Airborne transmission

Airborne transmission or aerosol transmission is

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

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

Overview

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

Airborne pathogens or

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

Some airborne diseases can affect non-humans. For example,

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

Transmission efficiency

Environmental factors influence the efficacy of airborne disease transmission; the most evident environmental conditions are

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

• Relative humidity (RH) plays an important role in the evaporation of droplets and the distance they travel. 30 μm droplets evaporate in seconds.^[29] The CDC recommends a minimum of 40% RH indoors^[30] 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^[31] (more important than total precipitation);^[32][33] mean daily sunshine hours;^[34] latitude and altitude^[32] 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.^[35]
• Climate affects temperature, winds and relative humidity, the main factors affecting the spread, duration and infectiousness of droplets containing infectious particles.
  Northern Hemisphere winter due to climate conditions that favour the infectiousness of the virus.^[27]
• Isolated weather events decrease the concentration of airborne fungal spores; a few days later, number of spores increases exponentially.^[36]
• 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.^[37]
• Proximity to large bodies of water such as rivers and lakes can enhance airborne disease.^[35]
• 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.^[4]
• Poor maintenance of air conditioning systems has led to outbreaks of Legionella pneumophila.^[38]
• Hospital-acquired airborne diseases are associated with poorly-resourced and maintained medical systems,^[39] which make isolation challenging.^{[citation needed]}
• Air conditioning may reduce transmission by removing contaminated air, but may also contribute to the spread of respiratory secretions inside a room.^[9]

Prevention

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

Preventive techniques can include disease-specific

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

The

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

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

See also

• Aeroplankton
• Basic reproduction number
• Miasma theory
• Vector (epidemiology)
• Waterborne diseases
• Zoonosis

References