Following several years of collaboration with major public health agencies, the World Health Organization has broadened its classification of airborne diseases — a move that could have significant influence over infection control policies in countries around the world.
The WHO outlined updated terminology in an April 18 report that moves away from previously relied upon descriptors such as "aerosols," which referred to smaller particles, and "droplets," which was used to describe larger particles.
The new consensus, reached after consulting with agencies from 2021 to 2023, recognizes people infected with a respiratory pathogen can expel infectious particles when breathing, talking, singing, spitting, coughing or sneezing. The particles should be described as "infectious respiratory particles."
"IRPs exist on a continuous spectrum of sizes, and no single cutoff points should be applied to distinguish smaller from larger particles," the WHO said.
The updated terminology reflects the consensus that, whether fine, micro particles or "droplets," either can be transmitted "through the air" — a phrase experts agreed can be used in a general way to characterize a disease where the main mode of transmission involves the pathogen traveling through or being suspended in the air.
The report further specifies that pathogens can be spread through the air in two main ways: "airborne transmission or inhalation," which refers to situations where IRPs are expelled and inhaled by another person, and "direct deposition," or cases where IRPs are directly deposited on the exposed mouth, nose or eyes of another person.
Reaching a consensus on the updated terminology involved a comprehensive, technical consultation process with several major health agencies, including the CDC. It follows significant controversy and confusion in the early days of the COVID-19 pandemic, when the WHO did not acknowledge that the virus that causes the disease could spread through the air.