While not strictly the same document, the WHO this week released a related document bringing new clarity for a(hopefully) shared terminology of airborne infection going forward.
From the executive summary:
"Terminology used to describe the transmission of pathogens through the air varies across scientific disciplines, organizations and the general public. While this has been the case for decades, during the coronavirus disease (COVID-19) pandemic, the terms ‘airborne’, ‘airborne transmission’ and ‘aerosol transmission’ were used in different ways by stakeholders in different scientific disciplines, which may have contributed to misleading information and confusion about how pathogens are transmitted in human populations.
...
The scope of what type of pathogens were covered in this consultation and the resulting
descriptors used in this document are as follows:
• Pathogens, contained within a particle (known as ‘infectious particles’), that travel through the air, when these infectious particles are carried by expired airflow (they are known as ‘infectious respiratory particles’ or IRPs), and which enter the human
respiratory tract (or are deposited on the mucosa of the mouth, nose or eye of another person) and;
• Pathogens from any source (including human, animal, environment), that cause
predominantly respiratory infections (e.g., Tuberculosis [TB], influenza, severe acute
respiratory syndrome [SARS], Middle East respiratory syndrome [MERS]), but as
well as those causing infections involving the respiratory and other organ systems (e.g. COVID-19, measles).
The following descriptors and stages have been defined by this extensively discussed consultation to characterize the transmission of pathogens through the air (under typical circumstances):
• Individuals infected with a pathogen, during the infectious stage of the disease (the source), can generate particles containing the pathogen, along with water and respiratory secretions. Such particles are herein described as potentially ‘infectious
particles’.
• These potentially infectious particles are carried by expired airflow, exit the infec-
tious person’s mouth/nose through breathing, talking, singing, spitting, coughing or
sneezing and enter the surrounding air. From this point, these particles are known as ‘infectious respiratory particles’ or IRPs.
• IRPs exist in a wide range of sizes (from sub-microns to millimetres in diameter).
The emitted IRPs are exhaled as a puff cloud (travelling first independently from air
currents and then dispersed and diluted further by background air movement in the room).
• IRPs exist on a continuous spectrum of sizes, and no single cut off points should be
applied to distinguish smaller from larger particles, this allows to move away from
the dichotomy of previous terms known as ‘aerosols’ (generally smaller particles) and ‘droplets’ (generally larger particles).
• Many environmental factors influence the way IRPs travel through air, such as ambi-
ent air temperature, velocity, humidity, sunlight (ultraviolet radiation), airflow distri-
bution within a space, and many other factors, and whether they retain viability and infectivity upon reaching other individuals.
...
The descriptor ‘transmission through the air’ can be used to describe the mode of trans-
mission of IRPs through the air.
Under the umbrella of the ‘through the air’, two descriptors can be used:
•
‘Airborne transmission/inhalation’: Occurs when IRPs expelled into the air as described above and enter, through inhalation, the respiratory tract of another person and may potentially cause infection. This form of transmission can occur
when the IRPs have travelled either short or long distances from the infectious person. The portal of entry of an IRP with respiratory tract tissue during airborne transmission can theoretically occur at any point along the human respiratory tract, but preferred sites of entry may be pathogen specific. It should be noted that the dis-
tance travelled depends on multiple factors including particle size, mode of expul-
sion and environmental conditions (such as airflow, humidity, temperature, setting, ventilation).
•‘Direct deposition’: Occurs when IRPs expelled into the air following a short-range
semi-ballistic trajectory, then directly deposited on the exposed facial mucosal sur-
faces (mouth, nose or eyes) of another person, thus, enter the human respiratory tract
via these portals and potentially cause infection."
https://iris.who.int/bitstream/handle/10665/376496/9789240089181-eng.pdf
A Concerned Scientist
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