Q3. Why can't we use a regular fan to remove aerosols?

While a regular fan might help a little, the wide air pattern it emits [view here in Part 2] will not create a physical barrier between the worker and the resident unless the fan is close to the interaction.  Such proximity would introduce operational discomfort for the worker, often getting in the way of maneuvers.  Worse, a regular fan does not use an air filter therefore it does not reduce airborne contaminants like the crosswind unit by eventually absorbing most of them.  Instead, a regular fan recirculates infectious microdroplets and aerosols back into the room keeping them suspended and available for infection.  Although a regular fan may offer a small degree of initial defense if used properly, it is advisable not to use a regular fan to mitigate airborne pathogen transmission because it would give a false sense of security and therefore should not be relied on. The W.H.O. stated the following (July 29, 2020):
  • "Can fans be used safely inside during the COVID-19 pandemic?  Air blowing from an infected person directly at another person in a closed space increases the risk of the virus spreading from one person to another."
This is relevant to the 'in-the-line-of-fire' scenario in the worst case condition when there is no air purification from the fan (see Question Q11 for an in-depth explanation).  The crosswind concept stipulates that the air stream must be aimed transversely between two people.  It is this important configuration that gives this solution such high efficacy, which is in apparent opposition to the W.H.O. statement above.
The W.H.O. also stated the following for this question:
  • "If COVID-19 is circulating in your community and the use of a table or pedestal fan is unavoidable, it is important to regularly bring in air from outside by opening windows or doors, while minimizing how much air blows from one person (or group of people) to another person (or group of people)."
The crosswind concept does this by filtering the air before intercepting the aerosols between two people, thus outside air  is not needed when the system is correctly deployed and used.