(Illustration by Hanna Barczyk)
In March 2020, the World Health Organization (WHO) infamously tweeted “FACT: #COVID19 is NOT airborne.” The medical and public-health establishments fervently believed that contagious respiratory diseases are predominantly spread by direct contact and large droplets (tiny particles of mucus and saliva produced when someone coughs or sneezes) and only very rarely via microscopic aerosols that can float through the air for hours and even infect people far away. It’s why we thought six-foot distance was sufficient, because that’s as far as large droplets were estimated to travel.
It took 18 months and significant protest by a committed band of scientists before the WHO formally announced that COVID-19 was, in fact, airborne. In his recent book Air-Borne, science writer Carl Zimmer poses a question that should haunt us: Why did it take so long? His account reveals not just a failed pandemic response but a century-old scientific blind spot that has shaped—and misshaped—our understanding of disease transmission.
When researchers William and Mildred Wells proposed in the 1930s that diseases spread through tiny airborne particles, they met fierce resistance from the medical establishment. Even when their protégé Richard Riley’s landmark 1950s research demonstrated that tuberculosis spread through air ducts in a Veterans Affairs hospital, the findings were initially dismissed and buried under the weight of the old orthodoxy. It wasn’t until well into the 1980s and 1990s that near unanimity was reached among major public-health organizations that tuberculosis was airborne—an acknowledgment likely driven by the resurgence of TB during the early HIV/AIDS epidemic in the United States. Even so, TB, along with measles, was treated as an exception to the droplet and direct-contact rule.
This resistance wasn’t mere stubbornness. It was the kind of intellectual inertia that often precedes paradigm shifts in science. Philosopher Thomas Kuhn described these moments as crises, when anomalies accumulate until the old framework can no longer contain them, and a new understanding bursts forth. What makes our current moment extraordinary is that we’re living through such a shift in real time, watching as the edifice of droplet theory crumbles and a new science of airborne transmission emerges from its ruins.
But we might be squandering this opportunity. Airborne-disease research has increased dramatically since the pandemic, but interest and attention have wavered. Funding for aerosol science and innovation, decimated further by recent government cuts, still pales in comparison with its striking disease burden: Respiratory infections are among the leading causes of morbidity and mortality globally. They resulted in 350 million Disability-Adjusted Life Years (DALYs) lost—a measure of years lost due to ill health, disability, or early death—and 11.3 million deaths in 2021 alone. COVID-19 has caused more than 27 million deaths globally, tuberculosis causes 1.6 million deaths every year, and influenza kills 700,000 annually. We now know that these and many other respiratory infections show considerable airborne transmission.
History demonstrates that paradigm shifts have the potential not just to correct errors, but also to unleash waves of innovation. In the late 19th century when germ theory (the understanding that microorganisms cause disease) finally displaced miasma theory (the prevailing belief that illness spread through poisonous vapors from rotting organic matter), medicine was entirely transformed. Suddenly, targeted interventions became possible. Antiseptic surgery, antibiotics, and vaccines all eventually flowed from this new understanding.
Yet innovation doesn’t always wait for paradigm shifts. Sometimes it stumbles forward in the dark, guided by intuition rather than understanding. Before germ theory, reformers drained swamps and improved sanitation—interventions that worked even if the reasoning was wrong. The measles vaccine was developed in 1963, a couple of decades before there was widespread consensus that measles was primarily airborne. These successes were real but haphazard. When the paradigm finally shifts, scattered victories can be systematized, accelerated, and made efficient. What was once serendipity becomes strategy.
Today, having just lived through another one of these shifts, we have yet to apply the full force of 21st-century science to airborne-disease transmission. In 2017, you might find 20 attendees at leading aerobiology gatherings. Today, several global conferences are dedicated to aerobiology and aerosol science, and perhaps a few thousand researchers are making steady progress in aerobiology and adjacent fields. But our understanding remains relatively rudimentary, and the field is still very small relative to its potential impact. We have at best rough estimates of what share of respiratory infections are transmitted via large droplets and fomites versus aerosols. The basic physics of how pathogens move through air, how they survive in different conditions, and how much interventions like ventilation and filtration reduce real-world infections are fundamental questions that remain largely unanswered.
Mounting Evidence
Future historians will likely mark the post-COVID era as a turning point in public health, because the pandemic forced humanity to confront its misconceptions about disease transmission. But they may also marvel at the opportunity we nearly missed: the chance to revolutionize our approach to respiratory diseases that kill millions annually.
