With spring comes hay fever, which means lots of people sneezing all around you (and in some cases, all over you). If you'd prefer not to be covered in disgusting snot droplets, you need to keep your distance from allergy sufferers — but how close is too close?
Snot picture from Shutterstock
During a good sneeze, up to 40,000 droplets of saliva may be expelled from the throat and some of them fly out of the mouth at speeds of up to over 60,000 kilometers per hour.
But how far do they actually travel? According to researchers at the Department of Mechanical Engineering in the University of Hong Kong, droplet nuclei expelled from the mouth through coughing or sneezing are capable of travelling surprising distances in indoor environments.
Our study reveals that for respiratory exhalation flows, the sizes of the largest droplets that would totally evaporate before falling 2 m away are between 60 and 100 microm, and these expelled large droplets are carried more than 6 m away by exhaled air at a velocity of 50 m/s (sneezing), more than 2 m away at a velocity of 10 m/s (coughing) and less than 1 m away at a velocity of 1 m/s (breathing).
In other words, you're probably not safe from a sneeze even if you're on the other side of the room. (The Asian practice of wearing face masks in public is starting to make a lot of sense.)
Naturally, the motion of the droplet is determined by the forces acting upon it, including gravity, buoyancy and drag. Smaller droplets tend to fall slower and follow the streamlines created by the exhaled air.
If you've ever wondered how big sneeze droplets can get, science is currently on the case. In a recent experiment, researchers from Tsinghua University in Beijing measured the size distribution of sneeze droplets using a laser particle size analyser.
Unlike the droplets exhaled by cough and speech, two types of distributions were observed for sneeze droplets: unimodal and bimodal distributions:
The result shows that the size of sneeze droplets of the unimodal distribution is significantly larger than that of cough and speech, whereas similarity can be found between the bimodal distribution and that of other respiratory activities. The results shown in this work can be a good help especially for the simulation studies on disease transmission through direct and indirect contacts.
Characterizations of particle size distribution of the droplets exhaled by sneeze [Journal of the Royal Society Interface]