Respiratory droplet

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Some infectious diseases can be spread via respiratory droplets expelled from the mouth and nose, as when a person sneezes. Sneeze.JPG
Some infectious diseases can be spread via respiratory droplets expelled from the mouth and nose, as when a person sneezes.

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 increase their number. [1] [2] [3]

Contents

Droplet sizes range from < 1 µm to 1000 µm, [1] [2] 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. [1] [2] 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"). [4]

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, [5] [3] and is not consistent with the standard definition of an aerosol.

Description

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. Plot of droplet sizes in our breath (when speaking).svg
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]

Formation

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]

Transport

Human cough: effect of wind speed on the transport of respiratory droplets. WindEffect.png
Human cough: effect of wind speed on the transport of respiratory droplets.

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]

Role in disease transmission

Illustration of a respiratory droplet, showing mucins (green), surfactant proteins and lipids (blue) and a coronavirus particle (pink) Respiratory Droplet with SARS-CoV-2.jpg
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]

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. [17] [18] It has been noted that during the 2002–2004 SARS outbreak, use of surgical masks and N95 respirators tended to decrease infections of healthcare workers. [19] However, surgical masks are much less good at filtering out small droplets/particles than N95 and similar respirators, so the respirators offer greater protection. [20] [21]

Also, higher ventilation 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. [8]

History

World-War-II-era UK public-health-education poster. Coughs and Sneezes Spread Diseases Art.IWMPST14133.jpg
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]

See also

Related Research Articles

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 previously infected. The term strictly refers to the transmission of microorganisms directly from one individual to another by one or more of the following means:

<span class="mw-page-title-main">Surgical mask</span> Mouth and nose cover against bacterial aerosols

A surgical mask, also known by other names such as a medical face mask or procedure mask, is a personal protective equipment used by healthcare professionals that serves as a mechanical barrier that interferes with direct airflow in and out of respiratory orifices. This helps reduce airborne transmission of pathogens and other aerosolized contaminants between the wearer and nearby people via respiratory droplets ejected when sneezing, coughing, forceful expiration or unintentionally spitting when talking, etc. Surgical masks may be labeled as surgical, isolation, dental or medical procedure masks.

<span class="mw-page-title-main">Bioaerosol</span> Airborne particles containing living organisms

Bioaerosols are a subcategory of particles released from terrestrial and marine ecosystems into the atmosphere. They consist of both living and non-living components, such as fungi, pollen, bacteria and viruses. Common sources of bioaerosols include soil, water, and sewage.

<span class="mw-page-title-main">Airborne transmission</span> Disease transmission by airborne particles

Airborne transmission 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 medicine. The relevant infectious agent may be viruses, bacteria, or fungi, and they may be spread through breathing, talking, coughing, sneezing, raising of dust, spraying of liquids, flushing toilets, or any activities which generate aerosol particles or droplets. This is the transmission of diseases via transmission of an infectious agent, and does not include diseases caused by air pollution.

Influenza prevention involves taking steps that one can use to decrease their chances of contracting flu viruses, such as the Pandemic H1N1/09 virus, responsible for the 2009 flu pandemic.

<span class="mw-page-title-main">Wells curve</span> Science of medicine

The Wells curve 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 toilet plume is the dispersal of microscopic particles as a result of flushing a toilet. Normal use of a toilet by healthy individuals is considered unlikely to be a major health risk. However this dynamic changes if an individual is fighting an illness and currently shedding out a virulent pathogen in their urine, feces or vomitus. There is indirect evidence that specific pathogens such as norovirus or SARS coronavirus could potentially be spread by toilet aerosols, but as of 2015, no direct experimental studies had clearly demonstrated or refuted actual disease transmission from toilet aerosols. It has been hypothesized that dispersal of pathogens may be reduced by closing the toilet lid before flushing, and by using toilets with lower flush energy.

<span class="mw-page-title-main">SARS-CoV-2</span> Virus that causes COVID-19

Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) is a strain of coronavirus that causes COVID-19, the respiratory illness responsible for the COVID-19 pandemic. The virus previously had the provisional name 2019 novel coronavirus (2019-nCoV), and has also been called human coronavirus 2019. First identified in the city of Wuhan, Hubei, China, the World Health Organization designated the outbreak a public health emergency of international concern from January 30, 2020, to May 5, 2023. SARS‑CoV‑2 is a positive-sense single-stranded RNA virus that is contagious in humans.

<span class="mw-page-title-main">Dental aerosol</span> Hazardous biological compound

A dental aerosol is an aerosol that is produced from dental instrument, dental handpieces, three-way syringes, and other high-speed instruments. These aerosols may remain suspended in the clinical environment. Dental aerosols can pose risks to the clinician, staff, and other patients. The heavier particles contained within the aerosols are likely to remain suspended in the air for relatively short period and settle quickly onto surfaces, however, the lighter particles may remain suspended for longer periods and may travel some distance from the source. These smaller particles are capable of becoming deposited in the lungs when inhaled and provide a route of diseases transmission. Different dental instruments produce varying quantities of aerosol, and therefore are likely to pose differing risks of dispersing microbes from the mouth. Air turbine dental handpieces generally produce more aerosol, with electric micromotor handpieces producing less, although this depends on the configuration of water coolant used by the handpiece.

