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A respiratory droplet is a small aqueous droplet produced by exhalation, consisting of saliva or
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...
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 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
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometr ...
l cells or
virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsk ...
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.


Description

Respiratory droplets from humans include various cells types (e.g.
epithelial cells Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercell ...
and cells of the immune system), physiological electrolytes contained in mucous and saliva (e.g. Na+, K+, Cl), and, potentially, various
pathogens In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a ger ...
. 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 A false dilemma, also referred to as false dichotomy or false binary, is an informal fallacy based on a premise that erroneously limits what options are available. The source of the fallacy lies not in an invalid form of inference but in a false ...
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.


Formation

Respiratory droplets can be produced in many ways. They can be produced naturally as a result of breathing, talking,
sneezing A sneeze (also known as sternutation) is a semi-autonomous, convulsive expulsion of air from the lungs through the nose and mouth, usually caused by foreign particles irritating the nasal mucosa. A sneeze expels air forcibly from the mouth and ...
,
cough A cough is a sudden expulsion of air through the large breathing passages that can help clear them of fluids, irritants, foreign particles and microbes. As a protective reflex, coughing can be repetitive with the cough reflex following three ph ...
ing, or
singing Singing is the act of creating musical sounds with the voice. A person who sings is called a singer, artist or vocalist (in jazz and/or popular music). Singers perform music (arias, recitatives, songs, etc.) that can be sung with or with ...
. They can also be artificially generated in a healthcare setting through aerosol-generating procedures such as
intubation Intubation (sometimes entubation) is a medical procedure involving the insertion of a tube into the body. Patients are generally anesthetized beforehand. Examples include tracheal intubation, and the balloon tamponade with a Sengstaken-Blake ...
,
cardiopulmonary resuscitation Cardiopulmonary resuscitation (CPR) is an emergency procedure consisting of chest compressions often combined with artificial ventilation in an effort to manually preserve intact brain function until further measures are taken to restore spon ...
(CPR),
bronchoscopy Bronchoscopy is an endoscopic technique of visualizing the inside of the airways for diagnostic and therapeutic purposes. An instrument (bronchoscope) is inserted into the airways, usually through the nose or mouth, or occasionally through a trac ...
, surgery, and autopsy. Similar droplets may be formed through
vomiting Vomiting (also known as emesis and throwing up) is the involuntary, forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose. Vomiting can be the result of ailments like food poisoning, gastroenteri ...
, flushing toilets, wet-cleaning surfaces, showering or using
tap water Tap water (also known as faucet water, running water, or municipal water) is water supplied through a tap, a water dispenser valve. In many countries, tap water usually has the quality of drinking water. Tap water is commonly used for drinking, ...
, or spraying
graywater Greywater (or grey water, sullage, also spelled gray water in the United States) refers to domestic wastewater generated in households or office buildings from streams without fecal contamination, i.e., all streams except for the wastewater fro ...
for agricultural purposes. Depending on the method of formation, respiratory droplets may also contain
salts In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively c ...
, cells, and
virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsk ...
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 The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...
that affects droplet formation.


Transport

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. 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. 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 The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies ...
. Advanced
Computational Fluid Dynamics Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid flows. Computers are used to perform the calculations required to simulate ...
(CFD) showed that at wind speeds varying from 4 to 15 km/h, respiratory droplets may travel up to 6 meters.


