Miasma theory

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Miasma_theory

 

An 1831 color lithograph by Robert Seymour depicts cholera as a robed, skeletal creature emanating a deadly black cloud.

The miasma theory (also called the miasmatic theory) is an abandoned medical theory that held that diseases—such as cholera, chlamydia, or the Black Death—were caused by a miasma (μίασμα, Ancient Greek for 'pollution'), a noxious form of "bad air", also known as night air. The theory held that epidemics were caused by miasma, emanating from rotting organic matter. Though miasma theory is typically associated with the spread of contagious diseases, some academics in the early nineteenth century suggested that the theory extended to other conditions as well, e.g. one could become obese by inhaling the odor of food.

The miasma theory was advanced by Hippocrates in the fourth century B.C. and accepted from ancient times in Europe and China. The theory was eventually abandoned by scientists and physicians after 1880, replaced by the germ theory of disease: specific germs, not miasma, caused specific diseases. However, cultural beliefs about getting rid of odor made the clean-up of waste a high priority for cities.

Etymology

The word miasma comes from ancient Greek and means 'pollution'. The idea also gave rise to the name malaria (literally 'bad air') through medieval Italian.

Views worldwide

Book of Sebastian Petrycy published in Kraków in 1613 about prevention against "bad air".

Miasma was considered to be a poisonous vapor or mist filled with particles from decomposed matter (miasmata) that caused illnesses. The miasmatic position was that diseases were the product of environmental factors such as contaminated water, foul air, and poor hygienic conditions. Such infection was not passed between individuals but would affect individuals within the locale that gave rise to such vapors. It was identifiable by its foul smell. It was also initially believed that miasmas were propagated through worms from ulcers within those affected by a plague.

Europe

In the fifth or fourth century BC, Hippocrates wrote about the effects of the environs over the human diseases:

Whoever wishes to investigate medicine properly, should proceed thus: in the first place to consider the seasons of the year, and what effects each of them produces for they are not at all alike, but differ much from themselves in regard to their changes. Then the winds, the hot and the cold, especially such as are common to all countries, and then such as are peculiar to each locality. We must also consider the qualities of the waters, for as they differ from one another in taste and weight, so also do they differ much in their qualities. In the same manner, when one comes into a city to which he is a stranger, he ought to consider its situation, how it lies as to the winds and the rising of the sun; for its influence is not the same whether it lies to the north or the south, to the rising or to the setting sun. These things one ought to consider most attentively, and concerning the waters which the inhabitants use, whether they be marshy and soft, or hard, and running from elevated and rocky situations, and then if saltish and unfit for cooking; and the ground, whether it be naked and deficient in water, or wooded and well watered, and whether it lies in a hollow, confined situation, or is elevated and cold; and the mode in which the inhabitants live, and what are their pursuits, whether they are fond of drinking and eating to excess, and given to indolence, or are fond of exercise and labor, and not given to excess in eating and drinking.

In the 1st century BC, the Roman architectural writer Vitruvius described the potential effects of miasma (Latin nebula) from fetid swamplands when visiting a city:

For when the morning breezes blow toward the town at sunrise, if they bring with them mist from marshes and, mingled with the mist, the poisonous breath of creatures of the marshes to be wafted into the bodies of the inhabitants, they will make the site unhealthy.

The miasmatic theory of disease remained popular in the Middle Ages and a sense of effluvia contributed to Robert Boyle's Suspicions about the Hidden Realities of the Air.

In the 1850s, miasma was used to explain the spread of cholera in London and in Paris, partly justifying Haussmann's later renovation of the French capital. The disease was said to be preventable by cleansing and scouring of the body and items. Dr. William Farr, the assistant commissioner for the 1851 London census, was an important supporter of the miasma theory. He believed that cholera was transmitted by air, and that there was a deadly concentration of miasmata near the River Thames' banks. Such a belief was in part accepted because of the general lack of air quality in urbanized areas. The wide acceptance of miasma theory during the cholera outbreaks overshadowed the partially correct theory brought forth by John Snow that cholera was spread through water. This slowed the response to the major outbreaks in the Soho district of London and other areas. The Crimean War nurse Florence Nightingale (1820–1910) was a proponent of the theory and worked to make hospitals sanitary and fresh-smelling. It was stated in 'Notes on Nursing for the Labouring Classes' (1860) that Nightingale would "keep the air [the patient] breathes as pure as the external air."

Fear of miasma registered in many early nineteenth-century warnings concerning what was termed "unhealthy fog". The presence of fog was thought to strongly indicate the presence of miasma. The miasmas were thought to behave like smoke or mist, blown with air currents, wafted by winds. It was thought that miasma didn't simply travel on air but changed the air through which it propagated; the atmosphere was infected by miasma, as diseased people were.

China

In China, miasma (Chinese: 瘴氣; pinyin: Zhàngqì; alternative names 瘴毒, 瘴癘) is an old concept of illness, used extensively by ancient Chinese local chronicles and works of literature. Miasma has different names in Chinese culture. Most of the explanations of miasma refer to it as a kind of sickness, or poison gas.

The ancient Chinese thought that miasma was related to the environment of parts of Southern China. The miasma was thought to be caused by the heat, moisture and the dead air in the Southern Chinese mountains. They thought that insects' waste polluted the air, the fog, and the water, and the virgin forest harbored a great environment for miasma to occur.

In descriptions by ancient travelers, soldiers, or local officials (most of them are men of letters) of the phenomenon of miasma, fog, haze, dust, gas, or poison geological gassing were always mentioned. The miasma was thought to have caused a lot of diseases such as the cold, influenza, heat strokes, malaria, or dysentery. In the medical history of China, malaria had been referred to by different names in different dynasty periods. Poisoning and psittacosis were also called miasma in ancient China because they did not accurately understand the cause of disease.

In the Sui dynasty, doctor Chao Yuanfang mentioned miasma in his book On Pathogen and Syndromes (諸病源候論). He thought that miasma in Southern China was similar to typhoid fever in Northern China. However, in his opinion, miasma was different from malaria and dysentery. In his book, he discussed dysentery in another chapter, and malaria in a single chapter. He also claimed that miasma caused various diseases, so he suggested that one should find apt and specific ways to resolve problems.

