The World Health Organization (WHO) has changed how it classifies pathogens that spread through the air, such as SARS-CoV-2. The redefinition has been two years in the making, and comes after criticism that during the COVID-19 pandemic, the WHO was too slow to acknowledge that COVID-19 was airborne. The change aims to provide clarity for future pandemics, but some researchers are not convinced.

Virologists now acknowledge that SARS-CoV-2 spreads mostly by airborne transmission of small particles that are inhaled and that can remain in the air for hours — a method that was previously called aerosol transmission. It also spreads through larger droplets of virus-containing particles on surfaces such as hands, or by being ejected over short distances.

However, the WHO didn’t publicly acknowledge the importance of airborne transmission until October 2020. Then WHO chief scientist Soumya Swaminathan said in November 2022 that this announcement should have been made much earlier on the basis of available evidence at the time. It didn’t amend its official document on COVID-19 transmission to include airborne transmission until December 2021. As a result, early infection-control and prevention advice focused mainly on surface cleaning, handwashing and social distancing, rather than mask-wearing and improved ventilation.

Some researchers say that earlier recognition of airborne transmission by the WHO could have saved lives, noting mounting evidence that pointed to airborne transmission before its official acknowledgement.

Technical report

To clarify the distinction between airborne, droplet and aerosol transmission, the WHO released a technical report on 18 April after more than two years of consultation with more than 100 specialists from a range of scientific disciplines. The report does away with the division between ‘droplets’ and ‘aerosols’, which were used during the pandemic to distinguish between particles above or below five micrometres in diameter. Instead, it proposes the term ‘infectious respiratory particles’ to describe all such particles, regardless of size.

However, the report still shies away from describing all pathogens that spread through the air as airborne. Instead, the document uses the umbrella term ‘through the air’ to describe any mode of transmission that “involves the pathogen travelling through or being suspended in the air”. That is then further broken down into two categories: ‘airborne transmission/inhalation’ is when infectious respiratory particles are inhaled from the air into the respiratory tract; ‘direct deposition’ is when such particles travel a short distance before landing directly on a person’s mouth, nose or eyes.

Under this terminology, COVID-19 would be recognized as spreading through the air by airborne inhalation/transmission, with a much smaller risk of transmission by direct deposition.

Up in the air

This classification is a mixed result for many scientists. “The positive thing which I see of this report is the fact that the report removes this division of five micrometres between aerosols and droplets,” says Lidia Morawska, an aerosol scientist at the Queensland University of Technology in Brisbane, Australia. That distinction was the justification for the emphasis on handwashing, distancing and surface-cleaning, rather than on mask-wearing and ventilation — a distinction that Morawska says was unscientific. It was also the classification that justified the WHO’s March 2020 assertion that COVID-19 was not airborne.

But the achievement of the 2024 report has been tarnished for some by the classification of ‘through the air’, instead of airborne.

“The simplest and clearest way to describe it is to call it airborne transmission,” says Linsey Marr, an environmental engineer at Virginia Tech in Blacksburg, whose research focuses on the transmission of infectious disease through aerosols and who was involved in the report. “We talk about waterborne transmission, blood-borne transmission, vector-borne transmission; it seems very logical and simple to me that we would call this airborne transmission.”

WHO chief scientist Jeremy Farrar, based in London, says that the term ‘through the air’ allows for the recognition of both airborne transmission in which infectious respiratory particles are inhaled, and the less common transmission through direct deposition. “It’s not a dichotomy of mutually exclusive transmission routes,” he says. The recognition of both ‘airborne’ and ‘direct deposition’ under the umbrella of ‘through the air’ avoided going back to the divide between aerosols and droplets, Farrar says. ‘Through the air’ is comparable with vector-borne and waterborne, he says, but acknowledges that “there is a direct deposition element, even if that is not the major way it transmits”.

Julian Tang, a clinical virologist at the University of Leicester, UK, who was also involved with the report, says that there was vigorous debate among scientists about the term ‘airborne’ and what it conveys. Some of the scientists argued that it would be problematic to use ‘airborne’ for all pathogens that spread through the air. “They think it’s too scary, too frightening, has too many connotations and it will cause panic, so they chose ‘through the air’,” Tang says.

Farrar says that he has no reticence to use the term ‘airborne’. He stresses that this technical report is merely the “base camp” of an ongoing process to refine terminology and its practical applications. “I’m not saying everybody is happy, and not everybody agrees on every word in the document, but at least people have agreed this is a baseline terminology.”

But Morawska, who was also consulted on the report, argues that the distinction between airborne and direct deposition is not justified. “It is just confusing the issues,” she says.

Even the co-chair of the group that developed the document, environmental engineer Yuguo Li at the University of Hong Kong, says that he prefers the term ‘airborne’ to ‘through the air’. However, he says that what’s important is how the concept is applied in practice. “In theory, airborne is a perfect umbrella term as waterborne and vector-borne, but it means different things to different people,” Li says. “The issue is not only about terminologies, it’s about the need to work with public-health experts, medical experts to understand those responses.”

Tang says that the document sets an important benchmark for how the world will respond to any future pandemics. “The next pandemic will most likely be a respiratory virus again because that’s normally the pathogen that mutates the fastest,” he says. The report’s clarity around transmission will help public-health providers to respond appropriately. “They will then consider masking early, they’ll consider ventilation early, they’ll consider all these precautions early because a precedent has been set already,” Tang says.

The report notes that measures to address through-the-air transmission must take into account both the risk posed by the infection itself — for example, disease severity — and the resources available, which might be limited in low-or middle-income countries.

Farrar thinks that this definition could save lives in future pandemics. “What’s needed now is moving on from base camp to now understanding actually, the non-pharmacological interventions that really matter, and if they work, and the evidence of it.”