The World Health Organization has issued a report that transforms how the world understands respiratory infections like covid-19, influenza, and measles.

Motivated by grave missteps in the pandemic, the WHO convened about 50 experts in virology, epidemiology, aerosol science, and bioengineering, among other specialties, who spent two years poring through the evidence on how airborne viruses and bacteria spread.

However, the WHO report stops short of prescribing actions that governments, hospitals, and the public should take in response. It remains to be seen how the Centers for Disease Control and Prevention will act on this information in its own guidance for infection control in health care settings.

The WHO concluded that airborne transmission occurs as sick people exhale pathogens that remain suspended in the air, contained in tiny particles of saliva and mucus that are inhaled by others.

While it may seem obvious, and some researchers have pushed for this acknowledgment for more than a decade, an alternative dogma persisted — which kept health authorities from saying that covid was airborne for many months into the pandemic.

Specifically, they relied on a traditional notion that respiratory viruses spread mainly through droplets spewed out of an infected person’s nose or mouth. These droplets infect others by landing directly in their mouth, nose, or eyes — or they get carried into these orifices on droplet-contaminated fingers. Although these routes of transmission still happen, particularly among young children, experts have concluded that many respiratory infections spread as people simply breathe in virus-laden air.

“This is a complete U-turn,” said Julian Tang, a clinical virologist at the University of Leicester in the United Kingdom, who advised the WHO on the report. He also helped the agency create an online tool to assess the risk of airborne transmission indoors.

Peg Seminario, an occupational health and safety specialist in Bethesda, Maryland, welcomed the shift after years of resistance from health authorities. “The dogma that droplets are a major mode of transmission is the ‘flat Earth’ position now,” she said. “Hurray! We are finally recognizing that the world is round.”

The change puts fresh emphasis on the need to improve ventilation indoors and stockpile quality face masks before the next airborne disease explodes. Far from a remote possibility, measles is on the rise this year and the H5N1 bird flu is spreading among cattle in several states. Scientists worry that as the H5N1 virus spends more time in mammals, it could evolve to more easily infect people and spread among them through the air.

Traditional beliefs on droplet transmission help explain why the WHO and the CDC focused so acutely on hand-washing and surface-cleaning at the beginning of the pandemic. Such advice overwhelmed recommendations for N95 masks that filter out most virus-laden particles suspended in the air. Employers denied many health care workers access to N95s, insisting that only those routinely working within feet of covid patients needed them. More than 3,600 health care workers died in the first year of the pandemic, many due to a lack of protection.

However, a committee advising the CDC appears poised to brush aside the updated science when it comes to its pending guidance on health care facilities.

Lisa Brosseau, an aerosol expert and a consultant at the Center for Infectious Disease Research and Policy in Minnesota, warns of a repeat of 2020 if that happens.

“The rubber hits the road when you make decisions on how to protect people,” Brosseau said. “Aerosol scientists may see this report as a big win because they think everything will now follow from the science. But that’s not how this works and there are still major barriers.”

Money is one. If a respiratory disease spreads through inhalation, it means that people can lower their risk of infection indoors through sometimes costly methods to clean the air, such as mechanical ventilation and using air purifiers, and wearing an N95 mask. The CDC has so far been reluctant to press for such measures, as it updates foundational guidelines on curbing airborne infections in hospitals, nursing homes, prisons, and other facilities that provide health care. This year, a committee advising the CDC released a draft guidance that differs significantly from the WHO report.

Whereas the WHO report doesn’t characterize airborne viruses and bacteria as traveling short distances or long, the CDC draft maintains those traditional categories. It prescribes looser-fitting surgical masks rather than N95s for pathogens that “spread predominantly over short distances.” Surgical masks block far fewer airborne virus particles than N95s, which cost roughly 10 times as much.

Researchers and health care workers have been outraged about the committee’s draft, filing letters and petitions to the CDC. They say it gets the science wrong and endangers health. “A separation between short- and long-range distance is totally artificial,” Tang said.

Airborne viruses travel much like cigarette smoke, he explained. The scent will be strongest beside a smoker, but those farther away will inhale more and more smoke if they remain in the room, especially when there’s no ventilation.

