#1 Air Cleaning: 


1-  Filtration

For filtration it is useful to understand 2 words:  MERV and HEPA 

MERV rating is an abbreviation for Minimum Efficiency Reporting Value.    

See US Govt MERV rating

HEPA filters are mechanical filters designed to remove particulates from air resulting in ‘cleaner’ air. A good HEPA filter will remove almost all particulates from an air flow according to the quality of the filter materials and construction. 

HEPA filter is often impractical in central HVAC systems due to the large pressure drop the dense filter material causes. Experiments indicate that less obstructive, medium-efficiency filters of MERV 13 and higher can filter virus particles yet with much lower associated system and operating costs than HEPA filters.

These tools are our current heroes because of their ability to remove the majority of air contaminants down to the actual COVID-19 coronavirus particles (0.12 microns or 125 nanometers  ie = submicron) [Reference]

2 - Activated Charcoal 
  • Has well understood small absorbent pores that chemically react to the pollutants to bond with the carbon and become trapped.
  • Particularly effective for fumes & smoke but more limited effect on virus particles.
3 - Germicidal Ultraviolet Light
  • Has been applied in ‘clean’ room environments for decades.
  • Its use in indoor air cleaners to reduce viral loads in moving air is currently warranted investigation.
4Ionic Filter
  • An air ionizer is built around a negative ion generator. This generator sends out a stream of negative ions that attract positive airborne dust and allergen particles in the air. As a result, these particles may become too heavy to remain airborne and fall to the floor or may be captured on a plate.
  • A potential health hazard as ozone is produced by ionizers as a by product.

Here is an example of ultra high filtration and strong air flow output portable air purifying unit (AirPura) used in tandem in a clinical setting which employs:

  • High capacity air volume flow (560CFM)
  • High grade HEPA filter
  • Germicidal UV light
  • Activated Charcoal filter
Photo with permission from PureAirWizard

#2 Air Flow [dilution]:

1-         Ducted Systems

Optimally indoor air quality can be controlled by ensuring clean air is flowing through the building’s central piping distribution of “air ducts”. Air is filtered as it enters the system.  There are classifications to rate the degree of filtration achieved known as MERV rating*  (ie - MERV 13 or greater is optimal) .

[*An air filter 's minimum efficiency reporting value (MERV) rating measures how effectively the filter stops dust and other contaminants from passing through the filter and into the air stream. Filters with higher MERV ratings trap small particles more effectively than filters with lower MERV ratings.]


HEPA is a well understood technology which does a splendid job of removing reliably very small particles ... much better than the routinely used paper filters in most homes and small buildings.

  • Unfortunately, HEPA requires more powerful motors to move the air through such fine filtering than current systems are designed for.
  • Thus expensive retrofit may be required to enable the use of such filtering.
  • In a setting where cost is not a constraint “clean room” and “laminar flow” techniques can be applied to ensure air quality.

2 – Non-ducted Systems

In many smaller indoor environments where no central piping distribution of air ducts exists, air flow to clean or remove contaminated air may rely upon portable air purifiers.    


Read the packaging data: you need to determine the volume output capacity = cfm or l/min. Quality manufacturers will provide the specific information of volume output.

Commonly Labeled with "room size" this relates to normal air circulation in the “pre covid” era when the commonly accepted rate of 2 air exchanges/hr was considered acceptable.  On the contrary, much higher air exchange rates are now sought and some manufacturers have responded with production of large volume portable air filters (200-600cu ft/ min) which are likely more effective in reducing virus particle concentrations accumulating in stagnant air.

As such room size asoften provided as consumer guidance is only a vague approximation of the air flow needs in the Covid-19 era and often leads to underpowered and misleading claims.

Remember: To achieve adequate air filtration of COVID-19 virus particles in portable air purifiers, very high flow rate is needed with powerful motors.

In Summary, effective air purification requires 2 components:

  • Effective air cleaning - Choose appropriate method
  • Adequate airflow (i.e. cubic feet /minute) - Size matters - there are only a few manufacturers of large volume portable air filters.

After all engineering options have been exhausted - PPE is the remaining option to ensure safety

Learn More about PPE