The Couple Who Got It Right, and Then Were Forgotten

William Firth Wells was an engineer at Harvard. His wife, Mildred Weeks Wells, was a physician. In the 1930s the two of them became fascinated by a deceptively simple question: when a person exhales, coughs, or speaks, what actually happens to the tiny droplets they release into the air?

Two forces act on any such particle. Gravity pulls it down. Evaporation shrinks it, making it lighter and letting it drift. The Wellses sampled air and plotted a curve of how those opposing forces play out, making it possible to predict how long a particle of any size stays aloft.

The answer they published in 1934 was clear. Particles larger than about 100 microns fall to the ground within seconds. Anything smaller can stay suspended, drifting on air currents, traveling well beyond the person who exhaled it.

That number, 100 microns, is the real threshold between a droplet that drops and an aerosol that lingers. It was measured, published, and correct. Within a few decades it would effectively vanish from public health guidance.

How 100 Microns Became 5

Wells spent much of his career studying tuberculosis, then still widely believed to spread by droplets. In 1962 he published evidence that TB is genuinely airborne. He died the following year.

Here is where the story turns. Wells's tuberculosis findings involved particles small enough to reach deep into the lungs, a range around five microns. That figure described how TB establishes infection in the deep lung. It was never meant to define what stays airborne. But after Wells died, the distinction blurred, and five microns quietly took the place of his 100-micron threshold as the general definition of "airborne."

Three separate ideas, what is small enough to breathe deep into the lungs, what stays suspended in air, and what causes infection, collapsed into a single number. Repeated in guidance documents and cited by one paper after another, the error sank deeper into medical orthodoxy with every retelling. With Wells gone, there was no one left to press the point.

The consequence was enormous. If only particles under five microns stay airborne, almost everything a person exhales appears to fall harmlessly to the floor within a couple of meters. Airborne transmission looks like a rare exception, and guidance built on that assumption emphasizes distance and surfaces while treating the air as a minor concern.

A Graduate Student Finds the Footnote That Broke Everything

When COVID-19 arrived, aerosol scientists watched official guidance repeat what they knew to be physically wrong. In its July 2020 brief on transmission, the World Health Organization drew the line between droplets that fall and aerosols that remain airborne at five microns and a distance of one to two meters. The Wellses had shown nearly ninety years earlier that the real figure was around 100 microns.

Environmental engineer Linsey Marr and her colleagues set out to find where the wrong number came from. The search fell to Katie Randall, a graduate student working with historian Tom Ewing. She went into the archives and searched two terms together: five microns, and tuberculosis. The trail led back through decades of citations to Wells, and to the moment his TB work had been flattened into a universal rule.

Marr, atmospheric chemist Jose-Luis Jimenez, and more than two hundred other scientists signed an open letter urging health authorities to acknowledge airborne spread. What eventually landed was a single slide showing the path of a five-micron particle released at mouth height. Under the old assumption it should have dropped harmlessly nearby. Instead it traveled hundreds of feet past the familiar six-foot rule. Speaking at Harvard Medical School soon after, Anthony Fauci acknowledged the five-micron distinction had been wrong for years.

We Built Our Buildings Around the Wrong Number

This is not a footnote. That number shaped how hospitals were designed, how ventilation codes were written, and what ordinary people were told to do to protect themselves. Six feet of distance, hand sanitizer, wiped surfaces: all reasonable measures, all built on the premise that the air between people was mostly safe.

Correct the number and the picture changes. Far more of what we exhale stays airborne than the old rule allowed. It travels farther and lingers longer, and a room can hold infectious material long after the person who released it has gone. Measles can remain in the air of a room for up to two hours after an infected person leaves.

For a single virus particle, roughly 0.1 microns across, a conventional filter is not much of an obstacle. The air moves and the particle moves with it, through the ductwork and into the next room. The science has been corrected. Our buildings mostly have not.

Designing for the Air We Actually Breathe

If pathogens travel farther and linger longer than we assumed for sixty years, then a defense that guards only the space around a device, or removes only particles above a certain size, is answering the old question. Treating the air properly means treating all of it, continuously, and destroying what is in it.

That is what ViroZap was built to do. It is an FDA 510(k) cleared air disinfection and detoxification system developed by Dr. D. Yogi Goswami, a USF Distinguished University Professor and ASHRAE Fellow with 423 published papers and 43 patents, alongside Lovely Goswami, with over 40 years in air purification research. It installs into a building's existing HVAC system and works on the air everywhere the air goes.

The Number Has Been Corrected. The Buildings Have Not.

There is something humbling about this story, not because anyone acted in bad faith, but because of how ordinary the failure was. A careful measurement was made. A second, separate finding was made by the same researcher. Someone conflated them, everyone else cited the conflation, and a small error repeated in good faith for sixty years changed how the world thought about the air it breathes. The record has since been set straight. What has not caught up is the built environment.

The air in a room was never as safe as the textbooks implied. It took a pandemic and a graduate student in an archive to prove it. The question now is what we do with the corrected number.

The science caught up. It is time our buildings did too.