The concept of miasma developed in several stages. First, before the Western Jin dynasty, the concept of miasma was gradually forming; at least, in the Eastern Han dynasty, there was no description of miasma. During the Eastern Jin, large numbers of northern people moved south, and miasma was then recognized by men of letters and nobility. After the Sui and the Tang dynasty, scholars-bureaucrats sent to be the local officials recorded and investigated miasma. As a result, the government became concerned about the severe cases and the causes of miasma by sending doctors to the areas of epidemic to research the disease and heal the patients. In the Ming dynasty and Qing dynasty, versions of local chronicles record different miasma in different places.

However, Southern China was highly developed in the Ming and Qing dynasties. The environment changed rapidly, and after the 19th century, western science and medical knowledge were introduced into China, and people knew how to distinguish and deal with the disease. The concept of miasma therefore faded out due to the progress of medicine in China.

Influence in Southern China

The terrifying miasma diseases in the southern regions of China made it the primary location for relegating officials and sending criminals to exile since the Qin-Han Dynasty. Poet Han Yu (韓愈) of the Tang dynasty, for example, wrote to his nephew who came to see him off after his banishment to the Chao Prefecture in his poem, En Route (左遷至藍關示姪孫湘):

At dawn I sent a single warning to the throne of the Nine Steps;
At evening I was banished to Chao Yang, eight thousand leagues.
Striving on behalf of a noble dynasty to expel an ignoble government,
How should I, withered and worn, deplore my future lot?
The clouds gather on Ch'in Mountains, I cannot see my home;
The snow bars the passes of Lan, my horse cannot go forward.
But I know that you will come from afar, to fulfil your set purpose,

And lovingly gather my bones, on the banks of that plague-stricken river.

The prevalent belief and predominant fear of the southern region with its "poisonous air and gases" is evident in historical documents.

Similar topics and feelings toward the miasma-infected south are often reflected in early Chinese poetry and records. Most scholars of the time agreed that the geological environments in the south had a direct impact on the population composition and growth. Many historical records reflect that females were less prone to miasma infection, and mortality rates were much higher in the south, especially for the men. This directly influenced agriculture cultivation and the southern economy, as men were the engine of agriculture production. Zhou Qufei (周去非), a local magistrate from the Southern Song Dynasty described in his treatise, Representative Answers from the South: "... The men are short and tan, while the women were plump and seldom came down with illness," and exclaimed at the populous female population in the Guangxi region.

This inherent environmental threat also prevented immigration from other regions. Hence, development in the damp and sultry south was much slower than in the north, where the dynasties' political power resided for much of early Chinese history.

India

In India, there was also a miasma theory . Gambir was considered the first antimiasmatic application. This gambir tree is found in Southern India and Sri Lanka.

Developments from 19th century onwards

Zymotic theory

Based on zymotic theory, people believed vapors called miasmata (singular: miasma) rose from the soil and spread diseases. Miasmata were believed to come from rotting vegetation and foul water—especially in swamps and urban ghettos.

Many people, especially the weak or infirm, avoided breathing night air by going indoors and keeping windows and doors shut. In addition to ideas associated with zymotic theory, there was also a general fear that cold or cool air spread disease. The fear of night air gradually disappeared as understanding about disease increased as well as with improvements in home heating and ventilation. Particularly important was the understanding that the agent spreading malaria was the mosquito (active at night) rather than miasmata.

Contagionism versus miasmatism

Prior to the late 19th century, night air was considered dangerous in most Western cultures. Throughout the 19th century, the medical community was divided on the explanation for disease proliferation. On one side were the contagionists, believing disease was passed through physical contact, while others believed disease was present in the air in the form of miasma, and thus could proliferate without physical contact. Two members of the latter group were Dr. Thomas S. Smith and Florence Nightingale.

Thomas Southwood Smith spent many years comparing the miasmatic theory to contagionism.

To assume the method of propagation by touch, whether by the person or of infected articles, and to overlook that by the corruption of the air, is at once to increase the real danger, from exposure to noxious effluvia, and to divert attention from the true means of remedy and prevention.

Florence Nightingale:

The idea of "contagion", as explaining the spread of disease, appears to have been adopted at a time when, from the neglect of sanitary arrangements, epidemics attacked whole masses of people, and when men had ceased to consider that nature had any laws for her guidance. Beginning with the poets and historians, the word finally made its way into scientific nomenclature, where it has remained ever since [...] a satisfactory explanation for pestilence and an adequate excuse for non-exertion to prevent its recurrence.

The current germ theory accounts for disease proliferation by both direct and indirect physical contact.

Influence on sanitary engineering reforms

Main article: Sanitary movement

In the early 19th century, the living conditions in industrialized cities in Britain were increasingly unsanitary. The population was growing at a much faster rate than the infrastructure could support. For example, the population of Manchester doubled within a single decade, leading to overcrowding and a significant increase in waste accumulation. The miasma theory of disease made sense to the sanitary reformers of the mid-19th century. Miasmas explained why cholera and other diseases were epidemic in places where the water was stagnant and foul-smelling. A leading sanitary reformer, London's Edwin Chadwick, asserted that "all smell is disease", and maintained that a fundamental change in the structure of sanitation systems was needed to combat increasing urban mortality rates.

Chadwick saw the problem of cholera and typhoid epidemics as being directly related to urbanization, and he proposed that new, independent sewerage systems should be connected to homes. Chadwick supported his proposal with reports from the London Statistical Society which showed dramatic increases in both morbidity and mortality rates since the beginning of urbanization in the early 19th century. Though Chadwick proposed reform on the basis of the miasma theory, his proposals did contribute to improvements in sanitation, such as preventing the reflux of noxious air from sewers back into houses by using separate drainage systems in the design of sanitation. That led, incidentally, to decreased outbreaks of cholera and thus helped to support the theory.

The miasma theory was consistent with the observation that disease was associated with poor sanitation, and hence foul odours, and that sanitary improvements reduced disease. However, it was inconsistent with the findings arising from microbiology and bacteriology in the later 19th century, which eventually led to the adoption of the germ theory of disease, although consensus was not reached immediately. Concerns over sewer gas, which was a major component of the miasma theory developed by Galen, and brought to prominence by the "Great Stink" in London in the summer of 1858, led proponents of the theory to observe that sewers enclosed the refuse of the human bowel, which medical science had discovered could teem with typhoid, cholera, and other microbes.

