Figure 1.

Droplet generation. A flash photo of a human sneeze, showing the expulsion of droplets that may be laden with infectious pathogens. Sneezing can produce as many as 40 000 droplets of 0.5–12 μm.¹⁰⁷ These particles can be expelled at a velocity of 100 m/s,¹⁰⁸ reaching distances of several metres. Smaller droplets with less mass are less influenced by gravity, and can be transported as a ‘cloud’ over greater distances by air flows. Larger droplets with more mass are more strongly influenced by gravity and less so by air flows, and move more ‘ballistically’, falling to the ground more quickly. Reproduced with the kind permission of Prof. Andrew Davidhazy, School of Photographic Arts and Sciences, Rochester Institute of Technology Rochester, NY, USA.