Respiratory particles can be divided into “droplets” and “aerosols” using one fixed particle-size cutoff.

For decades, infection-control guidance often sorted particles breathed, coughed, or sneezed from the respiratory tract into two groups.

Larger “droplets” were expected to fall quickly to nearby surfaces. Smaller “aerosols” were expected to remain suspended in the air and travel farther. A particular particle diameter was often used as the dividing line.

The problem was that there was no single agreed line. Different health agencies, medical traditions, and aerosol scientists used different size cutoffs.

The physics is also more gradual than the two-box picture suggests. A respiratory particle can evaporate after leaving the mouth or nose and become smaller. How long it stays airborne depends not only on its diameter but also on airflow, humidity, air movement, and the speed and direction with which it was emitted.

COVID-19 made these differences in terminology difficult to ignore because clinicians, engineers, aerosol scientists, and public-health agencies were often using the same words in different ways.

A WHO-led consultation published revised terminology in 2024. It treats infectious respiratory particles as a continuous spectrum and describes their behavior without relying on one universal droplet-versus-aerosol size boundary.

The older words can still be useful in context. What was superseded was the idea that one fixed diameter cleanly separates two physically distinct kinds of respiratory particle.