In 1846, the Nuisances Removal and Diseases Prevention Act was passed to identify whether the transmission of cholera was by air or by water. The act was used to encourage owners to clean their dwellings and connect them to sewers.

Even though eventually disproved by the understanding of bacteria and the discovery of viruses, the miasma theory helped establish the connection between poor sanitation and disease. That encouraged cleanliness and spurred public health reforms which, in Britain, led to the Public Health Acts of 1848 and 1858, and the Local Government Act of 1858. The latter of those enabled the instituting of investigations into the health and sanitary regulations of any town or place, upon the petition of residents or as a result of death rates exceeding the norm. Early medical and sanitary engineering reformers included Henry Austin, Joseph Bazalgette, Edwin Chadwick, Frank Forster, Thomas Hawksley, William Haywood, Henry Letheby, Robert Rawlinson, John Simon, John Snow and Thomas Wicksteed. Their efforts, and associated British regulatory improvements, were reported in the United States as early as 1865.

Particularly notable in 19th century sanitation reform is the work of Joseph Bazalgette, chief engineer to London's Metropolitan Board of Works. Encouraged by the Great Stink, Parliament sanctioned Bazalgette to design and construct a comprehensive system of sewers, which intercepted London's sewage and diverted it away from its water supply. The system helped purify London's water and saved the city from epidemics. In 1866, the last of the three great British cholera epidemics took hold in a small area of Whitechapel. However, the area was not yet connected to Bazalgette's system, and the confined area of the epidemic acted as testament to the efficiency of the system's design.

Years later, the influence of those sanitary reforms on Britain was described by Richard Rogers:

London was the first city to create a complex civic administration which could coordinate modern urban services, from public transport to housing, clean water to education. London's County Council was acknowledged as the most progressive metropolitan government in the world. Fifty years earlier, London had been the worst slum city of the industrialized world: over-crowded, congested, polluted and ridden with disease...

The miasma theory did contribute to containing disease in urban settlements, but did not allow the adoption of a suitable approach to the reuse of excreta in agriculture. It was a major factor in the practice of collecting human excreta from urban settlements and reusing them in the surrounding farmland. That type of resource recovery scheme was common in major cities in the 19th century before the introduction of sewer-based sanitation systems. Nowadays, the reuse of excreta, when done in a hygienic manner, is known as ecological sanitation, and is promoted as a way of "closing the loop".

Throughout the 19th century, concern about public health and sanitation, along with the influence of the miasma theory, were reasons for the advocacy of the then-controversial practice of cremation. If infectious diseases were spread by noxious gases emitted from decaying organic matter, that included decaying corpses. The public health argument for cremation faded with the eclipsing of the miasma theory of disease.

Replacement by germ theory

Although the connection between germ and disease was proposed quite early, it was not until the late 1800s that the germ theory was generally accepted. The miasmatic theory was challenged by John Snow, suggesting that there was some means by which the disease was spread via a poison or morbid material (orig: materies morbi) in the water. He suggested this before and in response to a cholera epidemic on Broad Street in central London in 1854. Because of the miasmatic theory's predominance among Italian scientists, the discovery in the same year by Filippo Pacini of the bacillus that caused the disease was completely ignored. It was not until 1876 that Robert Koch proved that the bacterium Bacillus anthracis caused anthrax, which brought a definitive end to miasma theory.

1854 Broad Street cholera outbreak

Main article: 1854 Broad Street cholera outbreak

The work of John Snow is notable for helping to make the connection between cholera and typhoid epidemics and contaminated water sources, which contributed to the eventual demise of miasma theory. During the cholera epidemic of 1854, Snow traced high mortality rates among the citizens of Soho to a water pump in Broad Street. Snow convinced the local government to remove the pump handle, which resulted in a marked decrease in cases of cholera in the area. In 1857, Snow submitted a paper to the British Medical Journal which attributed high numbers of cholera cases to water sources that were contaminated with human waste. Snow used statistical data to show that citizens who received their water from upstream sources were considerably less likely to develop cholera than those who received their water from downstream sources. Though his research supported his hypothesis that contaminated water, not foul air, was the source of cholera epidemics, a review committee concluded that Snow's findings were not significant enough to warrant change, and they were summarily dismissed. Additionally, other interests intervened in the process of reform. Many water companies and civic authorities pumped water directly from contaminated sources such as the Thames to public wells, and the idea of changing sources or implementing filtration techniques was an unattractive economic prospect. In the face of such economic interests, reform was slow to be adopted.

In 1855, John Snow made a testimony against the Amendment to the "Nuisances Removal and Diseases Prevention Act" that regularized air pollution of some industries. He claimed that:

That is possible; but I believe that the poison of the cholera is either swallowed in water, or got directly from some other person in the family, or in the room; I believe it is quite an exception for it to be conveyed in the air; though if the matter gets dry it may be wafted a short distance.

In the same year, William Farr, who was then the major supporter of the miasma theory, issued a report to criticize the germ theory. Farr and the Committee wrote that:

After careful inquiry, we see no reason to adopt this belief. We do not feel it established that the water was contaminated in the manner alleged; nor is there before us any sufficient evidence to show whether inhabitants of that district, drinking from that well, suffered in proportion more than other inhabitants of the district who drank from other sources.

Experiments by Louis Pasteur

The more formal experiments on the relationship between germ and disease were conducted by Louis Pasteur between 1860 and 1864. He discovered the pathology of the puerperal fever and the pyogenic vibrio in the blood, and suggested using boric acid to kill these microorganisms before and after confinement.

By 1866, eight years after the death of John Snow, William Farr publicly acknowledged that the miasma theory on the transmission of cholera was wrong, by his statistical justification on the death rate.

Anthrax

Robert Koch is widely known for his work with anthrax, discovering the causative agent of the fatal disease to be Bacillus anthracis. He published the discovery in a booklet as Die Ätiologie der Milzbrand-Krankheit, Begründet auf die Entwicklungsgeschichte des Bacillus Anthracis (The Etiology of Anthrax Disease, Based on the Developmental History of Bacillus Anthracis) in 1876 while working in Wöllstein. His publication in 1877 on the structure of anthrax bacterium marked the first photography of a bacterium. He discovered the formation of spores in anthrax bacteria, which could remain dormant under specific conditions. However, under optimal conditions, the spores were activated and caused disease. To determine this causative agent, he dry-fixed bacterial cultures onto glass slides, used dyes to stain the cultures, and observed them through a microscope. His work with anthrax is notable in that he was the first to link a specific microorganism with a specific disease, rejecting the idea of spontaneous generation and supporting the germ theory of disease.

