The COVID Report with David Blake: Aerosols

Monday, Sept. 21

On Sunday, the CDC changed its guidance on transmission of the coronavirus. It now offers that most transmission occurs through the air as aerosol (or small droplet) transmission.

Their updated guidance:

What does this mean?

First, it does not change epidemiological guidance. The epidemiologist studies how people are interacting when transmission occurs, and it still occurs mostly when people are in close proximity, particularly indoors, particularly unmasked, for 15 minutes or longer.

However, the knowledge that it is aerosol transmission means it can be attacked in a few simple ways.

  1. Move your activity outdoors. Air exchange rates outdoors are the golden standard. Public Health officials in Australia claim it to be 20 times safer than indoors (other authorities variously rate it up to 200 times safer).
  2. Filter your air. The small respiratory droplets that are aerosols can be filtered with MERV-13 HEPA filters.
  3. Disinfect your air. Another HVAC approach is applying UV in the air ducts to kill the virus. There are standard doses to use for standard airflows, an HVAC engineer should be consulted.
  4. Increase exchanges with outdoor air.
  5. Open your windows.

The are things you can do.

  1. Write an email to the facilities person in charge of buildings you occupy.
  2. Point out this change in CDC guidance (send them the link)
  3. Ask if the building airflow is being altered to protect its occupants better, and if so, how? 

Some of these changes are remarkably cost effective.

Here is a plot of provisional new cases for the three populous counties in the Central Savannah River Area. Things have been fairly flat for the last few weeks, with hospital occupancy at roughly one third of the peak levels.


Contact David Blake at