<span class="mw-page-title-main">Cloth face mask</span> Mask made of common textiles worn over the mouth and nose

A cloth face mask is a mask made of common textiles, usually cotton, worn over the mouth and nose. When more effective masks are not available, and when physical distancing is impossible, cloth face masks are recommended by public health agencies for disease "source control" in epidemic situations to protect others from virus laden droplets in infected mask wearers' breath, coughs, and sneezes. Because they are less effective than N95 masks, surgical masks, or physical distancing in protecting the wearer against viruses, they are not considered to be personal protective equipment by public health agencies. They are used by the general public in household and community settings as protection against both infectious diseases and particulate air pollution.

<span class="mw-page-title-main">Mechanical filter (respirator)</span> Air-filtering face masks or mask attachments

Mechanical filters are a class of filter for air-purifying respirators that mechanically stops particulates from reaching the wearer's nose and mouth. They come in multiple physical forms.

<span class="mw-page-title-main">Face masks during the COVID-19 pandemic</span> Health control procedure against COVID-19

During the COVID-19 pandemic, face masks or coverings, including N95, FFP2, surgical, and cloth masks, have been employed as public and personal health control measures against the spread of SARS-CoV-2, the virus that causes COVID-19.

Lydia Bourouiba is an Esther and Harold E. Edgerton Professor, an Associate Professor in the Civil and Environmental Engineering and Mechanical Engineering departments, and in the Institute for Medical Engineering and Science at the Massachusetts Institute of Technology. She is also a Harvard-MIT Health Sciences and Technology Faculty, and Affiliate Faculty of Harvard Medical School. She directs the Fluid Dynamics of Disease Transmission Laboratory at MIT.

<span class="mw-page-title-main">Linsey Marr</span> American scientist

Linsey Chen Marr is an American scientist who is the Charles P. Lunsford Professor of Civil and Environmental Engineering at Virginia Tech. Her research considers the interaction of nanomaterials and viruses with the atmosphere. During the COVID-19 pandemic Marr studied how SARS-CoV-2 and other airborne pathogens could be transported in air. In 2023, she was elected to the National Academy of Engineering and named a MacArthur Fellow.

<span class="mw-page-title-main">Source control (respiratory disease)</span> Strategy for reducing disease transmission

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 for use by the public only in epidemic situations when there are shortages of surgical masks. In addition, respiratory etiquette such as covering the mouth and nose with a tissue when coughing can be considered source control. In diseases transmitted by droplets or aerosols, understanding air flow, particle and aerosol transport may lead to rational infrastructural source control measures that minimize exposure of susceptible persons.

William Firth Wells was an American scientist and sanitary engineer. In his early career, he pioneered techniques for the aquaculture of oysters and clams. He is best known for his work on airborne infections. Wells identified that tuberculosis could be transmitted through air via the nuclei of evaporated respiratory droplets, and developed the Wells curve to describe what happens to respiratory droplets after they have been expelled into the air.

<span class="mw-page-title-main">Transmission of COVID-19</span> Mechanisms that spread coronavirus disease 2019

The transmission of COVID-19 is the passing of coronavirus disease 2019 from person to person. COVID-19 is mainly transmitted when people breathe in air contaminated by droplets/aerosols and small airborne particles containing the virus. Infected people exhale those particles as they breathe, talk, cough, sneeze, or sing. Transmission is more likely the closer people are. However, infection can occur over longer distances, particularly indoors.

Droplet nuclei are aerosols formed from the evaporation of respiratory droplets. They are generally smaller than 5 μm in diameter. Droplet nuclei are formed by the "dried residua of larger respiratory droplets". These particles are "the vehicle for airborne respiratory disease transmission, which are the dried-out residual of droplets possibly containing infectious pathogens". Diseases such as tuberculous and COVID-19 can be transmitted via droplet nuclei.

Lidia Morawska is a Polish–Australian physicist and distinguished professor at the School of Earth and Atmospheric Sciences, at the Queensland University of Technology and director of the International Laboratory for Air Quality and Health (ILAQH) at QUT. She is also co-director of the Australia-China Centre for Air Quality Science and Management, an adjunct professor at the Jinan University in China, and a Vice-Chancellor fellow at the Global Centre for Clean Air Research (GCARE), University of Surrey in the United Kingdom. Her work focuses on fundamental and applied research in the interdisciplinary field of air quality and its impact on human health, with a specific focus on atmospheric fine, ultrafine and nanoparticles. Since 2003, she expanded her interests to include also particles from human respiration activities and airborne infection transmission.

Nicole M. Bouvier is an American physician who is Professor of Medicine at Icahn School of Medicine at Mount Sinai. Her research considers the environmental and viral factors that impact respiratory transmission of influenza viruses.

References

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