In popular culture

• In Inuyasha, Naraku has the power of the miasma.
• In Inuyasha the Movie: Swords of an Honorable Ruler, as Sō'unga killed it the ogres, and according to Saya their corpses contained the miasma.
• In Episode 44 of Yashahime: Princess Half-Demon, the Grim Comet has the miasma.
• The video game Dwarf Fortress features miasma as a literal fog that emerges from rotten items and corpses. It reduces the happiness of the dwarves, though it doesn't necessarily cause them to fall ill.
• In Over the Garden Wall one of the characters is defeated by opening a window and letting the night air in.

Airborne transmission

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Airborne_transmission 

 

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 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.

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 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 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 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, aerosols/droplets 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).

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.

Common infections that spread by airborne transmission include SARS-CoV-2; measles morbillivirus, chickenpox virus; 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 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 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 monsoons.
• 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 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 (US) or FFP3 (EU) required. Use of FFP3 masks by staff managing patients with COVID-19 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 (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 competency plus the quantity of infectious particles ingested. Antibiotics may be used in dealing with airborne bacterial primary infections, such as pneumonic plague.

Aeroplankton

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Aeroplankton 

 

Sea spray containing marine microorganisms can be swept high into the atmosphere and may travel the globe before falling back to earth.

Aeroplankton (or aerial plankton) are tiny lifeforms that float and drift in the air, carried by wind. Most of the living things that make up aeroplankton are very small to microscopic in size, and many can be difficult to identify because of their tiny size. Scientists collect them for study in traps and sweep nets from aircraft, kites or balloons. The study of the dispersion of these particles is called Aerobiology.

Aeroplankton is made up mostly of microorganisms, including viruses, about 1,000 different species of bacteria, around 40,000 varieties of fungi, and hundreds of species of protists, algae, mosses, and liverworts that live some part of their life cycle as aeroplankton, often as spores, pollen, and wind-scattered seeds. Additionally, microorganisms are swept into the air from terrestrial dust storms, and an even larger amount of airborne marine microorganisms are propelled high into the atmosphere in sea spray. Aeroplankton deposits hundreds of millions of airborne viruses and tens of millions of bacteria every day on every square meter around the planet.

Small, drifting aeroplankton are found everywhere in the atmosphere, reaching concentration up to 10⁶ microbial cells per cubic metre. Processes such as aerosolisation and wind transport determine how the microorganisms are distributed in the atmosphere. Air mass circulation globally disperses vast numbers of the floating aerial organisms, which travel across and between continents, creating biogeographic patterns by surviving and settling in remote environments. As well as the colonization of pristine environments, the globetrotting behaviour of these organisms has human health consequences. Airborne microorganisms are also involved in cloud formation and precipitation, and play important roles in the formation of the phyllosphere, a vast terrestrial habitat involved in nutrient cycling.

Overview

The atmosphere is the least understood biome on Earth despite its critical role as a microbial transport medium. Recent studies have shown microorganisms are ubiquitous in the atmosphere and reach concentration up to 10⁶ microbial cells per cubic metre (28,000/cu ft)  and that they might be metabolically active. Different processes, such as aerosolisation, might be important in selecting which microorganisms exist in the atmosphere. Another process, microbial transport in the atmosphere, is critical for understanding the role microorganisms play in meteorology, atmospheric chemistry and public health.

Changes in species geographic distributions can have strong ecological and socioeconomic consequences. In the case of microorganisms, air mass circulation disperses vast amounts of individuals and interconnects remote environments. Airborne microorganisms can travel between continents, survive and settle on remote environments, which creates biogeographic patterns. The circulation of atmospheric microorganisms results in global health concerns and ecological processes such as widespread dispersal of both pathogens  and antibiotic resistances, cloud formation and precipitation, and colonization of pristine environments. Airborne microorganisms also play a role in the formation of the phyllosphere, which is one of the vastest habitats on the Earth's surface  involved in nutrient cycling.

The field of bioaerosol research studies the taxonomy and community composition of airborne microbial organisms, also referred to as the air microbiome. A recent series of technological and analytical advancements include high-volumetric air samplers, an ultra-low biomass processing pipeline, low-input DNA sequencing libraries, as well as high-throughput sequencing technologies. Applied in unison, these methods have enabled comprehensive and meaningful characterization of the airborne microbial organismal dynamics found in the near-surface atmosphere. Previous studies investigating bioaerosols using amplicon sequencing predominantly focussed on the bacterial fraction of the air microbiome, while fungal and plant pollen fractions frequently remained understudied. Airborne microbial organisms also impact agricultural productivity, as bacterial and fungal species distributed by air movement act as plant blights. Furthermore, atmospheric processes, such as cloud condensation and ice nucleation events were shown to depend on airborne microbial particles. Therefore, understanding the dynamics of microbial organisms in air is crucial for insights into the atmosphere as an ecosystem, but also will inform on human wellbeing and respiratory health.

In recent years, next generation DNA sequencing technologies, such as metabarcoding as well as coordinated metagenomics and metatranscriptomics studies, have been providing new insights into microbial ecosystem functioning, and the relationships that microorganisms maintain with their environment. There have been studies in soils, the ocean, the human gut, and elsewhere.

In the atmosphere, though, microbial gene expression and metabolic functioning remain largely unexplored, in part due to low biomass and sampling difficulties. So far, metagenomics has confirmed high fungal, bacterial, and viral biodiversity, and targeted genomics and transcriptomics towards ribosomal genes has supported earlier findings about the maintenance of metabolic activity in aerosols  and in clouds. In atmospheric chambers airborne bacteria have been consistently demonstrated to react to the presence of a carbon substrate by regulating ribosomal gene expressions.

Types

Pollen grains

See also: Palynology

Effective pollen dispersal is vital for maintenance of genetic diversity and fundamental for connectivity between spatially separated populations. An efficient transfer of the pollen guarantees successful reproduction in flowering plants. No matter how pollen is dispersed, the male-female recognition is possible by mutual contact of stigma and pollen surfaces. Cytochemical reactions are responsible for pollen binding to a specific stigma.

