The Flu May Linger in the Air, Just Like the Coronavirus

While good ventilation can dilute aerosols, it is far less effective against droplets, which are much wider and heavier — in the same way that a passing breeze would perturb the trajectory of a Ping-Pong ball, but not a cannonball.

The study points to a more “important role” for aerosolized flu transmission than some might assume, Dr. Marr said.

Determining the exact size of that role, however, is another matter entirely. “It’s very hard to conduct these human challenge studies and separate the different modes of transmission,” Dr. Marr said. That problem applies across respiratory viruses, including the coronavirus.

Part of the problem is the continuum on which aerosols and droplets exist. Though they go by different names, the two categories really belong to the same group: globs of fluid that come in varying sizes. Blobs less than five micrometers in diameter are termed aerosols, which can exit the airway at the slightest breath and waft away; anything larger is a droplet, hefty enough to fall to the ground within a few feet of its source. The boundary between them is somewhat arbitrary, though generally speaking, the smaller the particle, the farther it travels.

When people expel fluid from their airway, it tends to manifest in a mixture, some bigger, some smaller and everything in between, said Seema Lakdawala, who studies influenza transmission at the University of Pittsburgh.

Even after they exit an individual, these fluidic blobs remain dynamic. Large droplets, for instance, can disperse or evaporate into little aerosols in midair. Others might scatter onto a surface or a hand, lingering for minutes or hours before encountering someone new. And the rates at which all these events occur can shift, depending on the force with which someone, maybe a loud talker, expels these droplets or the amount of air flow in an area, Dr. Lakdawala said.

“Everyone thinks transmission is a very binary concept,” she added. “The reality is that there is a continuum of aerosols.”

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