Airborne transmission

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Infected people generate larger droplets and aerosols which can infect over longer distances Aerosol transmission.png
Infected people generate larger droplets and aerosols which can infect over longer distances

A poster outlining precautions for airborne transmission in healthcare settings. It is intended to be posted outside rooms of patients with an infection that can spread through airborne transmission. Airborne Precautions poster.pdf
A poster outlining precautions for airborne transmission in healthcare settings. It is intended to be posted outside rooms of patients with an infection that can spread through airborne transmission.
Video explainer on reducing airborne pathogen transmission indoors

Airborne transmission or aerosol transmission is transmission of an infectious disease through small particles suspended in the air. [2] 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.

Contents

Infectious aerosols: physical terminology

Aerosol transmission has traditionally been considered distinct from transmission by droplets, but this distinction is no longer used. [3] [4] Respiratory droplets were thought to rapidly fall to the ground after emission: [5] but smaller droplets and aerosols also contain live infectious agents, and can remain in the air longer and travel farther. [4] [6] [7] Individuals generate aerosols and droplets across a wide range of sizes and concentrations, and the amount produced varies widely by person and activity. [8] Larger droplets greater than 100 μm usually settle within 2 m. [8] [5] Smaller particles can carry airborne pathogens for extended periods of time. While the concentration of airborne pathogens is greater within 2m, they can travel farther and concentrate in a room. [4]

The traditional size cutoff of 5 μm between airborne and respiratory droplets has been discarded, as exhaled particles form a continuum of sizes whose fates depend on environmental conditions in addition to their initial sizes. This error has informed hospital based transmission based precautions for decades. [8] Indoor respiratory secretion transfer data suggest that droplets/aerosols in the 20 μm size range initially travel with the air flow from cough jets and air conditioning like aerosols, [9] but fall out gravitationally at a greater distance as "jet riders". [9] As this size range is most efficiently filtered out in the nasal mucosa, [10] the primordial infection site in COVID-19, aerosols/droplets [11] in this size range may contribute to driving the COVID-19 pandemic.

Overview

Airborne diseases can be transmitted from one individual to another through the air. The pathogens transmitted may be any kind of microbe, and they may be spread in aerosols, dust or droplets. The aerosols might be generated from sources of infection such as the bodily secretions of an infected individual, or biological wastes. Infectious aerosols may stay suspended in air currents long enough to travel for considerable distances; sneezes, for example, can easily project infectious droplets for dozens of feet (ten or more meters). [12]

Airborne pathogens or allergens typically enter the body via the nose, throat, sinuses and lungs. Inhalation of these pathogens affects the respiratory system and can then spread to the rest of the body. Sinus congestion, coughing and sore throats are examples of inflammation of the upper respiratory airway. Air pollution plays a significant role in airborne diseases. Pollutants can influence lung function by increasing air way inflammation. [13]

Common infections that spread by airborne transmission include SARS-CoV-2; [14] measles morbillivirus, [15] chickenpox virus; [16] Mycobacterium tuberculosis , influenza virus, enterovirus, norovirus and less commonly other species of coronavirus, adenovirus, and possibly respiratory syncytial virus. [17] Some pathogens which have more than one mode of transmission are also anisotropic, meaning that their different modes of transmission can cause different kinds of diseases, with different levels of severity. Two examples are the bacterias Yersinia pestis (which causes plague) and Francisella tularensis (which causes tularaemia), which both can cause severe pneumonia, if transmitted via the airborne route through inhalation. [18]

Poor ventilation enhances transmission by allowing aerosols to spread undisturbed in an indoor space. [19] Crowded rooms are more likely to contain an infected person. The longer a susceptible person stays in such a space, the greater chance of transmission. Airborne transmission is complex, and hard to demonstrate unequivocally [20] but the Wells-Riley model can be used to make simple estimates of infection probability. [21]

Some airborne diseases can affect non-humans. For example, Newcastle disease is an avian disease that affects many types of domestic poultry worldwide that is airborne. [22]

It has been suggested that airborne transmission should be classified as being either obligate, preferential, or opportunistic, although there is limited research that show the importance of each of these categories. [23] Obligate airborne infections spread only through aerosols; the most common example of this category is tuberculosis. Preferential airborne infections, such as chicken pox, can be obtained through different routes, but mainly by aerosols. Opportunistic airborne infections such as influenza typically transmit through other routes; however, under favourable conditions, aerosol transmission can occur. [24]

Transmission efficiency

Environmental factors influence the efficacy of airborne disease transmission; the most evident environmental conditions are temperature and relative humidity. [25] [26] The transmission of airborne diseases is affected by all the factors that influence temperature and humidity, in both meteorological (outdoor) and human (indoor) environments. Circumstances influencing the spread of droplets containing infectious particles can include pH, salinity, wind, air pollution, and solar radiation as well as human behavior. [27]

Airborne infections usually land in the respiratory system, with the agent present in aerosols (infectious particles < 5 µm in diameter). [28] This includes dry particles, often the remnant of an evaporated wet particle called nuclei, and wet particles.