Allergic diseases are considered to be one of the most important contemporary public health problems affecting up to 15–35% of humans worldwide. There is a body of evidence suggesting that allergic reactions induced by pollen are on the increase, particularly in highly industrial countries.

Colourised SEM image of pollen grains from common plants

 

Pollen grains observed in aeroplankton of South Europe

Fungal spores

Drawings of fungal spores found in air. Some cause asthma, such as Alternaria alternata. A drawing of a very small "dust" seed from the flower Orchis maculata is provided for comparison.

 

    A = ascospore, B = basidiospore, M = mitospore

Pteridophyta spores, including fern spores, in the air of Lublin

Fungi, a major element of atmospheric bioaerosols, are capable of existing and surviving in the air for extended periods of time. Both the spores and the mycelium may be dangerous for people suffering from allergies, causing various health issues including asthma. Apart from their negative impact on human health, atmospheric fungi may be dangerous for plants as sources of infection. Moreover, fungal organisms may be capable of creating additional toxins that are harmful to humans and animals, such as endotoxins or mycotoxins.

Considering this aspect, aeromycological research is considered capable of predicting future symptoms of plant diseases in both crops and wild plants. Fungi capable of travelling extensive distances with wind despite natural barriers, such as tall mountains, may be particularly relevant to understanding the role of fungi in plant disease. Notably, the presence of numerous fungal organisms pathogenic to plants has been determined in mountainous regions.

A wealth of correlative evidence suggests asthma is associated with fungi and triggered by elevated numbers of fungal spores in the environment. Intriguing are reports of thunderstorm asthma. In a now classic study from the United Kingdom, an outbreak of acute asthma was linked to increases in Didymella exitialis ascospores and Sporobolomyces basidiospores associated with a severe weather event. Thunderstorms are associated with spore plumes: when spore concentrations increase dramatically over a short period of time, for example from 20,000 spores/m³ to over 170,000 spores/m³ in 2 hours. However, other sources consider pollen or pollution as causes of thunderstorm asthma. Transoceanic and transcontinental dust events move large numbers of spores across vast distances and have the potential to impact public health, and similar correlative evidence links dust blown off the Sahara with pediatric emergency room admissions on the island of Trinidad.

Pteridophyte spores

Pteridophyte life cycle

Pteridophytes are vascular plants that disperse spores, such as fern spores, Pteridophyte spores are similar to pollen grains and fungal spores, and are also components of aeroplankton. Fungal spores usually rank first among bioaerosol constituents due to their count numbers which can reach to between 1,000 and 10,000 per cubic metre (28 and 283/cu ft), while pollen grains and fern spores can each reach to between 10 and 100 per cubic metre (0.28 and 2.83/cu ft).

Arthropods

Spider ballooning structures. Black, thick points represent the spider's body. Black lines represent ballooning threads.

Many small animals, mainly arthropods (such as insects and spiders), are also carried upwards into the atmosphere by air currents and may be found floating several thousand feet up. Aphids, for example, are frequently found at high altitudes.

Ballooning, sometimes called kiting, is a process by which spiders, and some other small invertebrates, move through the air by releasing one or more gossamer threads to catch the wind, causing them to become airborne at the mercy of air currents. A spider (usually limited to individuals of a small species), or spiderling after hatching, will climb as high as it can, stand on raised legs with its abdomen pointed upwards ("tiptoeing"), and then release several silk threads from its spinnerets into the air. These automatically form a triangular shaped parachute which carries the spider away on updrafts of winds where even the slightest of breezes will disperse the arachnid. The flexibility of their silk draglines can aid the aerodynamics of their flight, causing the spiders to drift an unpredictable and sometimes long distance. Even atmospheric samples collected from balloons at 5 km (3.1 mi) altitude and ships mid-ocean have reported spider landings. Mortality is high.

Enough lift for ballooning may occur, even in windless conditions, if an electrostatic charge gradient is present in the atmosphere.

Nematodes

Distribution modes and possible geographic ranges of nematodes 

Nematodes (roundworms), the most common animal taxon, are also found among aeroplankton. Nematodes are an essential trophic link between unicellular organisms like bacteria, and larger organisms such as tardigrades, copepods, flatworms, and fishes. For nematodes, anhydrobiosis is a widespread strategy allowing them to survive unfavorable conditions for months and even years. Accordingly, nematodes can be readily dispersed by wind. However, as reported by Vanschoenwinkel et al., nematodes account for only about one percent of wind-drifted animals. Among the habitats colonized by nematodes are those that are strongly exposed to wind erosion as e.g., the shorelines of permanent waters, soils, mosses, dead wood, and tree bark. In addition, within a few days of forming temporary waters such as phytotelmata were shown to be colonized by numerous nematode species.

Unicellular microorganisms

A stream of unicellular airborne microorganisms circles the planet above weather systems but below commercial air lanes. Some microorganisms are swept up from terrestrial dust storms, but most originate from marine microorganisms in sea spray. In 2018, scientists reported that hundreds of millions of viruses and tens of millions of bacteria are deposited daily on every square meter around the planet.

The presence of airborne cyanobacteria and microalgae as well as their negative impacts on human health have been documented by many researchers worldwide. However, studies on cyanobacteria and microalgae are few compared with those on other bacteria and viruses. Research is especially lacking on the presence and taxonomic composition of cyanobacteria and microalgae near economically important water bodies with much tourism. Research on airborne algae is especially important in tourist areas near water-bodies. Sunbathers are exposed to particularly high quantities of harmful cyanobacteria and microalgae. Additionally, harmful microalgae and cyanobacteria blooms tend to occur in both marine and freshwater reservoirs during summer. Previous work has shown that the Mediterranean Sea is dominated by the picocyanobacteria Synechococcus sp. and Synechocystis sp., which are responsible for the production of a group of hepatotoxins known as microcystins. Because most tourism occurs in summer, many tourists are exposed to the most extreme negative impacts of airborne microalgae.

Comparison of windborne and surface-water prokaryote (bacteria plus archaea) communities over the Red Sea, showing their relative abundance during two years of DNA sequencing.

