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 (speech droplet), sneezing, coughing, or vomiting, so they are always present in our breath, but speaking and coughing increase 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, or at intermediate size, may initially travel like aerosols but at a distance fall to the ground like droplets ("jet riders").

These droplets can contain infectious bacterial cells or virus particles they are important factors in the transmission of respiratory diseases. In some cases, in the study of disease transmission a distinction between what are called "respiratory droplets" and what are called "aerosols" is made, with only larger droplets referred to as "respiratory droplets" and smaller ones referred to as "aerosols" but this arbitrary distinction has never been supported experimentally or theoretically, and is not consistent with the standard definition of an aerosol.

Respiratory droplets from humans include various cells types (e.g. epithelial cells and cells of the immune system), physiological electrolytes contained in mucus and saliva (e.g. Na⁺, K⁺, Cl⁻), and, potentially, various pathogens.

Droplets that dry in the air become droplet nuclei which float as aerosols and can remain suspended in air for considerable periods of time.

The 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.

Respiratory droplets can be produced in many ways. They can be produced naturally as a result of breathing, talking, sneezing, coughing, or singing. They can also be artificially generated in a healthcare setting through aerosol-generating procedures such as intubation, cardiopulmonary resuscitation (CPR), bronchoscopy, surgery, and autopsy. Similar droplets may be formed through vomiting, flushing toilets, wet-cleaning surfaces, showering or using tap water, or spraying graywater for agricultural purposes.

Depending on the method of formation, respiratory droplets may also contain salts, cells, and virus particles. In the case of naturally produced droplets, they can originate from different locations in the respiratory tract, which may affect their content. There may also be differences between healthy and diseased individuals in their mucus content, quantity, and viscosity that affects droplet formation.