Installing air samplers in places such as day cares and airplanes, she frequently found the flu virus where the textbooks said it shouldnвЂ™t be-hiding in the air, most often in particles small enough to stay aloft for hours. Marr decided to collect some data of her own. The error meant that the medical community had a distorted picture of how people might get sick. вЂњIвЂ™d see the wrong number over and over again, and I just found that disturbing,вЂќ she says. Reality is far messier, with particles much larger than 5 microns staying afloat and behaving like aerosols, depending on heat, humidity, and airspeed. That much seemed obvious to her from everything she knew about how things move through air.

There was just one literally tiny problem: вЂњThe physics of it is all wrong,вЂќ Marr says. The WHO and the US Centers for Disease Control and Prevention also listed 5 microns as the fulcrum on which the droplet-aerosol dichotomy toggled. The more she looked, the more she found that number. By this definition, any infectious particle smaller than 5 microns in diameter is an aerosol anything bigger is a droplet. A micron is a unit of measurement equal to one-millionth of a meter. The books Marr flipped through drew the line between droplets and aerosols at 5 microns. Trapped in their group-specific jargon, the two camps on Zoom literally couldnвЂ™t understand one another. To them, the word airborne only applied to particles smaller than 5 microns. Now, though, the WHOвЂ™s advisers seemed to be saying those same laws didnвЂ™t apply to virus-laced respiratory particles. When it came to flecks of soot and ash belched out by smokestacks and tailpipes, the organization readily accepted the physics she was describing-that particles of many sizes can hang aloft, travel far, and be inhaled. Morawska had spent more than two decades advising a different branch of the WHO on the impacts of air pollution. вЂњYou just donвЂ™t argue with Lidia about physics,вЂќ she says. One of the WHO experts abruptly cut her off, telling her she was wrong, Marr recalls. At one point, Lidia Morawska, a revered atmospheric physicist who had arranged the meeting, tried to explain how far infectious particles of different sizes could potentially travel. They were trying to warn the WHO it was making a big mistake.

Just days before, the organization had tweeted вЂњFACT: #COVID19 is NOT airborne.вЂќ ThatвЂ™s why Marr was skipping her usual morning workout to join 35 other aerosol scientists.

But the WHO didnвЂ™t seem to have caught on. For people indoors, that posed a considerable risk. To her, the new coronavirus looked as if it could hang in the air, infecting anyone who breathed in enough of it. Marr is an aerosol scientist at Virginia Tech and one of the few in the world who also studies infectious diseases. It was just past 1 pm Geneva time on April 3, 2020, but in Blacksburg, Virginia, where Marr lives with her husband and two children, dawn was just beginning to break. She also saw a few people she didnвЂ™t know, including Maria Van Kerkhove, the World Health OrganizationвЂ™s technical lead for Covid-19, and other expert advisers to the WHO. On her computer screen, dozens of familiar faces began to appear. Early one morning, Linsey Marr tiptoed to her dining room table, slipped on a headset, and fired up Zoom.

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