Airborne bacteria are emitted by most Earth surfaces (plants, oceans, land, and urban areas) to the atmosphere via a variety of mechanical processes such as aeolian soil erosion, sea spray production, or mechanical disturbances including anthropogenic activities. Due to their relatively small size (the median aerodynamic diameter of bacteria-containing particles is around 2–4 μm), these can then be transported upward by turbulent fluxes  and carried by wind to long distances. As a consequence, bacteria are present in the air up to at least the lower stratosphere. Given that the atmosphere is a large conveyor belt that moves air over thousands of kilometers, microorganisms are disseminated globally. Airborne transport of microbes is therefore likely pervasive at the global scale, yet there have been only a limited number of studies that have looked at the spatial distribution of microbes across different geographical regions. One of the main difficulties is linked with the low microbial biomass associated with a high diversity existing in the atmosphere outdoor (~10²–10⁵ cells/m³) thus requiring reliable sampling procedures and controls. Furthermore, the site location and its environmental specificities have to be accounted for to some extent by considering chemical and meteorological variables.

The environmental role of airborne cyanobacteria and microalgae is only partly understood. While present in the air, cyanobacteria and microalgae can contribute to ice nucleation and cloud droplet formation. Cyanobacteria and microalgae can also impact human health. Depending on their size, airborne cyanobacteria and microalgae can be inhaled by humans and settle in different parts of the respiratory system, leading to the formation or intensification of numerous diseases and ailments, e.g., allergies, dermatitis, and rhinitis. According to Wiśniewska et al., these harmful microorganisms can constitute between 13% and 71% of sampled taxa. However, the interplay between microbes and atmospheric physical and chemical conditions is an open field of research that can only be fully addressed using multidisciplinary approaches.

Airborne microalgae and cyanobacteria are the most poorly studied organisms in aerobiology and phycology. This lack of knowledge may result from the lack of standard methods for both sampling and further analysis, especially quantitative analytical methods. Few studies have been performed to determine the number of cyanobacteria and microalgae in the atmosphere. However, it was shown in 2012 that the average quantity of atmospheric algae is between 100 and 1000 cells per cubic meter of air. As of 2019, about 350 taxa of cyanobacteria and microalgae have been documented in the atmosphere worldwide. Cyanobacteria and microalgae end up in the air as a consequence of their emission from soil, buildings, trees, and roofs.

Biological particles are known to represent a significant fraction (~20–70%) of the total number of aerosols > 0.2 μm, with large spatial and temporal variations. Among these, microorganisms are of particular interest in fields as diverse as epidemiology, including phytopathology, bioterrorism, forensic science, and public health, and environmental sciences, like microbial ecology, meteorology and climatology. More precisely concerning the latter, airborne microorganisms contribute to the pool of particles nucleating the condensation and crystallization of water and they are thus potentially involved in cloud formation and in the triggering of precipitation. Additionally, viable microbial cells act as chemical catalyzers interfering with atmospheric chemistry. The constant flux of bacteria from the atmosphere to the Earth's surface due to precipitation and dry deposition can also affect global biodiversity, but they are rarely taken into account when conducting ecological surveys. As stressed by these studies attempting to decipher and understand the spread of microbes over the planet, concerted data are needed for documenting the abundance and distribution of airborne microorganisms, including at remote and altitudes sites.

Bioaerosols

Bioaerosols, known also as primary biological aerosols, are the subset of atmospheric particles that are directly released from the biosphere into the atmosphere. They include living and dead organisms (e.g., algae, archaea, bacteria ), dispersal units (e.g., fungal spores and plant pollen ), and various fragments or excretions (e.g., plant debris and brochosomes). Bioaerosol particle diameters range from nanometers up to about a tenth of a millimeter. The upper limit of the aerosol particle size range is determined by rapid sedimentation, i.e., larger particles are too heavy to remain airborne for extended periods of time. Bioaerosols include living and dead organisms as well as their fragments and excrements emitted from the biosphere into the atmosphere. Included are archaea, fungi, microalgae, cyanobacteria, bacteria, viruses, plant cell debris, and pollen.

Historically, the first investigations of the occurrence and dispersion of microorganisms and spores in the air can be traced back to the early 19th century. Since then, the study of bioaerosols has come a long way, and air samples collected with aircraft, balloons, and rockets have shown that bioaerosols released from land and ocean surfaces can be transported over long distances and up to very high altitudes, i.e., between continents and beyond the troposphere.

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  Global bioaerosol cycling

  After emission from the biosphere, bioaerosol particles interact with other aerosol particles and trace gases in the atmosphere and can be involved in the formation of clouds and precipitation. After dry or wet deposition to the Earth's surface, viable bioparticles can contribute to biological reproduction and further emission. This feedback can be particularly efficient when coupled to the water cycle (bioprecipitation).

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  Global ecosystem interactions of bioaerosol particles

  Key aspects and areas of research required to determine and quantify the interactions and effects of biogenic aerosol particles in the Earth system, including primary biological aerosols directly emitted to the atmosphere and secondary organic aerosols formed upon oxidation and gas-to-particle conversion of volatile organic compounds.

Bioaerosols play a key role in the dispersal of reproductive units from plants and microbes (pollen, spores, etc.), for which the atmosphere enables transport over geographic barriers and long distances. Bioaerosols are thus highly relevant for the spread of organisms, allowing genetic exchange between habitats and geographic shifts of biomes. They are central elements in the development, evolution, and dynamics of ecosystems.

Transport and distribution

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  Factors controlling microbial communities of the planetary boundary layer 

Once aerosolized, microbial cells enter the planetary boundary layer, defined as the air layer near the ground directly influenced by the planetary surface. The concentration and taxonomic diversity of airborne microbial communities in the planetary boundary layer has been recently described,though the functional potential of airborne microbial communities remains unknown.

From the planetary boundary layer, the microbial community might eventually be transported upwards by air currents into the free troposphere (air layer above the planetary boundary layer) or even higher into the stratosphere. Microorganisms might undergo a selection process during their way up into the troposphere and the stratosphere.

Subject to gravity, aerosols (or particulate matter) as well as bioaerosols become concentrated in the lower part of the troposphere that is called the planetary boundary layer. Microbial concentrations thus usually show a vertical stratification from the bottom to the top of the troposphere with average estimated bacterial concentrations of 900 to 2 × 10⁷ cells per cubic metre in the planetary boundary layer  and 40 to 8 × 10⁴ cells per cubic metre in the highest part of the troposphere called the free troposphere. The troposphere is the most dynamic layer in terms of chemistry and physics of aerosols and harbors complex chemical reactions and meteorological phenomena that lead to the coexistence of a gas phase, liquid phases (i.e., cloud, rain, and fog water) and solid phases (i.e., microscopic particulate matter, sand dust). The various atmospheric phases represent multiple biological niches.

