Wells curve

Not to be confused with Wells-Riley model, which considers how airborne infectious particles can cause new infections..

The Wells curve (or Wells evaporation falling curve of droplets) is a diagram, developed by W. F. Wells in 1934, which describes what is expected to happen to small droplets once they have been exhaled into air. Coughing, sneezing, and other violent exhalations produce high numbers of respiratory droplets derived from saliva and/or respiratory mucus, with sizes ranging from about 1 μm to 2 mm. Wells' insight was that such droplets would have two distinct fates, depending on their sizes. The interplay of gravity and evaporation means that droplets larger than a humidity-determined threshold size would fall to the ground due to gravity, while droplets smaller than this size would quickly evaporate, leaving a dry residue that drifts in the air. Since droplets from an infected person may contain infectious bacteria or viruses, these processes influence transmission of respiratory diseases.

A traditional hard size cutoff of 5 μm between airborne and respiratory droplets has been criticized as a false dichotomy not grounded in science, as exhaled particles form a continuum of sizes whose fates depend on environmental conditions in addition to their initial sizes. However, it has informed hospital based transmission based precautions for decades.

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