The gas cloud that forms after a cough or a sneeze is a lot more complicated than originally thought, according to a team of researchers at Massachusetts Institute of Technology (MIT). It's more complicated than merely being filled with contagious germs and viruses that cause colds and influenza. The aerodynamics of these "multiphase turbulent buoyant cloud(s)" are actually opposite of what was expected and this turning upside-down of current thinking reveals these germ-filled gas clouds travel as much as 200 times farther than expected.

The results of the study could lead to a rethinking of ventilation systems, too. The high-speed imaging technologies and mathematical models used in the MIT study clearly demonstrate that gas clouds from coughs and sneezes can travel far enough to reach ventilation systems in schools, hospitals, and many workplaces. Current air circulation systems on airplanes and other modes of public transportation may need a design update, too.

It was originally assumed that the larger particles in a sneeze or cough cloud traveled farther, thanks to the law of momentum (mass times velocity). The MIT study indicates that these heavier mucus molecules actually fall out of the gas cloud first.

The original expectation was based on each expelled droplet behaving independently of the others. The gas cloud is actually quite dynamic, producing its own circulating environment in which different size droplets respond in different ways. When the gas cloud mingles with the air in a room, it grows. The ambient circulation in the room influences the behavior of the now-larger gas cloud.

The smaller droplets in the cloud remain airborne much longer than the larger ones. The smallest droplets were measured in micrometers, with one micrometer representing one-millionth of a meter. Different sizes of these invisible droplets travel at different distances according to their size:


  • 100 micrometers — the droplets travel five times farther than expected
  • 50 micrometers — could remain airborne long enough to enter ventilation units on the ceiling
  • 10 micrometers — travel 200 times farther than expected

In light of these findings, turning one's head away from others when coughing or sneezing may not be courtesy enough. The extreme distances these invisible particles travel make it all the more important to completely cover one's mouth and nose during a cough or a sneeze. It also makes avoiding other people's contagious airborne germs a lot trickier.

Members of the MIT research team are already engaged in further studies of droplet size, distribution, and all elements of the gas cloud. One question they hope to answer is where does the pathogen, the germ, actually end up. Another is how this new understanding may be applied to understanding the patterns of airborne epidemics as they spread across a population.

Source: Dizikes, Peter. "In the cloud: How coughs and sneezes float farther than you think." MIT News. Massachusetts Institute of Technology. Apr 8, 2104. Web. Apr 20, 2014.