Role in disease transmission

A common form of
disease transmission In medicine, public health, and biology, transmission is the passing of a pathogen causing communicable disease from an infected host individual or group to a particular individual or group, regardless of whether the other individual was previo ...
is by way of respiratory droplets, generated by coughing,
sneezing A sneeze (also known as sternutation) is a semi-autonomous, convulsive expulsion of air from the lungs through the nose and mouth, usually caused by foreign particles irritating the nasal mucosa. A sneeze expels air forcibly from the mouth and ...
, 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. Viruses spread by droplet transmission include
influenza virus ''Orthomyxoviridae'' (from Greek ὀρθός, ''orthós'' 'straight' + μύξα, ''mýxa'' 'mucus') is a family of negative-sense RNA viruses. It includes seven genera: ''Alphainfluenzavirus'', ''Betainfluenzavirus'', '' Gammainfluenzavirus'', ' ...
,
rhinovirus The rhinovirus (from the grc, ῥίς, rhis "nose", , romanized: "of the nose", and the la, vīrus) is the most common viral infectious agent in humans and is the predominant cause of the common cold. Rhinovirus infection proliferates in tem ...
,
respiratory syncytial virus Respiratory syncytial virus (RSV), also called human respiratory syncytial virus (hRSV) and human orthopneumovirus, is a common, contagious virus that causes infections of the respiratory tract. It is a negative-sense, single-stranded RNA virus. ...
,
enterovirus ''Enterovirus'' is a genus of positive-sense single-stranded RNA viruses associated with several human and mammalian diseases. Enteroviruses are named by their transmission-route through the intestine ('enteric' meaning intestinal). Serologic ...
, and
norovirus Norovirus, sometimes referred to as the winter vomiting disease, is the most common cause of gastroenteritis. Infection is characterized by non-bloody diarrhea, vomiting, and stomach pain. Fever or headaches may also occur. Symptoms usually devel ...
;
measles morbillivirus ''Measles morbillivirus'' (MeV), also called measles virus (MV), is a single-stranded, negative-sense, enveloped, non-segmented RNA virus of the genus '' Morbillivirus'' within the family '' Paramyxoviridae''. It is the cause of measles. Human ...
; and coronaviruses such as SARS coronavirus (SARS-CoV-1) and
SARS-CoV-2 Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) is a strain of coronavirus that causes COVID-19 (coronavirus disease 2019), the respiratory illness responsible for the ongoing COVID-19 pandemic. The virus previously had a ...
that causes
COVID-19 Coronavirus disease 2019 (COVID-19) is a contagious disease caused by a virus, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The first known case was identified in Wuhan, China, in December 2019. The disease quickly ...
. Bacterial and fungal infection agents may also be transmitted by respiratory droplets. By contrast, a limited number of diseases can be spread through
airborne transmission Airborne or aerosol transmission is transmission of an infectious disease through small particles suspended in the air. Infectious diseases capable of airborne transmission include many of considerable importance both in human and veterinary ...
after the respiratory droplet dries out. We all continuously breathe out these droplets, but in addition some medical procedures called aerosol-generating medical procedures also generate droplets. Ambient
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...
and
humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity dep ...
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.


Measures taken to reduce transmission

In a healthcare setting, precautions include housing a patient in an individual room, limiting their transport outside the room and using proper
personal protective equipment Personal protective equipment (PPE) is protective clothing, helmets, goggles, or other garments or equipment designed to protect the wearer's body from injury or infection. The hazards addressed by protective equipment include physical, e ...
. It has been noted that during the
2002–2004 SARS outbreak The 2002–2004 outbreak of SARS, caused by severe acute respiratory syndrome coronavirus (SARS-CoV or SARS-CoV-1), infected over 8,000 people from 29 countries and territories, and resulted in at least 774 deaths worldwide. The outbreak was ...
, use of surgical masks and
N95 respirator An N95 filtering facepiece respirator, commonly abbreviated N95 respirator, is a particulate-filtering facepiece respirator that meets the U.S. National Institute for Occupational Safety and Health (NIOSH) N95 classification of air filtrati ...
s tended to decrease infections of healthcare workers. However, surgical masks are much less good at filtering out small droplets/particles than N95 and similar respirators, so the respirators offer greater protection. Also, higher
ventilation Ventilation may refer to: * Ventilation (physiology), the movement of air between the environment and the lungs via inhalation and exhalation ** Mechanical ventilation, in medicine, using artificial methods to assist breathing *** Ventilator, a m ...
rates can be used as a hazard control to dilute and remove respiratory particles. However, if unfiltered or insufficiently filtered air is exhausted to another location, it can lead to spreading of an infection.


History

German bacteriologist
Carl Flügge Carl Georg Friedrich Wilhelm Flügge (12 September 1847 – 10 December 1923) was a German bacteriologist and hygienist. His finding that pathogens were present in expiratory droplets, the eponymous Flügge droplets, laid ground for the concept ...
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. 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. He developed the Wells curve, which describes how the size of respiratory droplets influences their fate and thus their ability to transmit disease.


See also

*
Basic reproduction number In epidemiology, the basic reproduction number, or basic reproductive number (sometimes called basic reproduction ratio or basic reproductive rate), denoted R_0 (pronounced ''R nought'' or ''R zero''), of an infection is the expected number of ...
*
Source control (respiratory disease) Source control is a strategy for reducing disease transmission by blocking respiratory secretions produced through speaking, coughing, sneezing or singing. Surgical masks are commonly used for this purpose, with cloth face masks recommended f ...


References

{{Reflist Disease transmission Particulates