Respiratory droplet

A respiratory droplet is a small aqueous droplet produced by exhalation, consisting of saliva or mucus and other matter derived from respiratory tract surfaces. Respiratory droplets are produced naturally as a result of breathing, speaking, sneezing, coughing, or vomiting, so they are always present in our breath, but speaking and coughing increases their number. Droplet sizes range from < 1 µm to 1000 µm, and in typical breath there are around 100 droplets per litre of breath. So for a breathing rate of 10 litres per minute this means roughly 1000 droplets per minute, the vast majority of which are a few micrometres across or smaller. As these droplets are suspended in air, they are all by definition aerosols. However, large droplets (larger than about 100 µm, but depending on conditions) rapidly fall to the ground or another surface and so are only briefly suspended, while droplets much smaller than 100 µm (which is most of them) fall only slowly and so form aerosols with lifetimes of minutes or more. As the droplets are so small they dry rapidly once in the surrounding air, shrink, and therefore remain suspended for a longer time