IAQ: Air Filtration

Definition of air filter: a porous substance that allows air to pass but retains suspended solid particles

What about masks?

There are three air filters featured in the above picture but only one is called an air filter, the one in the return air stream of the HVAC system. But, the vacuum cleaner bag and the medical mask are both air filters, also. All three are disposable (exception: the SonoMask) and all three must be replaced when they become dirty.

Related news article from the NY Post, 20 Apr 2021 | 8 different types of face masks and when to wear them: KN95, 3M, N95

An practical mask for the general public is the new washable SonoMask manufactured by Israeli company Sonovia. This human ‘air filter’ endures 55 weekly washings and can protect the wearer for over a full year. Available from Amazon.com individually or bulk discount. Embedded antibiotic material neutralizes pathogens. The reader is encouraged to consider all the information available on this page

Commercial air filters (including the vacuum cleaner bag) are rated according to their effectiveness at arresting airborne particles from the respective air streams.

Focus on Surgical Masks

Reference – Master of Science Thesis by Erin Sanchez, February 18 2010: Filtration Efficiency of Surgical Masks | Download this document in PDF format

Conclusions of thesis, page 42 “Surgical masks are more appropriate for droplets of larger size such as droplets resulting from sneezing and coughing…Based on the data healthcare workers should not use surgical masks as personal protective equipment…”

Question: If surgical masks are inappropriate for the personal protection of a healthcare worker, why would they be appropriate for your personal protection against an airborne virus?

Air Filter Ratings

Over the years, the HVAC industry has variously applied the dust arrestance, Dispersed Oil Particulate (Dust Spot) (DOP) efficiency, Minimum Efficiency Rating Value (MERV), and High Efficiency Particle Air filter (HEPA), and Ultra Low Penetration Air filter (ULPA). The MERV rating is the most commonly applied today.

MERV Ratings Chart

My personal preference to satisfy both filter effectiveness and to be economical is the MERV-13, as a minimum, near ideal, the MERV-16. Learn more from Engineered Systems magazine article (11 Dec 2020) Can HVAC Air Filters ‘Catch’ the SARS-CoV-2 Virus?

Pressure Drop

For a given filter dimension, the higher the MERV rating, the higher will be the pressure drop for a given configuration (number of pleats, depth of pleats), as demonstrated in the follow table.

Source | University of South Florida: Media Velocity Considerations in Pleated Air

Download this excel worksheet to calculate face velocity of pleated filters.

For residential, commercial and educational facilities, filters have been selected to protect the equipment. In some cases, the filters have been selected to reduce housekeeping labor. When filters are selected to protect people, the equipment will be protected and housekeeping (dusting, sweeping) will be minimized.

Improving Filtration

Simply replacing a low MERV-rated filter with a higher-rated product may seriously affect the fan and may even result in filter blow-out due to excessive pressure drop. Naturally, if you elect to upgrade your filter at the air handling unit, consider replacing the OEM narrow-gauge filter rack with a wider rack to take advantage of the relatively lower pressure drop of a deeper filter module. Consult the manufacturer of the air handling unit before upgrading the filter rack.

Price Industries has an excellent, easy to implement solution, the FDM: Filtered Diffuser Module. The FDM utilizes the T-bar of a suspended ceiling for tool-less installation. The existing diffusers remain in place.
The air handling unit continues to need a filter to protect the equipment, but it can be an inexpensive MERV 8 product.

By having an FDM at each diffuser, the advantage is lower overall system pressure drop and ease of maintenance.


To effectively capture airborne pathogens, filter performance is of paramount importance. If important, the post filter service inspection should be deemed critical.

Is the filter pointed in the right direction? Are there any gaps between filter modules? What is the condition of the cooling coil face?

I advise a record filter service record that includes the date of service, the name of the technician, a check list, and the rating of the replacement filter. A MERV nothing can look a lot like a MERV-13.

Is Air Travel Safe?

Forbes Magazine, 15 Oct 2020 | Study: Risk Of Covid-19 Transmission On Planes ‘Virtually Nonexistent’ For Mask-Wearers

  • Passengers wearing masks are at a very low risk of contracting Covid-19 on planes, even during packed flights, according to a recent study from the Department of Defense and United Airlines
  • the results of the study showed that the aggressive air filtration and circulation systems on planes lead to the near-immediate dispersal of particles carrying the infection
  • it would take a minimum of 54 hours of sitting next to someone with Covid-19 to be exposed to an infectious dose


Published by John White

A lifetime (over 50 years) of experiences with automation and control systems ranging from aerospace navigation, radar, and ordinance delivery systems to the world's first robotic drilling machine for the oil patch, to process-control systems, energy management systems and general problem-solving. At present, my focus is on self-funding HVAC retrofit projects and indoor air quality with a view to preventing infections from airborne pathogens.

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