An airborne transmission is disease transmission through small particulates that can be transmitted through the air over time and distance. Diseases capable of airborne transmission include many of considerable importance both in human and veterinary medicine. The relevant pathogens 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. Human airborne diseases do not include conditions caused by air pollution such as volatile organic compounds (VOCs), gases and any airborne particles.
Airborne transmission traditionally has been considered distinct from transmission by droplets, based on misunderstandings of the physical behaviour of particles of various sizes. Respiratory droplets were thought to be large enough to fall to the ground rapidly after being produced; however, if particles of any size can be inhaled, they are essentially airborne particles.
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 fall to the ground, and settle within 2 m. Particles can carry airborne pathogens for extended periods of time. There is a greater concentration of airborne pathogens within 2m; however, they can travel farther and build up in concentration in a room.
The traditional hard size cutoff of 5 μm between airborne and respiratory droplets has been criticized as a false dichotomy not grounded in science, as exhaled particles form a continuum of sizes whose fates depend on environmental conditions in addition to their initial sizes. However, it has informed hospital based transmission based precautions for decades. Recent data on indoor respiratory secretion transfer 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 larger distance. 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.
Airborne diseases include any that are caused via transmission through the air. Many airborne diseases are of great medical importance. The pathogens transmitted may be any kind of microbe, and they may be spread in aerosols, dust or liquids. The aerosols might be generated from sources of infection such as the bodily secretions of an infected animal or person, or biological wastes such as accumulate in lofts, caves, garbage and the like. Such infected aerosols may stay suspended in air currents long enough to travel for considerable distances; sneezes, for example, can easily project infectious droplets the full length of a bus.
Airborne pathogens or allergens often cause inflammation in the nose, throat, sinuses and the lungs. This is caused by the inhalation of these pathogens that affect a person's respiratory system or even the rest of the body. Sinus congestion, coughing and sore throats are examples of inflammation of the upper respiratory air way due to these airborne agents. Air pollution plays a significant role in airborne diseases which is linked to asthma. Pollutants are said to influence lung function by increasing air way inflammation.
Many common infections can spread by airborne transmission at least in some cases, including but not limited to: COVID-19; measles morbillivirus, chickenpox virus; Mycobacterium tuberculosis, influenza virus, enterovirus, norovirus and less commonly coronavirus, adenovirus, and possibly respiratory syncytial virus. Transmission likely often starts with inhalation of the infectious agent (eg the SARS-CoV-2 virus for COVID-19) and is expected to be most likely after long stays in crowded, poorly ventilated rooms. Crowded rooms are more likely to contain an infected person, with poor ventilation the infectious agent will persist in the room air, and the longer the stay, the more air a susceptible person breathes in. Airborne transmission is complex, and hard to demonstrate unequivocally but there is a simple model that can be used to make simple estimates of the probability of becoming infected: the Wells-Riley model.
Airborne diseases can also affect non-humans. For example, Newcastle disease is an avian disease that affects many types of domestic poultry worldwide which is transmitted via airborne contamination. Often, airborne pathogens or allergens cause inflammation in the nose, throat, sinuses, and the upper airway lungs. Upper airway inflammation causes coughing congestion, and sore throat. This is caused by the inhalation of these pathogens that affect a person's respiratory system or even the rest of the body. Sinus congestion, coughing and sore throats are examples of inflammation of the upper respiratory air way due to these airborne agents.
Airborne transmission can be classified into three categories; obligate, preferential, and opportunistic. The first category, obligate airborne infections, are only through aerosols and 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. The last category, opportunistic airborne infections, typically transmit through other routes; however, under favourable conditions, aerosol transmission can occur. An example of an opportunistic airborne infection includes influenza. Because the drying process often damages the pathogens, the number of diseases that can be spread through an airborne route is limited.
Routes of transmission
Airborne infections usually occur by the respiratory route, with the agent present in aerosols (infectious particles < 5 µm in diameter). This includes dry particles, often the remainders of an evaporated wet particle called nuclei, and wet particles. This kind of infection usually requires independent ventilation during treatment. e.g., tuberculosis.
Relative Humidity (RH)
Relative humidity (RH) plays an important role in the evaporation of droplets and the distance they travel. The 30 μm droplets evaporate in seconds.
The CDC recommends a minimum of 40% RH indoors. Maintaining indoor relative humidity >40% will 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, there is more virus with infectious potential in the air.
Environmental factors influence the efficacy of airborne disease transmission; the most evident environmental conditions are temperature and relative humidity. The sum of all the factors that influence temperature and humidity, either meteorological (outdoor) or human (indoor), as well as other circumstances influencing the spread of droplets containing infectious particles, as winds, or human behavior, influence the transmission of airborne diseases.
- Rainfall, the number of rainy days being more important than total precipitation; mean daily sunshine hours; latitude and altitude are relevant when assessing the possibility of spread of any airborne infection. Some infrequent or exceptional events influence the dissemination of airborne diseases, including tropical storms, hurricanes, typhoons, or monsoons.
Climate determines temperature, winds and relative humidity, the main factors affecting the spread, duration and infectiousness of droplets containing infectious particles. The influenza virus is spread easily in the Northern Hemisphere winter due to climate conditions which favour the infectiousness of the virus.
- After isolated weather events, the concentration of airborne fungal spores is decreased; a few days later, an exponentially increased number of spores is found, compared to normal conditions.
- Socioeconomics has a minor role in airborne disease transmission. In cities, the spread of airborne disease is more rapid 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 be a cause of some outbreaks of airborne disease.
- Poor maintenance of air conditioning systems has led to outbreaks of Legionella pneumophila.
- Hospital-acquired airborne diseases are associated with poorly-resourced medical systems, which make isolation challenging.
Some ways to prevent airborne diseases include disease-specific immunization, wearing a respirator and limiting time spent in the presence of any patient likely to be a source of infection. Wearing a face mask can also lower the risk of transmission, as it slows down the air flow between individuals. Exposure to a patient or animal with an airborne disease does not guarantee contracting the disease, as infection is dependent on host immune system competency plus the quantity of infectious particles ingested.
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 the transmission of airborne infections.
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 when their operator has a respiratory infection, as they were spreading a much higher number of aerosol particles than e.g. the act of shouting.
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Airborne transmission occurs by dissemination of either airborne droplet nuclei or small particles in the respirable size range containing infectious agents that remain infective over time and distance
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