Global technical consultation report on proposed terminology for pathogens that transmit through the air

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.

This global technical consultation report brings together viewpoints from experts spanning a range of disciplines with the key objective of seeking consensus regarding the terminology used to describe the transmission of pathogens through the air that can potentially cause infection in humans.

This consultation aimed to identify terminology that could be understood and accepted by different technical disciplines. The agreed process was to develop a consensus document that could be endorsed by global agencies and entities. Despite the complex discussions and challenges, significant progress was made during the consultation process, particularly the consensus on a set of descriptors to describe how pathogens are transmitted through the air and the related modes of transmission. WHO recognizes the important areas where consensus was not achieved and will continue to address these areas in follow-up consultations.

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).

Global technical consultation report on proposed terminology for pathogens that transmit through the air

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 infectious 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 ambient air temperature, velocity, humidity, sunlight (ultraviolet radiation), airflow distribution within a space, and many other factors, and whether they retain viability and infectivity upon reaching other individuals.

The descriptor ‘through the air’ can be used in a general way to characterize an infectious disease where the main mode of transmission involves the pathogen travelling through or being suspended in the air. This has similarity with other public health descriptors of infectious diseases, such as ‘waterborne’ and ‘bloodborne’, that refer to the main medium through which a specific disease is transmitted, and as commonly understood by the scientific, clinical, public health communities and the general public.

The descriptor ‘transmission through the air’ can be used to describe the mode of transmission 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 distance travelled depends on multiple factors including particle size, mode of expulsion 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 surfaces (mouth, nose or eyes) of another person, thus, enter the human respiratory tract via these portals and potentially cause infection.