Possible processes in the way atmospheric microbial communities can distribute themselves have recently been investigated in meteorology, seasons, surface conditions  and global air circulation.

Over space and time

Microorganisms attached to aerosols can travel intercontinental distances, survive, and further colonize remote environments. Airborne microbes are influenced by environmental and climatic patterns that are predicted to change in the near future, with unknown consequences. Airborne microbial communities play significant roles in public health and meteorological processes, so it is important to understand how these communities are distributed over time and space.

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  Atmosphere layers, temperature and airborne emission sources

Most studies have focused on laboratory cultivation to identify possible metabolic functions of microbial strains of atmospheric origin, mainly from cloud water. Given that cultivable organisms represent about 1% of the entire microbial community, culture-independent techniques and especially metagenomic studies applied to atmospheric microbiology have the potential to provide additional information on the selection and genetic adaptation of airborne microorganisms.

There are some metagenomic studies on airborne microbial communities over specific sites. Metagenomic investigations of complex microbial communities in many ecosystems (for example, soil, seawater, lakes, feces and sludge) have provided evidence that microorganism functional signatures reflect the abiotic conditions of their environment, with different relative abundances of specific microbial functional classes. This observed correlation of microbial-community functional potential and the physical and chemical characteristics of their environments could have resulted from genetic modifications (microbial adaptation ) and/or physical selection. The latter refers to the death of sensitive cells and the survival of resistant or previously adapted cells. This physical selection can occur when microorganisms are exposed to physiologically adverse conditions.

The presence of a specific microbial functional signature in the atmosphere has not been investigated yet. Microbial strains of airborne origin have been shown to survive and develop under conditions typically found in cloud water (i.e., high concentrations of H₂O₂, typical cloud carbonaceous sources, ultraviolet – UV – radiation etc. While atmospheric chemicals might lead to some microbial adaptation, physical and unfavorable conditions of the atmosphere such as UV radiation, low water content and cold temperatures might select which microorganisms can survive in the atmosphere. From the pool of microbial cells being aerosolized from Earth's surfaces, these adverse conditions might act as a filter in selecting cells already resistant to unfavorable physical conditions. Fungal cells and especially fungal spores might be particularly adapted to survive in the atmosphere due to their innate resistance  and might behave differently than bacterial cells. Still, the proportion and nature (i.e., fungi versus bacteria) of microbial cells that are resistant to the harsh atmospheric conditions within airborne microbial communities are unknown.

Airborne microbial transport is central to dispersal outcomes  and several studies have demonstrated diverse microbial biosignatures are recoverable from the atmosphere. Microbial transport has been shown to occur across inter-continental distances above terrestrial habitats. Variation has been recorded seasonally, with underlying land use, and due to stochastic weather events such as dust storms. There is evidence specific bacterial taxa (e.g., Actinomycetota and some Gammaproteobacteria) are preferentially aerosolized from oceans.

Over urban areas

Dust storms as a source of aerosolized bacteria

As a result of rapid industrialization and urbanization, global megacities have been impacted by extensive and intense particulate matter pollution events, which have potential human health consequences. Severe particulate matter pollution is associated with chronic obstructive pulmonary disease and asthma, as well as risks for early death. While the chemical components of particulate matter pollution and their impacts on human health have been widely studied, the potential impact of pollutant-associated microbes remains unclear. Airborne microbial exposure, including exposure to dust-associated organisms, has been established to both protect against and exacerbate certain diseases. Understanding the temporal dynamics of the taxonomic and functional diversity of microorganisms in urban air, especially during smog events, will improve understanding of the potential microbe-associated health consequences.ion and metagenomic library preparation have enabled low biomass environments to be subject to shotgun sequencing analysis. In 2020, Qin et al. used shotgun sequencing analysis to reveal a great diversity of microbial species and antibiotic resistant genes in Beijing's particulate matter, largely consistent with a recent study. The data suggest that potential pathogen and antibiotic resistance burden increases with increasing pollution levels and that severe smog events promote the exposure. In addition, the particulate matter also contained several bacteria that harbored antibiotic resistant genes flanked by mobile genetic elements, which could be associated with horizontal gene transfer. Many of these bacteria were typical or putative members of the human microbiome.

Dispersal

Dispersal is a vital component of an organism's life-history, and the potential for dispersal determines the distribution, abundance, and thus, the community dynamics of species at different sites.  A new habitat must first be reached before filters such as organismal abilities and adaptations, the quality of a habitat, and the established biological community determine the colonization efficiency of a species. While larger animals can cover distances on their own and actively seek suitable habitats, small (<2 mm) organisms are often passively dispersed, resulting in their more ubiquitous occurrence. While active dispersal accounts for rather predictable distribution patterns, passive dispersal leads to a more randomized immigration of organisms. Mechanisms for passive dispersal are the transport on (epizoochory) or in (endozoochory) larger animals (e.g., flying insects, birds, or mammals) and the erosion by wind.

A propagule is any material that functions in propagating an organism to the next stage in its life cycle, such as by dispersal. The propagule is usually distinct in form from the parent organism. Propagules are produced by plants (in the form of seeds or spores), fungi (in the form of spores), and bacteria (for example endospores or microbial cysts). Often cited as an important requirement for effective wind dispersal is the presence of propagules (e.g., resting eggs, cysts, ephippia, juvenile and adult resting stages), which also enables organisms to survive unfavorable environmental conditions until they enter a suitable habitat. These dispersal units can be blown from surfaces such as soil, moss, and the desiccated sediments of temporary or intermittent waters. The passively dispersed organisms are typically pioneer colonizers.

