This error caused the medical community to have a distorted picture of how people spread Covid.
first many years of her career studying air pollution, just as Morawska had. But her priorities began to change in the late 2000s, when Marr sent her oldest child off to day care. That winter, she noticed how waves of runny noses, chest colds, and flu swept through the classrooms, despite the staff’s rigorous disinfection routines. “Could these common infections actually be in the air?” she wondered. Marr picked up a few introductory medical textbooks to satisfy her curiosity.
There was just one literally tiny problem: “The physics of it is all wrong,” Marr says. That much seemed obvious to her from everything she knew about how things move through air. Reality is far messier, with particles much larger than 5 microns staying afloat and behaving like aerosols, depending on heat, humidity, and airspeed. “I’d see the wrong number over and over again, and I just found that disturbing,” she says.
Thinking it might help her overcome this resistance, she’d try from time to time to figure out where the flawed 5-micron figure had come from. But she always got stuck. The medical textbooks simply stated it as fact, without a citation, as if it were pulled from the air itself. Eventually she got tired of trying, her research and life moved on, and the 5-micron mystery faded into the background. Until, that is, December 2019, when a paper crossed her desk from the lab of Yuguo Li.
Scientists use a rotating drum to aerosolize viruses and study how well they survive under different conditions.Marr told him about her concerns with the 5-micron boundary and suggested that their two issues might be linked. If the 6-foot guideline was built off of an incorrect definition of droplets, the 5-micron error wasn’t just some arcane detail. It seemed to sit at the heart of the WHO’s and the CDC’s flawed guidance. Finding its origin suddenly became a priority.
She tried another tack. Everyone agreed that tuberculosis was airborne. So she plugged “5 microns” and “tuberculosis” into a search of the CDC’s archives. She scrolled and scrolled until she reached the earliest document on tuberculosis prevention that mentioned aerosol size. It cited an out-of-print book written by a Harvard engineer named William Firth Wells. Published in 1955, it was calledIn the Before Times, she would have acquired the book through interlibrary loan.
But at the same time, Langmuir was growing increasingly preoccupied by the threat of biological warfare. He worried about enemies carpeting US cities in airborne pathogens. In March 1951, just months after the start of the Korean War, Langmuir published a report in which he simultaneously disparaged Wells’ belief in airborne infection and credited his work as being foundational to understanding the physics of airborne infection.
The groundbreaking results were published in 1962. Wells died in September of the following year. A month later, Langmuir mentioned the late engineer in a speech to public health workers. It was Wells, he said, that they had to thank for illuminating their inadequate response to a growing epidemic of tuberculosis. He emphasized that the problematic particles—the ones they had to worry about—were smaller than 5 microns.
The news made headlines. And it provoked a strong backlash. Prominent public health personalities rushed to defend the WHO. Twitter fights ensued. Saskia Popescu, an infection-prevention epidemiologist who is now a biodefense professor at George Mason University, was willing to buy the idea that people were getting Covid by breathing in aerosols, but only at close range. That’s not airborne in the way public health people use the word.
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