News Excerpt:

After a drawn-out global controversy over the coronavirus, the WHO has updated its classification of how pathogens spread through the air.

Reason for the change in terminology?

• The change aims to help identify, communicate about and respond to existing and new airborne pathogens. 
• It also hopes to clear confusion about airborne transmission that arose during the Covid-19 pandemic due to differing views on whether the coronavirus spread via smaller aerosols or larger droplets.

Earlier Terminologies:

• Before the pandemic, the WHO and other agencies typically recognized a few ways diseases could spread. 
• “contact transmission,” in which someone picked up a pathogen either by touching an infected person directly or through contact with a contaminated surface.
• “Droplet transmission” involved the short-range spread of diseases when people coughed or sneezed droplets larger than 5 microns (five millionths of a metre), which then landed directly on a victim’s mouth, eyes or nose.
• “Airborne transmission” referred to just a handful of diseases that spread in droplets smaller than 5 microns, floating for long distances until someone inhaled them.
• Varying terminology “highlighted gaps in common understanding and contributed to challenges in public communication and efforts to curb transmission” of Covid-19.

New definition of airborne pathogens and diseases:

• Under the new terminology, all particles expelled from the mouth or nose of an infected person, regardless of size, are called “infectious respiratory particles” or IRPs.
• According to the WHO, the spectrum of IRP sizes is continuous, meaning there is no strict cut-off between smaller and larger particles.
• The descriptor “through the air” characterises diseases where the main transmission mode involves IRPs travelling through or being suspended in air. 
• Within this new definition, WHO defines two types of transmission. 
• First, airborne transmission or inhalation, when IRPs are expelled into the air and inhaled by others at short or long distances, depending on factors like airflow and ventilation.
• Second, direct deposition, when IRPs expelled by an infectious person are deposited on the exposed mouth, nose or eyes of a nearby person.

Difference between old and new terminologies:

• Earlier agencies have historically required high levels of proof before calling diseases airborne, which required very stringent containment measures; the new definition says the risk of exposure and severity of disease should also be considered.
• Past disagreements also centred around whether infectious particles were "droplets" or "aerosols" based on size, which the new definition moves away from.

Global implications of the change:

• This would have legal, logistic, operational and financial consequences that have global implications with regards to equity and access.
• The terminology sets common language that engineers, physicists, clinicians, and epidemiologists will be able to apply in determining the broad implications of airborne particles, along with the effectiveness of countermeasures like face masks and respirators.