The original aerosol research was performed in the 1930s and the 100 micron number was correctly cited for a bit.

Years later, some research into TB discovered only the smallest particles, 5 microns and smaller, could get deep into the lungs and cause TB.

A paper mistakenly used the 5 micron number as the max size of droplets that remain in the air and other papers in public health repeated the mistake.

What's interesting is that the mistake only propagated among public health researchers. Aerosol physicists were using the right number all along and it was only when aerosol physicists started looking at SARS-CoV-2 that they noticed public health was using the wrong number.

Turns out, public health was never in the habit of talking to aerosol physicists and aersol physicists were mainly focused on pollution. It was just a weird case of siloing that the experts didn't talk to each other until a pandemic forced them to.