Preventing Airborne Spread of Covid-19 and Other Respiratory Diseases

Why it took so long to acknowledge that Covid-19 is airborne—and what we need to do now.

By Michael Eisenstein

On the top floor of the Baltimore Veterans Administration Hospital, Richard Riley and his colleague William Wells were hard at work installing rows of guinea pig cages. The year was 1954, and the two researchers—both infectious disease specialists at the School—were laying the groundwork for a study that would turn established dogma on its head.

The medical world was convinced that tuberculosis was transmitted primarily by surface contamination and person-to-person contact, but Riley and Wells proved otherwise. Up in the hospital penthouse, they assembled ventilation ductwork that delivered air from a TB ward below to the guinea pig cages above. One set of cages received unadulterated air from the ward, while the others received air sterilized by ultraviolet light. In a seminal 1962 publication based on this work, Riley’s team would report 63 cases of infection in the first set of cages—and none in the second.

“That was accepted as a demonstration that [TB] was carried by small aerosol particles in the air,” says Raymond Tellier, MD, MSc, an associate professor on the Infectious Diseases team at McGill University Health Centre. He adds that researchers have subsequently obtained evidence that numerous other common respiratory infections like measles and influenza exhibit similar modes of transmission.

So why were we all wiping down our groceries, hiding behind Plexiglas, and slathering our hands with sanitizer when COVID-19 arrived?

The answer comes down to misconceptions about this disease’s mode of transmission that undermined the global public health response. “The CDC and the WHO insisted for almost a year that the virus wasn’t airborne, or that airborne was not the main route of transmission,” says Ana Rule, PhD ’05, MHS ’98, an assistant professor in Environmental Health and Engineering.


The problem is that many scientists drew the wrong conclusions from Riley and Wells. Their data found that airborne liquid particles 2–5 microns in diameter were the primary vehicle for the spread of TB. But some scientists incorrectly took from these studies that only particles this tiny could be airborne and spread over meaningful distances from an infected individual—a consequential error described in a 2021 Wired article.

A consensus subsequently emerged that most non-TB respiratory pathogens exit the mouths and noses of infected individuals in larger fluid particles classified as “droplets,” a category encompassing any exhaled particle with a diameter larger than 5 microns. These droplets were further presumed to be more strongly subject to the effects of gravity and quickly fall to the ground, rather than being carried aloft over longer ranges like smaller “aerosol” particles. This misunderstanding persisted for six decades and fueled a focus on contaminated surfaces and short-range transmission as primary drivers of how respiratory infections spread.

Getting Particular

»Airborne: Any particles that can float and be borne aloft on air currents

»Aerosol: Infinitesimal particles that can stay aloft even after an infected person has left

»Droplet: Particles of various sizes; larger droplets fall quickly to the ground

This size threshold is nonsensical to researchers specializing in air quality and aerosols. “This idea that droplets larger than 5 microns don’t go anywhere in the air has been thoroughly debunked,” says William Bahnfleth, PhD, a professor of architectural engineering at Penn State University. Indeed, several infectious disease researchers have demonstrated that pathogens including influenza, measles, and even SARS-CoV, the predecessor to the virus that causes COVID-19, can travel over considerable distances, potentially moving from one room—or even one floor—to another if carried by ventilation. Rule conducted one such study just a few years before COVID-19 arrived, examining the contributions of droplet-based spread of influenza in a hospital setting versus airborne spread at a distance. “We tried to evaluate if this close contact was more important than farther away and we found no difference,” says Rule.

This disconnect between assumptions about aerosol behavior and the reality of airborne transmission led to mixed messages at the onset of the pandemic. Most notably, there was an excessive focus on surface hygiene, putting up barriers, and setting distances rather than on masking and how air circulation—or lack thereof—contributes to the spread of SARS-CoV-2.

Today, the importance of airborne transmission is much more widely recognized, thanks to a large number of studies over the past two years that have documented both such spread and the protective efficacy of countermeasures like masks and ventilation. “We know that particles of up to 100 microns can remain in the air and be carried by coughing or talking or even singing at a very appreciable distance well beyond the canonical two meters,” says Tellier. With these insights, researchers are getting a handle on how to predict SARS-CoV-2 transmission risk based on airflow conditions, such as a detailed mathematical model developed recently by a multi-institutional team including Bahnfleth that draws on many of the principles first recognized by Riley and Wells.


The risk of infection is far greater indoors than outside, where moving air typically dilutes and removes virus-laden aerosols shortly after they leave a person’s mouth or nose. Indoors, people are at the mercy of mechanical ventilation systems to achieve regular air circulation.

This importance of robust ventilation is particularly salient in schools, which last year jettisoned remote and hybrid models and fully restored in-person education. “For schools that have mechanical ventilation systems, they should push it to the maximum amount of filtration that the systems will allow, while making plans to improve ventilation and filtration,” says Gigi Gronvall, PhD, an associate professor in Environmental Health and Engineering and a senior scholar at the Center for Health Security, which published a 2021 report on air quality in schools. Importantly, the Biden administration’s American Rescue Plan Act allocated $112 billion to help bolster U.S. schools in their response to and recovery from the pandemic—explicitly including investments for improving air quality.

But people also congregate in offices, restaurants, theaters, and many other indoor locations. With the exception of health care facilities, most of these do not achieve the high rate of air circulation required to keep airborne infection at bay. Bahnfleth believes it simply will not be feasible to bring many of these facilities up to spec without considerable effort and expense. “The vast majority of buildings that are going to be around 30 or 40 years from now already exist,” he says, “and it’s a massive infrastructure redo to change them.”

The good news is that there are affordable countermeasures that can complement inadequate ventilation (see sidebar), including that stalwart of the COVID-19 years: masks.


In the face of growing public pressure, most U.S. states with masking requirements lifted them earlier this year except in limited settings, like hospitals—and even the CDC has shifted policy so that masking is only recommended after hospital occupancy has begun to swell.

If the pandemic had played out differently, this would be fine. But vaccination coverage remains disappointing in the U.S.—as of early April, only 65.7% of Americans were fully vaccinated, and fewer than half of those had received the recommended first booster. This means that masks will remain an important layer of personal defense against the further spread of COVID-19 until improved ventilation becomes a higher priority. “Good ventilation is going to be better than no ventilation, and any mask is better than no mask,” says Rule. “I think we have established that also through this pandemic.”

In the big picture, the importance of improving air quality goes far beyond this pandemic. Good ventilation and filtration could provide a critical line of defense against the seasonal colds that routinely tear through schools and workplaces every winter, as well as toxic pollutants from industrial sources or natural disasters like wildfires. The Center for Health Security’s schools report calls for the creation of a federal task force to tackle this issue, and in mid-March, the Biden administration announced the Clean Air in Buildings Challenge, which aims to combat the spread of infection through improved ventilation. Gronvall sees a historic opportunity for course correction after decades of ignoring airborne sources of health risk. “COVID has really demonstrated that we need to pay more attention to the air we breathe,” she says.