Air Purification Strategies

Featured: COVID-19 Air Disinfection Strategies

Air Purification Technologies

There are many different solutions for treatment of air using the building’s HVAC systems. We will focus on the following three methods:

HVAC Filtration

Ultraviolet Germicidal Irradiation

Bipolar Ionization

The COVID-19 pandemic has created unprecedented changes in lifestyles, highlighting the need to create safe environments in every part of society permanently. From work to shopping, daily routines have come to a complete standstill because there is no current system to allow people to congregate safely in the midst of an outbreak. Using various technologies, there are many contaminants that can be neutralized and/or removed from the air. Airborne contaminants include but are not limited to molds, viruses, bacteria, and allergens. These pathogens all diminish the wellbeing, safety, and health of those who congregate in public buildings, shopping centers, manufacturing facilities, offices, schools, healthcare facilities, multifamily living spaces, etc.. These now unsafe spaces have brought to light the dire need for a disinfection and purification system, which will inevitably become essential and practical in the future.

HVAC Filtration

HVAC systems require filtration to meet indoor air quality requirements as well as protecting the internal components of the air handling systems such as heating and cooling coils. Minimum Efficiency Reporting Value, commonly known as MERV, is a measurement scale designed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) to report the effectiveness of air filters. The higher the rating, the more efficient the filter is at removing particles from the airstream.

This chart groups MERV ratings by particle size ranges:


The respiratory droplets that a person infected with COVID-19 releases when they cough, or sneeze contain the viruses and can be caught in a properly selected filter. Droplet sizes can range in size from 0.5 μm to 15 μm. Based on the chart above, the required MERV would be in the range of 13-16. A filter with fine filter media (MERV-A) versus a synthetic charged media would be recommended for extended filter life and consistent efficiencies.

ASHRAE has issued “Position Document on Infectious Aerosols” (4-14-20) with HVAC strategies for Non-Healthcare type facilities with the following recommendations:

For an existing HVAC system, the air handling unit would need to be evaluated to ensure it has the static pressure capacity and the filter frames and space necessary to install a filter with higher efficiency. There are several options for MERV 13-A filters, such as bag filters which tend to be very long (need more space in the unit) and have lower pressure loss or a 12” deep rigid filter that will fit into many existing air handling units.

The efficacy of reducing airborne contaminates, including droplets containing viruses, by upgrading to higher efficiency filtration depends on many factors, such as air change rate and the effectiveness of the supply and return/exhaust distribution.

ROUGH COSTS: A typical MERV 13-A filter cost is about 0.10 $/CFM.

Install Minimum of MERV 13-(A) Filters or highest level achievable

Add in duct and/or upper-room UVGI

Increase outdoor air ventilation (disable demand-controlled ventilation and open outdoor air dampers to 100% as indoor and outdoor conditions permit)

Keep HVAC Systems Running 24/7

Add portable room air cleaners with HEPA

Bypass energy recovery ventilation systems that leak potentially contaminated exhaust air back into the outdoor air supply

Ultraviolet Germicidal Irradiation

Ultraviolet germicidal irradiation (UVGI) is a disinfection method that uses short-wavelength ultraviolet (UV-C) light to kill or inactivate microorganisms by destroying nucleic acids and disrupting their DNA. There are several different applications of UVGI to irradiate microorganisms from a space or on surfaces.

The following are some applications using UVGI:

Upper Room Disinfection

  • UVGI fixtures are mounted in the room above the occupied zone which directs the UV-C horizontally to create a disinfection zone. Natural air currents or forced air movement brings contaminated air into the disinfection zone.
  • Safety is a concern since the UV lamps are in the occupied space.
  • Tests can be performed to determine exposure in the occupied zone.

In-Duct Air Disinfection

  • Deactivates airborne microorganisms “on the fly”.
  • Usually mounted in an air handling unit to deactivate microorganisms in the airstream as well as on the surfaces of filters and coils.
  • Typical target effectiveness for single pass inactivation of about 85%.
  • Low impact to system static pressure.
  • Electrical consumption is approximately 0.02 W/CFM.

In-Duct Surface Disinfection

  • Irradiate coil and/or filter surfaces to control growth on fibrous or metallic surfaces. This reduces the efficiency of the coil from a pressure loss and heat conductance standpoint.
  • Cooling coil drain pan and the wetted surfaces of a cooling coil are breeding grounds for microorganisms, the UVGI will mitigate growth when installed downstream of the coil.
  • This installation is not intended to deactivate microorganisms on the fly, but only on surfaces.

In-Room Surface Disinfection

  • Permanently installed fixtures or portable units typically used in healthcare applications to disinfect a space. Permanently installed systems may have occupied and unoccupied modes to increase UV-C intensity when the room is not occupied for a more effective disinfection.
  • Portable units can be used safely when people are not present in the space being treated. Units have timers to start and stop the disinfection cycle.
ROUGH COSTS: A typical UVGI system for surface disinfection within an AHU cost is about 0.20 $/CFM.
Bipolar Ionization

Bipolar Ionization in an emerging technology used to purify air using glass and metal ion-generating tubes. These systems are designed to increase the levels of both positive and negative ions in indoor spaces to levels naturally found in very clean environments.

The bipolar process causes particles to become agglomerated since particles are attracted to other particles which causes them to become heavier. The heavier particles will either drop out of the breathing zone to the floor or potentially caught in the air handling unit filtration system. Volatile organic compounds, chemicals, gases, and odors also interact with the bipolar ions causing them to be broken down into immeasurable amounts of elements like carbon dioxide and water vapor. Bipolar ions also impact microorganisms such as viruses, bacteria, molds, and allergens. The positive and negative ions surround the hemagglutinin (surface proteins that form on organisms and trigger infections) and change into highly reactive hydroxyl radicals. This strips a hydrogen molecule from the hemagglutinin and changes into water. The ions destroy the virus surface structure on the molecular level.

The major difference between this technology, filtration systems and UVGI systems is that the bipolar ions seek out the particles, whereas in a passive system like a filter, the system relies on the HVAC system to convey the particles back to the filter or UVGI.

Recent third-party laboratory and field testing has demonstrated the effectiveness of bi-polar ionization, exhibiting an 85.8% dust particle decay rate as compared to 12.8% natural decay rate without the system. (source: Wright, M.D., Holden, N.K., Shallcross, D.E., & Henshaw, D.L. (2014). Indoor and outdoor atmospheric ion mobility spectra, diurnal variation, and relationship with meteorological parameters. Journal of Geophysical Research: Atmospheres.)

Lab tests conducted by Syracuse University’s Building Energy and Environmental Systems Laboratory revealed reductions of over 90% for all of the most common indoor VOCs tested. (source: Guo, B. (2018). Full-scale chamber testing of air cleaner performance for the removal of volatile organic compounds. Building Energy and Environmental Systems Laboratory, Department of Mechanical and Aerospace Engineering, Syracuse University).

When testing against three bioaerosolized microorganisms, the bi-polar ionization system reduced prevalence by 99.98%. (source: AntiMicrobial Test Lab. (n.d.) Relative performance of AtmosAir Matterhorn when tested against bioaerosolized microorganisms).

This technology is easily applied to an HVAC system in the air handling unit or installed in the ductwork. There are also systems that can be installed directly in a room. The number of bipolar ion-generating tubes is dependent on the airflow and the particulars of the space. The system releases bipolar ions directly in the duct or system and into the spaces served in adequate quantities to ensure the reactions with particles, microorganisms, VOCs, mold spores, odors, and other contaminates occur.

ROUGH COSTS: A Bipolar Ionization system for an AHU would cost about 0.40 to 0.50 $/CFM.