Why sneeze and cough droplets travel farther than classical models predicted

Settled in 2014 by Lydia Bourouiba, Eline Dehandschoewercker, John W. M. Bush (MIT).

Wells's 1934 evaporation-falling curve, which classified respiratory droplets as either falling quickly to the ground or evaporating into tiny airborne nuclei, became the textbook basis for disease-transmission distance estimates for 80 years, yet it consistently underpredicted how far droplets from real coughs and sneezes could travel. Bourouiba, Dehandschoewercker and Bush filmed and modeled real sneezes and coughs, showing that expelled droplets do not fall independently but are carried within a fast-moving, multiphase turbulent gas cloud that keeps small droplets suspended and moving far longer than Wells's simple ballistic-evaporation picture allowed, extending effective transmission range well beyond earlier estimates. This cloud-dynamics model became especially influential during the COVID-19 pandemic for reassessing safe-distance guidelines.

A single sneeze is not a spray of independent droplets falling one by one, but a coherent turbulent gas cloud that carries its cargo of droplets like a tiny weather system.

Explanations that circulated

• Each droplet from a sneeze falls and evaporates independently, following simple ballistic trajectories as in Wells's original two-category (large droplet vs. droplet nuclei) model
• Sneeze/cough transmission range depends only on room humidity and droplet size, with no role for the exhaled air itself

References

• Violent expiratory events: on coughing and sneezing, Lydia Bourouiba, Eline Dehandschoewercker, John W. M. Bush, Journal of Fluid Mechanics, 2014