Airborne transmission

 ( Disease Transmission ) 

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, Tropical cyclone, , or .
• 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 Spore; 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.
• Air conditioning may reduce transmission by removing contaminated air, but may also contribute to the spread of respiratory secretions inside a room.
• The new findings reveal that understanding airflow patterns is even more crucial than simply increasing air changes per hour. During the COVID-19 pandemic, the common advice was to maximize ventilation, but this may not always be the most effective approach. A room can be well-prepared to prevent the spread of infectious diseases even at a low ACH. This insight could lead to safer building designs and significant energy savings during future pandemics.

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 HEPA, reduce detectable levels of virus and other , 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 (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. may be used in dealing with airborne bacterial primary infections, such as pneumonic plague.