COVID- 19 airborne transmission
What is airborne transmission?
- Airborne transmission is defined as the spread of an infectious agent caused by the dissemination of droplet nuclei (aerosols) that remain infectious when suspended in air over long distances and time.
- Airborne transmission of SARS-CoV-2 can occur during medical procedures that generate aerosols (“aerosol generating procedures”).
- WHO, together with the scientific community, has been actively discussing and evaluating whether SARS-CoV-2 may also spread through aerosols in the absence of aerosol generating procedures, particularly in indoor settings with poor ventilation.
Mechanism of transmission through aerosols
- Possible mechanisms of SARS-CoV-2 transmission through aerosols suggest that
1) a number of respiratory droplets generate microscopic aerosols (<5 µm) by evaporating, and
2) normal breathing and talking results in exhaled aerosols.
- Thus, a susceptible person could inhale aerosols, and could become infected if the aerosols contain the virus in sufficient quantities to cause infection within the recipient.
- However, the proportion of exhaled droplet nuclei or of respiratory droplets that evaporate to generate aerosols, and the infectious dose of viable SARS-CoV-2 required to cause infection in another person are not known, but it has been studied for other respiratory viruses.
What are aerosols? How different are they from respiratory droplets?
- Aerosol is a term used to broadly refer to particles suspended in the air; they could include fine dust, mist, or smoke.
- In the context of transmission of viruses, as in this case, aerosols are read as micro droplets, much smaller (5 microns or less) than respiratory droplets, and take a longer time to drop to the floor.
- They will be expelled by people breathing, laughing or singing, as against respiratory droplets that are expelled with forceful acts such as sneezing or coughing.
- As they remain suspended in the air for longer, an individual who is COVID-19 positive is likely to infect people standing even at a distance of 1-2 m in a small, poorly ventilated room which poses the risk that people sharing such environments can potentially inhale these viruses, resulting in infection and disease.
Why in the news?
- 239 scientists from 32 countries signed an open letter addressed to the World Health Organisation that said COVID-19 is also transmitted via aerosols.
However, WHO waits for more robust evidence on the principle of aerosol transmission
- Provide sufficient and effective ventilation as far as possible in public buildings, schools and hospitals
- Avoid overcrowding in public buildings and transportation systems
- Supplement general ventilation with airborne infection controls such as local exhaust, high efficiency air filtration, and germicidal ultraviolet lights.
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