For scientists and students, the opportunity is clear: An entire field awaits exploration, with fundamental questions that could shape human health for generations. For funders and governments, the chance to catalyze progress in a neglected but critical area has rarely been more apparent. The tools of modern science can now be brought to bear on questions we didn’t even know to ask five years ago.
The path forward requires concrete action across multiple fronts. Consider far-UVC light, a set of wavelengths that researchers have found to be highly effective at inactivating airborne pathogens without harming humans. Unlike conventional germicidal ultraviolet light, which isn’t safe for direct human exposure, far-UVC isn’t able to penetrate our skin or eyes, meaning it can safely disinfect densely occupied spaces significantly more effectively than ventilation and filters. But limited research budgets and investment have slowed progress, rendering one of the most promising technologies in airborne-disease mitigation frustratingly out of reach for widespread deployment.
Or consider K-12 education, where we’re sitting on one of the most cost-effective health and education interventions discovered and largely ignoring it. When the 2015 Aliso Canyon methane leak in Los Angeles forced the gas company to install air filters in nearby schools, economist Michael Gilraine found something remarkable: Students’ test scores soared. Math scores jumped by 0.20 standard deviations, English scores by 0.18, with gains that persisted and even grew over time. To grasp the magnitude, consider that Tennessee’s famous class-size-reduction experiment achieved similar gains after four years at roughly $7,000 per student in today’s dollars. The air filters? They cost about $30-$50 per student the first year, less than 1 percent of Tennessee’s investment, and then as little as a few dollars per student each year thereafter.
The evidence keeps mounting. A recent randomized controlled trial found that air purifiers cut student absenteeism by 12.5 percent. Even brief exposure matters: Just two hours of breathing filtered air improves executive function scores.
Reducing respiratory infections, allergies, and asthma triggers keeps students and staff healthier and in school. Particulate matter that students and staff inhale crosses into the bloodstream, triggers systemic inflammation, and can even breach the blood-brain barrier. Fewer sick days means more instructional time, less reliance on substitutes, and better instructional continuity. If the Aliso Canyon effect sizes replicate, air cleaning would dramatically outperform celebrated interventions like the Perry Preschool Project, high-dosage tutoring, and Head Start—for a fraction of the cost.
The impact would not be confined to the walls of the classroom. As parents of young children know well, infectious diseases that spread at school quickly make their way home, infecting parents, who go on to infect their colleagues, their friends, and their broader social networks. Evidence is growing that this dynamic means that school-age children are drivers of annual flu epidemics.
Yet walk into most American schools today and you’ll find air filters gathering dust in storage closets, relics of a pandemic that people desperately want to forget.
Three Areas of Focus
The sudden paradigm shift on airborne disease and indoor air quality affords leaders and innovators in global public health, infectious disease, and epidemiology historic opportunities for impact. Three sets of recommendations come to mind.
First, advance the basic science of airborne transmission. Researchers should pursue foundational questions to which we still do not know the answers: How precisely do different pathogens survive in aerosols? How does humidity affect transmission? What rates of air cleaning prevent infection in the real world? If many respiratory infections are primarily airborne, as increasingly appears to be the case, then our entire approach to public health needs rethinking. Hospital infection control, school design, workplace safety, and building codes were all built on the foundation of droplet theory.
Second, invest in developing and researching next-generation air-cleaning technologies. Today’s solutions are often box fans with high-efficiency particulate air (HEPA) filters or simply opening windows—both critical but limited in their effectiveness. We need quieter and more efficient devices. We must ensure that funding for the next generation of air purifiers and promising new technologies like far-UVC is commensurate with their immense potential.
Third, bridge the gap between R&D and real-world deployment. Philanthropists can fund projects and research that demonstrate the health and productivity benefits of clean indoor air in places like schools, nursing homes, and mass-transit facilities. Support organizations advocating for updated building codes. Create prizes for innovation in affordable air-quality monitoring and purification.
We stand at the threshold of what could be the next great leap in our fight against infectious disease. The paradigm has shifted, and old certainties have crumbled. Rarely do we live through an opportunity to build out an entirely new field so consequential for health and well-being. Let us be the generation that discovers how to make sure we all breathe safe air.
Read more stories by David Carel.