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, Farrar said.

“In the first hours, days, and weeks of a new respiratory pandemic, it’s issues like, ‘do masks work?’ And, if masks do work, ‘what type of mask,’ which will have enormous impact on the ability to protect communities and health-care workers,” he said.

Old Teachings

In March 2020, the WHO issued guidance for health workers caring for COVID patients asserting that the virus spread primarily through direct contact and respiratory “droplets” measuring 5-10 micrometers in diameter. The determination — based on decades-old infection-control teachings — meant that face masks were deemed an acceptable option during respirator shortages, unless the patient was undergoing a medical procedure likely to generate aerosols.

The WHO also recommended distancing of more than 1 meter — within which these droplets were thought to fall to the ground — along with hand-washing, surface cleaning and sneezing into elbows. With personal protective equipment in short supply, people were advised not to not wear masks unless they were sick or taking care of someone who was.

The new terminology acknowledges that infectious respiratory particles exist on a continuum spectrum of sizes, and recommends against applying definitive cut off points to distinguish smaller from larger particles.

“That’s a positive step,” said physicist Lidia Morawska, a distinguished professor in the school of earth and atmospheric sciences at Australia’s Queensland University of Technology, who worked on the WHO report.

In early 2020, Morawska led an international group of 36 scientists which warned about airborne spread and called on the United Nations agency to change its guidance on COVID transmission.

The WHO should have acknowledged airborne spread “much earlier, based on the available evidence,” the body’s former chief scientist Soumya Swaminathan told Science before leaving the post in late 2022.

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