WHO finally answers the airborne question

An updated definition of airborne transmission no longer includes a cut-off particle size.

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A World Health Organisation technical consultation group has finally delivered its report on the transmission of pathogens through the air – and not before time.  

Prompted by questions over what is and is not an airborne pathogen that arose early in the pandemic, the WHO convened a global technical consultation back in November 2021.  

Alongside the experts in epidemiology, microbiology and infection prevention, the working group also included bioengineers, aerobiologists and aerosol science experts.  

The goal was to come up with a consensus statement on generic terminology and descriptors for the transmission of pathogens through the air that could be understood across all disciplines.  

Despite the seemingly simple objective, the process was reportedly fraught with difficulty – and has resulted in a definition that must be taken with a grain of salt when it comes to practical infection control measures, as the group acknowledged.  

“As anticipated, it was challenging to achieve consensus on all aspects of this topic where experts had mutually exclusive and diametrically opposed positions regarding the supporting science, some of which still remain,” the final report reads.  

Without further ado, the three broad categories settled on were: airborne transmission/inhalation, direct deposition and contact transmission. This last was only added “for completeness”.  

Airborne transmission is defined as occurring when infectious respiratory particles are expelled into the air and enter the respiratory tract of another person through inhalation.  

“This form of transmission can occur when the [particles] have travelled either short or long distances from the infectious person,” the final report said. 

“The portal of entry of [a particle] 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 distance travelled may depend on multiple factors including particle size, mode of expulsion and environmental conditions (such as airflow, humidity, temperature, setting, ventilation, etc.).” 

The typical scenario of airborne transmission outlined by the working group was when a disease was transmitted between people who were not necessarily in close quarters.  

Direct deposition is defined as being when an infectious respiratory particle is expelled into the air and follows a short-range ballistic trajectory to be deposited on the facial mucosa of another person.  

This would include scenarios where two people are talking, and a small droplet of spit flies from the infected person onto the other person’s eye or mouth.  

Crucially, there is now no particle size cutoff included in this definition. 

Contact transmission covers transmission when droplets have landed on a surface, like a table or hand. 

The working group also posited the use of the term “through the air transmission” to characterise all infectious diseases that can spread via airborne transmission or direct deposition, similar to the way the terms “waterborne” and “bloodborne” were used.  

Using this broader term allows for the range of infectious respiratory particles on a spectrum of sizes over both short and long distances.  

The biggest immediate practical implication is that, because the definition of airborne is now vaguer, more diseases may require airborne precautions – think negative air pressure rooms, N95 respirators and other PPE.  

The expert group was quick to clarify that the new definitions left room to weigh up the risks and benefits of various infection control approaches.  

“There is NO suggestion from this consultative process that to mitigate the risk of shortrange airborne transmission full ‘airborne precautions’ [as they are currently known] should be used in all settings, for all pathogens, and by persons with any infection and disease risk levels where this mode of transmission is known or suspected,” they said.  

An annex to the report outlines the areas of disagreement between working group members, which included the potential “legal, logistic, operational and financial consequences” of recognising that smaller respiratory particles are capable of being transmitted at short and long range. 

Droplet precautions should still be taken to prevent direct deposition, the expert group wrote. 

“Most importantly, while discussions during the consultation were based on the available best science, it was agreed it was important to balance scientific insights with availability, access, affordability and other practical realities to minimise health inequity and avoid potential consequences such as the ability to access PPE,” the report read.