Respiratory droplet

Respiratory droplets from humans include various cells types (e.g. epithelial cells and cells of the immune system), physiological electrolytes contained in mucous and saliva (e.g. Na⁺, K⁺, Cl⁻), and, potentially, various pathogens.[6]

Droplets that dry in the air become droplet nuclei which float as aerosols and can remain suspended in air for considerable periods of time.[6]

The probability density function for droplets in the breath of someone speaking, as a function of diameter. Note that both axes are log scales, we breathe out droplets ranging in size from less than a micrometre to around a millimetre, and that we breathe out many more droplets around a micrometre across than larger droplets. Only the largest droplets, around a millimetre in size are visible, we cannot see the smaller ones.

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.[7]

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.[6] Similar droplets may be formed through vomiting, flushing toilets, wet-cleaning surfaces, showering or using tap water, or spraying graywater for agricultural purposes.[8]

Depending on the method of formation, respiratory droplets may also contain salts, cells, and virus particles.[6] In the case of naturally produced droplets, they can originate from different locations in the respiratory tract, which may affect their content.[8] There may also be differences between healthy and diseased individuals in their mucus content, quantity, and viscosity that affects droplet formation.[9]

Human cough: effect of wind speed on the transport of respiratory droplets.[10]

Different methods of formation create droplets of different size and initial speed, which affect their transport and fate in the air. As described by the Wells curve, the largest droplets fall sufficiently fast that they usually settle to the ground or another surface before drying out, and droplets smaller than 100 μm will rapidly dry out, before settling on a surface.[6][8] Once dry, they become solid droplet nuclei consisting of the non-volatile matter initially in the droplet. Respiratory droplets can also interact with other particles of non-biological origin in the air, which are more numerous than them.[8] When people are in close contact, liquid droplets produced by one person may be inhaled by another person; droplets larger than 10 μm tend to remain trapped in the nose and throat while smaller droplets will penetrate to the lower respiratory system.[9]

Advanced Computational Fluid Dynamics (CFD) showed that at wind speeds varying from 4 to 15 km/h, respiratory droplets may travel up to 6 meters. [10][11]

Illustration of a respiratory droplet, showing mucins (green), surfactant proteins and lipids (blue) and a coronavirus particle (pink)

A common form of disease transmission is by way of respiratory droplets, generated by coughing, sneezing, or talking. Respiratory droplet transmission is the usual route for respiratory infections. Transmission can occur when respiratory droplets reach susceptible mucosal surfaces, such as in the eyes, nose or mouth. This can also happen indirectly via contact with contaminated surfaces when hands then touch the face. Respiratory droplets are large and cannot remain suspended in the air for long, and are usually dispersed over short distances.[12]

Viruses spread by droplet transmission include influenza virus, rhinovirus, respiratory syncytial virus, enterovirus, and norovirus;[13] measles morbillivirus;[14] and coronaviruses such as SARS coronavirus (SARS-CoV-1)[13][14] and SARS-CoV-2 that causes COVID-19.[15][16] Bacterial and fungal infection agents may also be transmitted by respiratory droplets.[6] By contrast, a limited number of diseases can be spread through airborne transmission after the respiratory droplet dries out.[14] We all continuously breathe out these droplets, but in addition some medical procedures called aerosol-generating medical procedures also generate droplets.[6]

Ambient temperature and humidity affect the survivability of bioaerosols because as the droplet evaporates and becomes smaller, it provides less protection for the infectious agents it may contain. In general, viruses with a lipid envelope are more stable in dry air, while those without an envelope are more stable in moist air. Viruses are also generally more stable at low air temperatures.[8]

World-War-II-era UK public-health-education poster.

German bacteriologist Carl Flügge in 1899 was the first to show that microorganisms in droplets expelled from the respiratory tract are a means of disease transmission. In the early 20th century, the term Flügge droplet was sometimes used for particles that are large enough to not completely dry out, roughly those larger than 100 μm.[22]

Flügge's concept of droplets as primary source and vector for respiratory transmission of diseases prevailed into the 1930s until William F. Wells differentiated between large and small droplets.[11][23] He developed the Wells curve, which describes how the size of respiratory droplets influences their fate and thus their ability to transmit disease.[24]

• Basic reproduction number
• Source control (respiratory disease)