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
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...
and
veterinary medicine. The relevant
infectious agent may be
virus
A virus is a wikt:submicroscopic, submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and ...
es,
bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
, or
fungi
A fungus (plural, : fungi or funguses) is any member of the group of Eukaryote, eukaryotic organisms that includes microorganisms such as yeasts and Mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified ...
, 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
Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different type ...
.
Aerosol transmission has traditionally been considered distinct from transmission by
droplets, but this distinction is no longer used.
Respiratory droplets were thought to rapidly fall to the ground after emission:
but smaller droplets and aerosols also contain live infectious agents, and can remain in the air longer and travel farther.
Individuals generate aerosols and droplets across a wide range of sizes and concentrations, and the amount produced varies widely by person and activity.
Larger droplets greater than 100
μm
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
usually settle within 2 m.
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.
The traditional size cutoff of 5
μm
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
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.
Indoor respiratory secretion transfer data suggest that droplets/aerosols in the 20
μm
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
size range initially travel with the air flow from cough jets and air conditioning like aerosols,
but fall out gravitationally at a greater distance as "jet riders". As this size range is most efficiently filtered out in the
nasal mucosa, the primordial infection site in
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 quickl ...
, aerosols/droplets in this size range may contribute to driving the
COVID-19 pandemic
The COVID-19 pandemic, also known as the coronavirus pandemic, is an ongoing global pandemic of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The novel virus was first identified ...
.
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
An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable d ...
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;
sneeze
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 a ...
s, for example, can easily project infectious droplets for dozens of feet (ten or more meters).
Airborne pathogens or
allergens typically enter the body via the
nose
A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for respiration alongside the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next pass ...
,
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.
Common infections that spread by airborne transmission include
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 No ...
;
measles morbillivirus,
chickenpox virus
Varicella-zoster virus (VZV), also known as human herpesvirus 3 (HHV-3, HHV3) or ''Human alphaherpesvirus 3'' (taxonomically), is one of nine known herpes viruses that can infect humans. It causes chickenpox (varicella) commonly affecting chil ...
;
''
Mycobacterium tuberculosis'',
influenza virus,
enterovirus,
norovirus and less commonly other species of
coronavirus,
adenovirus, and possibly
respiratory syncytial virus.
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.
Poor ventilation enhances transmission by allowing aerosols to spread undisturbed in an indoor space. 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 but the
Wells-Riley model 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.
Wells-Riley can also be applied to other diseases transmitted in the air ...
can be used to make simple estimates of infection probability.
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.
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. 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.
Transmission
Environmental factors influence the efficacy of airborne disease transmission; the most evident environmental conditions are
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer.
Thermometers are calibrated in various temperature scales that historically have relied on ...
and
relative humidity.
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.
Airborne infections usually land in the respiratory system, with the agent present in aerosols (infectious particles < 5 µm in diameter). This includes dry particles, often the remnant of an evaporated wet particle called nuclei, and wet particles.
* Relative humidity (RH) plays an important role in the evaporation of droplets and the distance they travel. 30 μm droplets evaporate in seconds.
The CDC recommends a minimum of 40% RH indoors
to significantly reduce the infectivity of aerosolized virus. An ideal humidity for preventing aerosol respiratory viral transmission at room temperature appears to be between 40% and 60% RH. If the relative humidity goes below 35% RH, infectious virus stays longer in the air.
* The number of rainy days
(more important than total precipitation);
mean daily sunshine hours;
latitude and altitude
are relevant when assessing the possibility of spread of airborne disease. Some infrequent or exceptional events influence the dissemination of airborne diseases, including tropical storms,
hurricanes,
typhoons, or
monsoon
A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal osci ...
s.
* Climate affects temperature, winds and relative humidity, the main factors affecting the spread, duration and infectiousness of droplets containing infectious particles.
The influenza virus spreads easily in the
Northern Hemisphere winter due to climate conditions that favour the infectiousness of the virus.
* Isolated weather events decrease the concentration of airborne
fungal spores; a few days later, number of spores increases exponentially.
* Socioeconomics has a minor role in airborne disease transmission. In cities, airborne disease spreads more rapidly than in rural areas and urban outskirts. Rural areas generally favor higher airborne fungal dissemination.
* Proximity to large bodies of water such as rivers and lakes can enhance airborne disease.
* A direct association between insufficient ventilation rates and increased COVID-19 transmission has been observed. Prior to COVID-19, standards for ventilation systems focused more on supplying sufficient oxygen to a room, rather than disease-related aspects of air quality.
* Poor maintenance of air conditioning systems has led to outbreaks of ''
Legionella pneumophila''.
* Hospital-acquired airborne diseases are associated with poorly-resourced and maintained medical systems, which make
isolation challenging.
* Air conditioning may reduce transmission by removing contaminated air, but may also contribute to the spread of respiratory secretions inside a room.
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).
Preventive techniques can include disease-specific
immunization
Immunization, or immunisation, is the process by which an individual's immune system becomes fortified against an infectious agent (known as the immunogen).
When this system is exposed to molecules that are foreign to the body, called ''non-s ...
as well as nonpharmaceutical interventions such as wearing a
respirator and limiting time spent in the presence of infected individuals.
Wearing a face mask can lower the risk of airborne transmission to the extent that it limits the transfer of airborne particles between individuals. 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
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 filtrat ...
(US) or
FFP3 (EU) required. Use of FFP3 masks by staff managing patients with
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 quickl ...
reduced acquisition of COVID-19 by staff members.
Engineering solutions which aim to control or eliminate exposure to a hazard are higher on the
hierarchy of control than
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, ...
(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.
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.
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. Many public health specialists recommend physical distancing (also known as
social distancing) to reduce transmission.
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.
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.
In addition the risk for infection is also dependent on host
immune system
The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as Tumor immunology, cancer cells and objects such ...
competency plus the quantity of infectious particles ingested.
Antibiotic
An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy, ...
s may be used in dealing with airborne bacterial primary infections, such as
pneumonic plague.
See also
*
Aeroplankton
*
Basic reproduction number
*
Miasma theory
*
Vector (epidemiology)
*
Waterborne diseases
*
Zoonosis
References
{{DEFAULTSORT:Airborne Disease
Disease transmission
Articles containing video clips