People who have respiratory illnesses often breathe out particles that are infectious. What’s the size range of these “infectious respiratory particles”?
These particles, which WHO abbreviates as IRPs, range in size but are generally quite small.
Infectious respiratory particles move through the air in what WHO refers to as a “turbulent puff cloud” — a mix of gasses and respiratory particles from the lungs. Once the cloud slows down sufficiently, what happens?
Once the particles spread out in an indoor space, you’re more likely to encounter some fraction of them. To pick up an infection, however, you need to inhale a sufficient amount.
In the new report, WHO identifies two ways infectious respiratory particles move through the air. The first is airborne transmission or inhalation. In this scenario, where might the particles enter someone’s body?
It’s worth noting that other types of pathogens may prefer different sites of entry.
Another way that infectious respiratory particles can be transmitted through the air is “direct deposition.” What defines this mode of transmission?
With direct deposition, the particles deposit on the mucous membranes of the mouth, nose or eyes.
Many factors influence how particles spread and their potential impact, including evaporation, environmental conditions and speed of expulsion. Which of the following activities leads to a “more forceful expiration” in WHO’s parlance?
All of these activities are able to propel particles forcefully. Coughing is another one.
If you’re exposed to an airborne pathogen, are you automatically infected?
Infection “can only occur after the expelled [infectious respiratory particles] enter the respiratory tract, come into contact with the respiratory tissues, followed by multiplication of the infectious pathogens within a susceptible person,” according to the report. That is, the full chain of transmission is required.
The transmission of infectious respiratory particles is more likely indoors than outdoors. Why?
In general, when the particles are dispersed rather than concentrated, you are less likely to be exposed to enough of them to become infected.
The new report says it’s not a good idea to divide particles into aerosols (smaller) and droplets (larger). Why is this?
WHO says size cutoffs are fairly arbitrary, and all particles should be considered together.
For completeness, the report discusses “direct contact” and “indirect contact” as modes of transmission that don’t involve a route through the air. How do these modes differ?
Direct contact refers to the practice of exchanging an infectious agent through a direct physical transfer.
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