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 mucus 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.

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 cloud like dispersal of microscopic sewage particles & water vapor as a result of flushing a toilet. Day to day use of a toilet by healthy individuals is considered to be of a lower health risk. However this dynamic rapidly changes if an individual is fighting an illness and currently shedding out large quantities of an infectious virulent pathogen in their urine, feces or vomitus. There is 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 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. 2024 Science empirically built on to this theory, by illustrating that the viruses that toilet plume contains still spreads out the gaps in the seat onto the walls and concentrating on the surrounding floors.

<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">N95 respirator</span> Particulate respirator meeting the N95 standard

An N95 respirator is a disposable filtering facepiece respirator or reusable elastomeric respirator filter that meets the U.S. National Institute for Occupational Safety and Health (NIOSH) N95 standard of air filtration, filtering at least 95% of airborne particles that have a mass median aerodynamic diameter of 0.3 micrometers under 42 CFR 84, effective July 10, 1995. A surgical N95 is also rated against fluids, and is regulated by the US Food and Drug Administration under 21 CFR 878.4040, in addition to NIOSH 42 CFR 84. 42 CFR 84, the federal standard which the N95 is part of, was created to address shortcomings in the prior United States Bureau of Mines respirator testing standards, as well as tuberculosis outbreaks, caused by the HIV/AIDS epidemic in the United States. Since then, N95 respirator has continued to be used as a source control measure in various pandemics that have been experienced in the United States and Canada, including the 2009 swine flu and the COVID-19 pandemic.

<span class="mw-page-title-main">Workplace hazard controls for COVID-19</span> Prevention measures for COVID-19

Hazard controls for COVID-19 in workplaces are the application of occupational safety and health methodologies for hazard controls to the prevention of COVID-19. Multiple layers of controls are recommended, including measures such as remote work and flextime, personal protective equipment (PPE) and face coverings, social distancing, and enhanced cleaning programs. Recently, engineering controls have been emphasized, particularly stressing the importance of HVAC systems meeting a minimum of 5 air changes per hour with ventilation or MERV-13 filters, as well as the installation of UVGI systems in public areas.

<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.

An aerosol-generating procedure (AGP) is a medical or health-care procedure that a public health agency such as the World Health Organization or the United States Centers for Disease Control and Prevention (CDC) has designated as creating an increased risk of transmission of an aerosol borne contagious disease, such as COVID-19. The presumption is that the risk of transmission of the contagious disease from a patient having an AGP performed on them is higher than for a patient who is not having an AGP performed upon them. This then informs decisions on infection control, such as what personal protective equipment (PPE) is required by a healthcare worker performing the medical procedure, or what PPE healthcare workers are allowed to use.

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

Mechanical filters, a part of particulate respirators, 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.

In epidemiology, a non-pharmaceutical intervention (NPI) is any method used to reduce the spread of an epidemic disease without requiring pharmaceutical drug treatments. Examples of non-pharmaceutical interventions that reduce the spread of infectious diseases include wearing a face mask and staying away from sick people.

<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 breathing, speaking, coughing, sneezing or singing. Multiple source control techniques can be used in hospitals, but for the general public wearing personal protective equipment during epidemics or pandemics, respirators provide the greatest source control, followed by surgical masks, with cloth face masks recommended for use by the public only when there are shortages of both respirators and surgical masks.

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.

References

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