Prevention

A layered risk-management approach to slowing the spread of a transmissible disease attempts to minimize risk through multiple layers of interventions. Each intervention has the potential to reduce risk. A layered approach can include interventions by individuals (e.g. mask wearing, hand hygiene), institutions (e.g. surface disinfection, ventilation, and air filtration measures to control the indoor environment), the medical system (e.g. vaccination) and public health at the population level (e.g. testing, quarantine, and contact tracing). [4]

Preventive techniques can include disease-specific immunization as well as nonpharmaceutical interventions such as wearing a respirator and limiting time spent in the presence of infected individuals. [40] Wearing a face mask can lower the risk of airborne transmission to the extent that it limits the transfer of airborne particles between individuals. [41] The type of mask that is effective against airborne transmission is dependent on the size of the particles. While fluid-resistant surgical masks prevent large droplet inhalation, smaller particles which form aerosols require a higher level of protection with filtration masks rated at N95 (US) or FFP3 (EU) required. [42] Use of FFP3 masks by staff managing patients with COVID-19 reduced acquisition of COVID-19 by staff members. [43]

Engineering solutions which aim to control or eliminate exposure to a hazard are higher on the hierarchy of control than personal protective equipment (PPE). At the level of physically based engineering interventions, effective ventilation and high frequency air changes, or air filtration through high efficiency particulate filters, reduce detectable levels of virus and other bioaerosols, improving conditions for everyone in an area. [4] [44] Portable air filters, such as those tested in Conway Morris A et al. present a readily deployable solution when existing ventilation is inadequate, for instance in repurposed COVID-19 hospital facilities. [44]

The United States Centers for Disease Control and Prevention (CDC) advises the public about vaccination and following careful hygiene and sanitation protocols for airborne disease prevention. [45] Many public health specialists recommend physical distancing (also known as social distancing) to reduce transmission. [46]

A 2011 study concluded that vuvuzelas (a type of air horn popular e.g. with fans at football games) presented a particularly high risk of airborne transmission, as they were spreading a much higher number of aerosol particles than e.g., the act of shouting. [47]

Exposure does not guarantee infection. The generation of aerosols, adequate transport of aerosols through the air, inhalation by a susceptible host, and deposition in the respiratory tract are all important factors contributing to the over-all risk for infection. Furthermore, the infective ability of the virus must be maintained throughout all these stages. [48] In addition the risk for infection is also dependent on host immune system competency plus the quantity of infectious particles ingested. [40] Antibiotics may be used in dealing with airborne bacterial primary infections, such as pneumonic plague. [49]

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">Natural reservoir</span> Type of population in infectious disease ecology

In infectious disease ecology and epidemiology, a natural reservoir, also known as a disease reservoir or a reservoir of infection, is the population of organisms or the specific environment in which an infectious pathogen naturally lives and reproduces, or upon which the pathogen primarily depends for its survival. A reservoir is usually a living host of a certain species, such as an animal or a plant, inside of which a pathogen survives, often without causing disease for the reservoir itself. By some definitions a reservoir may also be an environment external to an organism, such as a volume of contaminated air or water.

An emergent virus is a virus that is either newly appeared, notably increasing in incidence/geographic range or has the potential to increase in the near future. Emergent viruses are a leading cause of emerging infectious diseases and raise public health challenges globally, given their potential to cause outbreaks of disease which can lead to epidemics and pandemics. As well as causing disease, emergent viruses can also have severe economic implications. Recent examples include the SARS-related coronaviruses, which have caused the 2002-2004 outbreak of SARS (SARS-CoV-1) and the 2019–21 pandemic of COVID-19 (SARS-CoV-2). Other examples include the human immunodeficiency virus which causes HIV/AIDS; the viruses responsible for Ebola; the H5N1 influenza virus responsible for avian flu; and H1N1/09, which caused the 2009 swine flu pandemic. Viral emergence in humans is often a consequence of zoonosis, which involves a cross-species jump of a viral disease into humans from other animals. As zoonotic viruses exist in animal reservoirs, they are much more difficult to eradicate and can therefore establish persistent infections in human populations.

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.

A fomite or fomes is any inanimate object that, when contaminated with or exposed to infectious agents, can transfer disease to a new host.

A number of possible health hazards of air travel have been investigated. Risks include induced explosive diarrhoea, a Pringle chip dipped in tomato sauce flying a UFO, travel sickness and more

<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.

<span class="mw-page-title-main">Respiratory droplet</span> Type of particle formed by breathing

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.

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">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.

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.

The Wells-Riley model is a simple model of the airborne transmission of infectious diseases, developed by William F. Wells and Richard L. Riley for tuberculosis and measles.

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.

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