See this Wired article.
TLDR: WHO and CDC guidance about COVID (and flu, for that matter) has been based on it being spread by large (over 5 microns) droplets that travel only a few feet, not by smaller (under 5 microns) aerosol particles that can hang in the air for minutes or hours, and travel hundreds of feet. But in fact, particles larger than 5 microns can hang in the air for minutes or hours, and travel hundreds of feet; a better dividing line for that purpose would be 100 microns, and there was research to that effect over 80 years ago. It turns out the 5-micron threshold comes from somebody in the 1950's writing about tuberculosis, which (unlike most infectious agents) is only dangerous if it gets deep into the lungs, and the nose filters out most particles over 5 microns, preventing them from getting deep into the lungs. There was never any evidence that 5 microns was a meaningful distinction between "droplets" and "aerosols". One researcher in the 1960's conflated several unrelated statements by another researcher, and we've been giving bad public-health advice based on that mistake ever since, affecting not only COVID, but influenza and other infectious diseases.

For related thoughts on chasing down the history of statements in academic research, see "The Problem of Ruritanian Purple Feathers", Tournaments Illuminated 77 (Winter 1985), reprinted in Knowne Worlde Handbook but apparently not on the Web anywhere.
TLDR: WHO and CDC guidance about COVID (and flu, for that matter) has been based on it being spread by large (over 5 microns) droplets that travel only a few feet, not by smaller (under 5 microns) aerosol particles that can hang in the air for minutes or hours, and travel hundreds of feet. But in fact, particles larger than 5 microns can hang in the air for minutes or hours, and travel hundreds of feet; a better dividing line for that purpose would be 100 microns, and there was research to that effect over 80 years ago. It turns out the 5-micron threshold comes from somebody in the 1950's writing about tuberculosis, which (unlike most infectious agents) is only dangerous if it gets deep into the lungs, and the nose filters out most particles over 5 microns, preventing them from getting deep into the lungs. There was never any evidence that 5 microns was a meaningful distinction between "droplets" and "aerosols". One researcher in the 1960's conflated several unrelated statements by another researcher, and we've been giving bad public-health advice based on that mistake ever since, affecting not only COVID, but influenza and other infectious diseases.

For related thoughts on chasing down the history of statements in academic research, see "The Problem of Ruritanian Purple Feathers", Tournaments Illuminated 77 (Winter 1985), reprinted in Knowne Worlde Handbook but apparently not on the Web anywhere.