W.H.O. Broadens Definition of Airborne Diseases

In the early days of the Covid pandemic, a team of scientists called on the World Health Organization to acknowledge that the disease could spread through the air.

Initially, the agency rebuffed them, despite growing evidence that coronavirus-laden droplets stuck around in the air, making indoor spaces hotbeds of infection. The researchers responded with a public campaign, which helped persuade the World Health Organization to finally acknowledge, in late 2021, that Covid was airborne.

In the wake of the controversy, the agency also asked a group of advisers — including some of its scientific critics — to update its formal guidelines for classifying the ways that pathogens spread. After more than two years of discussion, that group has published a report laying out new definitions that could have significant implications for countries around the world that depend on the agency to set policies to curb the spread of disease.

The W.H.O.’s previous stance was that only a handful of pathogens — those that travel in small droplets and spread across long distances, like tuberculosis — could be considered airborne. But the new report suggests broader categories that do not rely on droplet size or distance spread. Such changes were contentious because they raised the prospect that more diseases might now demand costly control measures, such as hospital isolation rooms and protective gear.

“It’s an important first step,” said Dr. Ed Nardell, a tuberculosis expert at Harvard Medical School and a member of the group. “We really have a start, with agreed-upon terminology, even if everybody’s not happy with it.”

Before the pandemic, the W.H.O. and other agencies typically recognized a few ways diseases could spread. One was by “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 meter), 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.

When Covid emerged, the agency said it was likely spreading over short distances, either through contact or droplet transmission.

But Yuguo Li, a mechanical engineer at Hong Kong University, and many other critics worried that the W.H.O. was overlooking the possibility that Covid could spread through the air. As the pandemic progressed, the scientists found evidence in outbreaks that the coronavirus might indeed be able to spread over long distances in floating droplets. (Some scientists questioned the strength of those studies.)

The W.H.O. formed the new advisory group in November 2021 and asked Dr. Li to be a co-chair. At the group’s meetings, Dr. Li and others argued that the agency had relied on false dichotomies.

For example, there is little scientific basis for the 5-micron threshold for small droplets. Larger droplets can also stay afloat for long periods of time.

The researchers also argued that short-range infections were not proof that a disease spreads only through coughs and sneezes. Infected people can also exhale droplets through breathing or talking that are then inhaled by others nearby.

The new report divides transmission routes into ones that involve contact, and others that involve the air. The group agreed to call the second route “through the air transmission.”

Linsey Marr, an environmental engineer at Virginia Tech and a member of the advisory group, found that phrase more awkward than a simpler term like airborne transmission.

“I find it very clunky,” she said. “But we were looking for the lowest common denominator terminology that everyone could live with.”

The report further specified that pathogens can spread through the air in two ways. One is “direct deposition,” which refers to droplets that hit the mucus membranes of the mouth, eyes or nose. The other is “airborne transmission/inhalation,” in which droplets are inhaled.

After the scientists came up with the new terminology, the W.H.O. obtained agreements from the U.S. Centers for Disease Control and Prevention, as well as its counterparts in Africa, China and Europe, to adopt the same definitions.

“It’s a pretty important statement of agreement to work together,” said Dr. Jeremy Farrar, the W.H.O.’s chief scientist.

But the new report did not make any recommendations for how the agencies should stop diseases from spreading by these different routes. The authors acknowledged that they had been unable to reach a consensus on this issue.

Traditionally, hospital guidelines for controlling airborne diseases have called for expensive measures such as isolation rooms with negative air pressure, as well as N95 respirators and other protective gear to avoid inhaling fine droplets. But it is not clear which diseases warrant that kind of control, or what efforts should be taken outside of hospitals.

Dr. Walter Zingg, an infectious disease expert at the University of Zurich and a member of the advisory group, said the old categories offered more straightforward guidance. Staying a few feet from someone coughing and sneezing was once thought to be an effective way to avoid droplet transmission, for example.

“It was simplistic and probably not true in a way, but it served a purpose,” he said. “Now we have to bring other variables to the table.”

Dr. Farrar said such guidelines should be based on clear experimental evidence, though that evidence is still scarce for many diseases. Scientists are still debating, for instance, the extent that influenza, which has been studied for more than a century, spreads by air.

“We know a certain amount, but we’re not absolutely sure,” Dr. Farrar said. “That’s the sort of work we desperately need for influenza.”