However, wind-drifted species vary in their vagility (probability to be transported with the wind), with the weight and form of the propagules, and therefore, the wind speed required for their transport, determining the dispersal distance. For example, in nematodes, resting eggs are less effectively transported by wind than other life stages, while organisms in anhydrobiosis are lighter and thus more readily transported than hydrated forms. Because different organisms are, for the most part, not dispersed over the same distances, source habitats are also important, with the number of organisms contained in air declining with increasing distance from the source system. The distances covered by small animals range from a few meters, to hundreds, to thousands of meters. While the wind dispersal of aquatic organisms is possible even during the wet phase of a transiently aquatic habitat, during the dry stages a larger number of dormant propagules are exposed to wind and thus dispersed. Freshwater organisms that must "cross the dry ocean"  to enter new aquatic island systems will be passively dispersed more successfully than terrestrial taxa. Numerous taxa from both soil and freshwater systems have been captured from the air (e.g., bacteria, several algae, ciliates, flagellates, rotifers, crustaceans, mites, and tardigrades). While these have been qualitatively well studied, accurate estimates of their dispersal rates are lacking.

Clouds

Impact of microbial activity on clouds 

Biological processes and their targets are indicated by green arrows, while red arrows indicate abiotic processes.

EPS: Exopolysaccharide              SOA: Secondary organic aerosol
Based on coordinated metagenomics/metatranscriptomics

The outdoor atmosphere harbors diverse microbial assemblages composed of bacteria, fungi and viruses  whose functioning remains largely unexplored. While the occasional presence of human pathogens or opportunists can cause potential hazard, in general the vast majority of airborne microbes originate from natural environments like soil or plants, with large spatial and temporal variations of biomass and biodiversity. Once ripped off and aerosolized from surfaces by mechanical disturbances such as those generated by wind, raindrop impacts or water bubbling, microbial cells are transported upward by turbulent fluxes. They remain aloft for an average of ~3 days, a time long enough for being transported across oceans and continents  until being finally deposited, eventually helped by water condensation and precipitation processes; microbial aerosols themselves can contribute to form clouds and trigger precipitation by serving as cloud condensation nuclei  and ice nuclei.

Living airborne microorganisms may end up concretizing aerial dispersion by colonizing their new habitat, provided that they survive their journey from emission to deposition. Bacterial survival is indeed naturally impaired during atmospheric transport, but a fraction remains viable. At high altitude, the peculiar environments offered by cloud droplets are thus regarded in some aspects as temporary microbial habitats, providing water and nutrients to airborne living cells. In addition, the detection of low levels of heterotrophy  raises questions about microbial functioning in cloud water and its potential influence on the chemical reactivity of these complex and dynamic environments. The metabolic functioning of microbial cells in clouds is still albeit unknown, while fundamental for apprehending microbial life conditions during long distance aerial transport and their geochemical and ecological impacts.

Aerosols affect cloud formation, thereby influencing sunlight irradiation and precipitation, but the extent to which and the manner in which they influence climate remains uncertain. Marine aerosols consist of a complex mixture of sea salt, non-sea-salt sulfate and organic molecules and can function as nuclei for cloud condensation, influencing the radiation balance and, hence, climate. For example, biogenic aerosols in remote marine environments (for example, the Southern Ocean) can increase the number and size of cloud droplets, having similar effects on climate as aerosols in highly polluted regions. Specifically, phytoplankton emit dimethylsulfide, and its derivate sulfate promotes cloud condensation. Understanding the ways in which marine phytoplankton contribute to aerosols will allow better predictions of how changing ocean conditions will affect clouds and feed back on climate. In addition, the atmosphere itself contains about 10²² microbial cells, and determining the ability of atmospheric microorganisms to grow and form aggregates will be valuable for assessing their influence on climate.

After the tantalizing detection of phosphine (PH₃) in the atmosphere of the Venus planet, and in the absence of a known and plausible chemical mechanism to explain the formation of this molecule, Greaves et al. speculated in 2020 that microorganisms might be present in suspension in the Venus atmosphere. They have formulated the hypothesis of the microbial formation of phosphine, envisaging the possibility of a liveable window in the Venusian clouds at a certain altitude with an acceptable temperature range for microbial life. However, in 2021 Hallsworth et al. examined the conditions required to support the life of extremophile microorganisms in the clouds at high altitude in the Venus atmosphere where favorable temperature conditions might prevail. Beside the presence of sulfuric acid in the clouds which already represent a major challenge for the survival of most of microorganisms, they came to the conclusion that the Venus atmosphere is too dry to host microbial life. They determined a water activity ≤ 0.004, two orders of magnitude below the 0.585 limit for known extremophiles.

Airborne microbiomes

See also: Microbiome

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  Biological habitats (niches) in aeromicrobiology 

While the physical and chemical properties of airborne particulate matter have been extensively studied, their associated airborne microbiome remains largely unexplored. Microbiomes are defined as characteristic microbial communities, which include prokaryotes, fungi, protozoa, other micro-eukaryotes and viruses, that occupy well-defined habitats. The term microbiome is broader than other terms, for example, microbial communities, microbial population, microbiota or microbial flora, as microbiome refers to both its composition (the microorganisms involved) and its functions (their members' activities and interactions with the host/environment), which contribute to ecosystem functions.

Throughout Earth's history, microbial communities have changed the climate, and climate has shaped microbial communities. Microorganisms can modify ecosystem processes or biogeochemistry on a global scale, and we start to uncover their role and potential involvement in changing the climate. However, the effects of climate change on microbial communities (i.e., diversity, dynamics, or distribution) are rarely addressed. In the case of fungal aerobiota, its composition is likely influenced by dispersal ability, rather than season or climate. Indeed, the origin of air masses from marine, terrestrial, or anthropogenic-impacted environments, mainly shapes the atmospheric air microbiome. However, recent studies have shown that meteorological factors and seasonality influence the composition of airborne bacterial communities. This evidence suggests that climatic conditions may act as an environmental filter for the aeroplankton, selecting a subset of species from the regional pool, and raises the question of the relative importance of the different factors affecting both bacterial and eukaryal aeroplankton.

In 2020, Archer et al. reported evidence for a dynamic microbial presence at the ocean–atmosphere interface at the Great Barrier Reef, and identified air mass trajectories over oceanic and continental surfaces associated with observed shifts in airborne bacterial and fungal diversity. Relative abundance of shared taxa between air and coral microbiomes varied between 2.2 and 8.8% and included those identified as part of the core coral microbiome. Above marine systems, the abundance of microorganisms decreases exponentially with distance from land, but relatively little is known about potential patterns in biodiversity for airborne microorganisms above the oceans. Here we test the hypothesis that persistent microbial inputs to the ocean–atmosphere interface of the Great Barrier Reef ecosystem vary according to surface cover (i.e. land vs. ocean) during transit of the air-mass.