Sneezing is something everybody does and it's easy to forget that the violent ejection of bodily fluids that occurs when we sneeze is an almost perfect way for us to unwittingly spread our contagious diseases.

To better understand how this process works – the physics of a sneeze, if you will – scientists in the US have filmed high-speed videos of two people sneezing about 50 times. What they've found is something we didn't know before: sneeze droplets are formed within a "high-propulsion sneeze cloud" outside of the mouth and the respiratory tract.

"Droplets are not all already formed and neatly distributed in size at the exit of the mouth, as previously assumed in the literature," said Lydia Bourouiba, head of the Fluid Dynamics of Disease Transmission Laboratory at the Massachusetts Institute of Technology (MIT).

Rather, droplets churn through the air in a complex cascading breakup after they've exited the respiratory tract and passed over the lips. In addition to filming video to capture the sneeze dynamics, the researchers also used data extraction algorithms and 3D visualisation techniques to understand how our snot and saliva fly through the air.

But why go to such trouble to chart sneeze formation? Well, there's actually a pretty good reason. Infectious diseases like measles, influenza, or SARS can all be spread through sneezing, with the viruses suspended in droplets that can be inhaled, swallowed, or deposited onto surfaces within a room or outside.

While scientists don't know for sure how far a sneeze can spread, Bourouiba and her team discovered last year that sneezes can transport pathogen-carrying droplets much further than had previously been suspected – and not only in larger droplets that you can see or feel. Over the course of several minutes, smaller droplets hanging in the air in gas form can travel the length of a room and even reach ventilation ducts at ceiling heights.

"This is a major blind spot when designing public health control and prevention policies, particularly when urgent measures are needed during epidemics or pandemics," said Bourouiba.

The new research was presented this month at the American Physical Society's annual meeting of the Division of Fluid Dynamics, and is set to be published in Experiments in Fluids.