Likewise, people open windows when they burn toast so that smoke dissipates before filling the kitchen and setting off an alarm. “You think viruses stop after 3 feet and drop to the ground?” Tang said of the classical notion of distance. “That is absurd.”

The CDC’s advisory committee is comprised primarily of infection control researchers at large hospital systems, while the WHO consulted a diverse group of scientists looking at many different types of studies. For example, one analysis examined the puff clouds expelled by singers, and musicians playing clarinets, French horns, saxophones, and trumpets. Another reviewed 16 investigations into covid outbreaks at restaurants, a gym, a food processing factory, and other venues, finding that insufficient ventilation probably made them worse than they would otherwise be.

In response to the outcry, the CDC returned the draft to its committee for review, asking it to reconsider its advice. Meetings from an expanded working group have since been held privately. But the National Nurses United union obtained notes of the conversations through a public records request to the agency. The records suggest a push for more lax protection. “It may be difficult as far as compliance is concerned to not have surgical masks as an option,” said one unidentified member, according to notes from the committee’s March 14 discussion. Another warned that “supply and compliance would be difficult.”

The nurses’ union, far from echoing such concerns, wrote on its website, “The Work Group has prioritized employer costs and profits (often under the umbrella of ‘feasibility’ and ‘flexibility’) over robust protections.” Jane Thomason, the union’s lead industrial hygienist, said the meeting records suggest the CDC group is working backward, molding its definitions of airborne transmission to fit the outcome it prefers.

Tang expects resistance to the WHO report. “Infection control people who have built their careers on this will object,” he said. “It takes a long time to change people’s way of thinking.”

The CDC declined to comment on how the WHO’s shift might influence its final policies on infection control in health facilities, which might not be completed this year. Creating policies to protect people from inhaling airborne viruses is complicated by the number of factors that influence how they spread indoors, such as ventilation, temperature, and the size of the space.

Adding to the complexity, policymakers must weigh the toll of various ailments, ranging from covid to colds to tuberculosis, against the burden of protection. And tolls often depend on context, such as whether an outbreak happens in a school or a cancer ward.

“What is the level of mortality that people will accept without precautions?” Tang said. “That’s another question.”

KFF Health News is a national newsroom that produces in-depth journalism about health issues and is one of the core operating programs at KFF—an independent source of health policy research, polling, and journalism. Learn more about KFF.

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KFF Health News Peggy Girshman reporting fellow Jackie Fortiér joined a boat tour to spotlight a review of microbes in the Anacostia River, a step toward making the river healthier and swimmable. The story was featured on WAMU’s “Health Hub” on Feb. 26.

On a bright October day, high schoolers from Francis L. Cardozo Education Campus piled into a boat on the Anacostia River in Washington, D.C. Most had never been on the water before.

Their guide, Trey Sherard of the Anacostia Riverkeeper, started the tour with a well-rehearsed safety talk. The nonprofit advocates for the protection of the river.

A boy with tousled black hair casually dipped his fingers in the water.

“Don’t touch it!” Sherard yelled.

Why was Sherard being so stern? Was it dangerously cold? Were there biting fish?

Because of the sewage.

“We get less sewage than we used to. Sewage is a code word for what?” Sherard asked the teenagers.

“Poop!” one student piped up.

“Human poop,” Sherard said. “Notice I didn’t say we get none. I said we get what? Less.”

Tours like this are designed to get young people interested in the river’s ecology, but it’s a fine line to tread — interacting with the water can make people sick. Because of the health risks, swimming hasn’t been legal in the Anacostia for more than half a century. The polluted water can cause gastrointestinal and respiratory illnesses, as well as eye, nose, and skin infections.

The river is the cleanest it’s been in years, according to environmental experts, but they still advise you not to take a dip in the Anacostia — not yet, at least.

About 40 million people in the U.S. live in a community with a combined sewer system, where wastewater and stormwater flow through the same pipes. When pipe capacities are reached after heavy rains, the overflow sends raw wastewater into the rivers instead of to a treatment plant.

Federal regulations, including sections of the Clean Water Act, require municipalities such as Washington to reduce at least 85% of this pollution or face steep fines.

