Epidemiologists have long observed that most respiratory bugs require close contact to spread. Yet in this small space, a lot can happen. A sick person may cough droplets on your face, emit small aerosols that you inhale, or shake your hand, which you then use to rub your nose. Any of these mechanisms can transmit the virus. “Technically, it’s very difficult to separate them and see which one is causing the infection,” says Marr. For long-range infections, only the smallest particles might be to blame. Up close, however, particles of all sizes were involved. Yet, for decades, droplets were seen as the primary culprit.
Marr decided to collect his own data. Installing air samplers in places such as daycares and airplanes, she frequently found the flu virus where textbooks said it shouldn’t be – hiding in the air, most often in particles small enough to stay in the air for hours. And there was enough of it to make people sick.
In 2011, that should have been major news. Instead, major medical journals rejected his manuscript. Even as she conducted new experiments that added evidence for the idea that the flu infects people through aerosols, only one niche editor, The Journal of the Royal Society interface, was always receptive to her work. In the compartmentalized world of academia, aerosols have always been the domain of engineers and physicists, and pathogens a purely medical concern; Marr was one of the few people who tried to bridge the gap. “I was definitely marginal,” she says.
Thinking that it might help her overcome this resistance, she occasionally tried to figure out where the faulty 5 micron number was coming from. But she always got stuck. The medical textbooks simply stated it as a fact, without a quote, as if it was taken from the air itself. Eventually, she got tired of trying, her research and her life evolved, and the mystery of 5 microns faded into the background. Until December 2019, when a paper passed through his office from Yuguo Li’s lab.
An indoor air researcher at the University of Hong Kong, Li made a name for himself during the first SARS outbreak in 2003. His investigation into an outbreak in the Amoy Gardens apartment complex provided the strongest evidence that a coronavirus could be airborne. But in the decades that followed, he had also struggled to convince the public health community that their risk calculation was off. Finally, he decided to do some math. Li’s stylish simulations showed that when a person coughed or sneezed, the large droplets were too few, and the targets – an open mouth, nostrils, eyes – too small to account for much infection. Li’s team therefore concluded that the public health facility was behind schedule and that most colds, flu, and other respiratory illnesses must instead be spread by aerosols.
Their findings, they argued, revealed the error of the 5 micron limit. And they had gone further, tracing the issue back to a decades-old document the CDC released for hospitals. Marr couldn’t help but feel a wave of excitement. A newspaper had asked her to review Li’s article, and she didn’t hide her feelings as she sketched out her response. On January 22, 2020, she wrote: “This work is extremely important to challenge existing dogma about how infectious diseases are transmitted through droplets and aerosols.”
Even as she wrote her note, the implications of Li’s work were far from theoretical. Hours later, Chinese government officials halted all travel in and out of the city of Wuhan, in a desperate attempt to contain a still-unknown respiratory disease plaguing the 11 million megalopolis. inhabitants. As the pandemic has halted country after country, the WHO and CDC have told people to wash their hands, scrub surfaces and maintain social distancing. They didn’t say anything about the masks or the dangers of being inside.
