The short of it is that there was some early research done in TB transmission and they found that the nose was really good at blocking particles larger than 5 m

The short of it is that there was some early research done in TB transmission and they found that the nose was really good at blocking particles larger than 5 microns but it showed that 100 micron particles would be suspended in the air as an aerosol and there was subsequent research that used the 5 microns as gospel for aerosol cutoff even though particles greater than that could be considered aerosol and that cutoff stuck.