1) Where did the 5-μm cutoff for aerosols in disease transmission come from? (It should really be ~100 μm)

2) Where did the 6' rule come from?

@katierandall @EThomasEwing @jljcolorado Bourouiba & I traced the history in this preprint https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3829873 /1

"tracing the origins of the 5 μm threshold...ultimately revealed a conflation between various understandings and definitions of 'aerosols.' Most contemporary sources use this threshold only to explain which particles stay suspended in the air for longer times,..." /2

"yet the 5 μm distinction is clearly not based on what stays airborne but on what reaches deepest in the lungs...." /3

"It is this conflation of particle transport through the air and particle deposition in the lungs that appears to be the source of the error in distinguishing between droplet and aerosol transmission routes as defined by a 5 μm threshold." /4

Problems: this fosters misunderstanding about infectious particles not remaining airborne and codifies a particle size based on tuberculosis that does not apply to other diseases /5

CDC has historically recommended 3', even for SARS-CoV-1. Based mainly on studies showing bacteria grew on petri dishes up to a certain distance away from a person talking and limitations in visualizing small particles. /6