To achieve compliance, Washington launched a $2.6 billion infrastructure project in 2011. DC Water’s Clean Rivers Project will eventually build multiple miles-long underground storage basins to capture stormwater and wastewater and pump it to treatment plants once heavy rains have subsided.

The Anacostia tunnel is the first of these storage basins to be completed. It can collect 190 million gallons of bacteria-laden wastewater for later treatment, said Moussa Wone, vice president of the Clean Rivers Project.

Climate change is causing more intense rainstorms in Washington, so even after construction is complete in 2030, Wone said, untreated stormwater will be discharged into the river, though much less frequently.

“On the Anacostia, we’re going to be reducing the frequency of overflows from 82 to two in an average year,” Wone said.

But while the Anacostia sewershed covers 176 square miles, he noted, only 17% is in Washington.

“The other 83% is outside the district,” Wone said. “We can do our part, but everybody else has to do their part also.”

Upstream in Maryland’s Montgomery and Prince George’s counties, miles of sewer lines are in the process of being upgraded to divert raw sewage to a treatment plant instead of the river.

The data shows that poop is a problem for river health — but knowing what kind of poop it is matters. Scientists monitor E. coli to indicate the presence of feces in river water, but since the bacteria live in the guts of most warm-blooded animals, the source is difficult to determine.

“Is it human feces? Or is it deer? Is it gulls’? Is it dogs’?” said Amy Sapkota, a professor of environmental and occupational health at the University of Maryland.

Bacterial levels can fluctuate across the river even without rainstorms. An Anacostia Riverkeeper report found that in 2023 just three of nine sites sampled along the Washington portion of the watershed had consistently low E. coli levels throughout the summer season.

Sapkota is heading a new bacterial monitoring program measuring the amount of E. coli that different animal species deposit along the river.

The team uses microbial source tracking to analyze samples of river water taken from different locations each month by volunteers. The molecular approach enables scientists to target specific gene sequences associated with fecal bacteria and determine whether the bacteria come from humans or wildlife. Microbial source tracking also measures fecal pollution levels by source.

“We can quantify the levels of different bacterial targets that may be coming from a human fecal source or an animal fecal source,” Sapkota said.

Her team expects to have preliminary results this year.

The health risk to humans from river water will never be zero, Sapkota said, but based on her team’s research, smart city planning and retooled infrastructure could lessen the level of harmful bacteria in the water.

“Let’s say that we’re finding that actually there’s a lot of deer fecal signatures in our results,” Sapkota said. “Maybe this points to the fact that we need more green buffers along the river that can help prevent fecal contaminants from wildlife from entering the river during stormwater events.”

Washington is hoping to recoup some of the cost of building green spaces and other river cleanup. In January, the office of D.C. Attorney General Brian Schwalb filed a lawsuit seeking unspecified damages from the federal government over decades of alleged pollution of the Anacostia River.

Brenda Lee Richardson, coordinator of the Anacostia Parks & Community Collaborative, said the efforts to cut down on trash and sewage are paying off. She sees a river on the mend, with more plant and animal life sprouting up.

“The ecosystem seems a lot greener,” she said. “There’s stuff in the river now that wasn’t there before.”

But any changes to the waterfront need to be done with residents of both sides of the river in mind, she said.

“We want there to be some sense of equity as it relates to who has access,” she said. “When I look at who is recreating, it’s not people who look like me.”

Richardson has lived for 40 years in Ward 8 — a predominantly Black area on the east side of the river whose residents are generally less affluent than those on the west side. She and her neighbors don’t consider the Anacostia a place to get out and play, she said.

As the water quality slowly improves, Richardson said, she hopes the Anacostia’s reputation is also rehabilitated. Even if it’s not safe to swim in, Richardson enjoys boating trips like the one with the Anacostia Riverkeeper.

“To see all those creatures along the way and the greenery. It was comforting,” she said. “So rather than take a pill to settle my nerves, I can just go down the river.”

KFF Health News is a national newsroom that produces in-depth journalism about health issues and is one of the core operating programs at KFF—an independent source of health policy research, polling, and journalism. Learn more about KFF.

Subscribe to KFF Health News’ free Morning Briefing.

This article first appeared on KFF Health News and is republished here under a Creative Commons license.