1. Engineer clean buildings 

Assess and identify the hazards and apply engineering controls to avoid and minimize them.

2. If engineering fails, humans must wear PPE

Only when all engineering potentials have been applied there may be hazards which should be managed by personal protective equipment.

3. PPE failure requires medical intervention 

To monitor the effectiveness of and the biological impact in the event of failure of personal protective equipment - medical diagnosis and management of adverse effects and injuries may be required.

We rely on engineering to create safe environments. Only when these fail to do their job we must try to avoid injury and biological damage by using equipment designed to render the hazard harmless. Failure of protective equipment leads to the need for medical (drugs) and  ICU management of the consequences.

1 ) The initial public health message of managing COVID-19 by avoiding droplets has clearly shifted to the  searching for ways to avoid its microdroplet transmission.
The initial strategies of social distancing and wearing a droplet containment mask are not sufficient to avoid transmission when indoors.

2) Covid-19 viral transmission did not fade with seasonal weather change as many had hoped

3) Our existing engineering of buildings and mass transit are not suited to containing and limiting transmission of microdroplet COVID-19 virus.
There is urgent need to optimize indoor air quality through ventilation and filtration as the two key means known to be effective in reducing viral transmission among humans.

The COVID-19 Journey - Overview of the COVID Pandemic