and coughing can generate aerosols or that it is possible to recover viral RNA from aerosol based transmission; infection depends on the route of exposure, the size of the inoculum, duration of exposure, and host defenses.


Both small and large droplets can interact with and be trapped within the turbulent gas cloud.


Methods of air disinfection include natural or mechanical ventilation of existing room air using filters UV and upper room germicidal features.


For effective air disinfection, ventilation with 6-12 room changes/hour  is recommended.

Droplets that settle along the trajectory can contaminate surfaces, while the rest remain trapped and clustered in the moving cloud.

When the cloud and its droplet payload lose momentum and coherence, remaining droplets within the cloud evaporate, produce residues or droplet nuclei that may stay suspended in the air for hours, following airflow patterns imposed by ventilation or climate control systems.

The evaporation of pathogen laden droplets depends on ambient temperature and humidity commit conditions and on the inner dynamics of the turbulent puff cloud coupled with the composition of the liquid exhaled by the patient.





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