Gas Phase Air Filtration Improves Breathing Air

Sept. 1, 2007
Controlling gaseous pollutants requires specialized air-filtration products

Not all gaseous contaminants - including volatile organic compounds (VOCs) - can be identified by smell. In fact, controlling VOCs and other gaseous contaminants is even more important when there are no odors to trigger complaints.

Most gaseous contaminants can be removed by source removal/reduction, proper ventilation, and an effective gas phase air filtration system. Gas phase air filtration is usually most effective when used with either source control or ventilation; however, filtration may be the only approach when the source of pollution is outside and the gaseous contaminants are brought in through the building's fresh-air ventilation system.

Controlling gaseous pollutants re­­quires specialized air-filtration products. Traditional particulate air filters - and even HEPA filters - are not effective at removing gaseous contaminants. 

While a variety of materials may be used, most gas phase air filters are made with activated carbon. Of all the adsorbents known, activated carbon is one of the strongest physical adsorbents, so it is excellent for creating a fresh, clean-smelling environment by removing odors and other gaseous contaminants. Activated carbon acts like a sponge: It is highly porous and has a vast amount of surface area. As odor molecules come into contact with the carbon, they are drawn into the carbon and held into place. Carbon-based odor molecules have a high affinity for bonding with the activated carbon because it lowers overall surface energy. 

Gas phase filtration may be recommended for:

  • Newly constructed buildings (2 years following completion).
  • Newly remodeled buildings (new wallpaper, paint, carpets, etc.).
  • Newly installed furnishings.
  • Photocopying areas.
  • Areas where solvents are used (laboratories, nail salons, spas, etc.).
  • Areas where source control and ventilation control have not resolved odor issues.
  • Areas where tenants complain of eye and respiratory irritation.

In addition, ASHRAE's Ventilation Standard 62-1989 makes a provision to reduce the amount of ventilation air required in a building if effective gas phase filtration is employed. This also may be an opportunity to reduce heating and cooling costs.

When gaseous contaminant filtration is indicated, effective filtration systems employ a combination of particulate and gas phase filtration technologies. In two-stage HVAC filtration systems, particulate pre-filters in the MERV 7 to 8 range protect higher-efficiency final filters. In environments with significant gaseous contaminant levels, final filters may be replaced with granular bed or deep pleat carbon filters. If doing so, it's important that the HVAC system has adequate fan capacity to handle the increased airflow restriction of these filters.

Some filters with activated carbon are dual-layer filters. The upstream layer provides particulate filtration and protects the carbon layer from particulate loading to ensure maximum odor removal. The downstream layer absorbs and retains gaseous contaminants from the air stream.

In more typical scenarios, gaseous contaminant levels are low to moderate, and single-stage pleated gas phase filters may be used. These filters are more expensive than traditional pleated filters, but they remove particulates and gases. Be sure to select a filter that has a particulate filtration level of at least MERV 7 and, again, be sure that the HVAC system has adequate fan capacity.

With the growing concern about indoor air quality, building owners and managers should consider letting tenants know that the building's filtration system has been upgraded to remove particulate and gaseous contaminants. This may be one of the most cost-effective and valuable upgrades you can make to a building.  

Ronald Cox is a certified air filtration specialist at Roswell, GA-based Kimberly-Clark Filtration Products (

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