Indoor Air Resources

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What You Can’t Smell Might Hurt You, Part 2

May 22 small image

By: LInda Barlow, freelance writer  |   05.21.14

Did you know that particulate air filters do nothing to reduce gaseous contaminants in a building? An effective gas phase air filtration system is a crucial component of a multi-pronged strategy for removing harmful gases from the breathing air inside buildings.

Typical air filters used in commercial buildings capture particulate contamination but do nothing to reduce harmful gases. These gases – including VOCs – may cause a variety of health effects that can lead to a drain on personal productivity, increased absenteeism and additional medical costs. 


Controlling Gaseous Pollutants

Air cleaning, via air filtration, is usually most effective when used in conjunction with either source control or ventilation. However, filtration may be the only approach when the source of pollution is outside of the building and the gaseous contaminants are brought in through the building’s fresh air ventilation system.


Gas phase filtration may be recommended in any of the following scenarios:

  • Newly constructed buildings
  • Newly remodeled buildings (new wallpaper, paint, carpets, etc.)
  • Newly installed furnishings
  • Areas where large volumes of photocopying are conducted
  • Areas where solvents are used (laboratories, nail salons, spas, etc.)
  • When source control and ventilation control have not resolved odor issues
  • When occupants complain of eye and respiratory irritation


Strength of Activated Carbon

Controlling gaseous pollutants requires specialized air filtration products. Particulate-only air filters, including HEPA filters, are not effective at removing gaseous contaminants.


While a variety of materials, such as silica gel, activated alumina, and porous clay may be used, most gas phase air filters are made with activated carbon. Activated carbon is made from a variety of high carbon-content substances including coal, wood, coconut shells and bamboo. 


Activated Carbon: A Magnet for Odors

On a microscopic level, activated carbon looks and acts like a natural sponge. Activated carbon particles are highly porous and have a vast amount of surface area. In fact, one gram of activated carbon may have a surface area exceeding 1,000 square meters[LB2] .


As odor molecules come into contact with the carbon, they are drawn into the carbon and held into place by a variety of forces – similar to a magnetic attraction. Carbon-based odor molecules have a high affinity for bonding with the activated carbon because it lowers overall surface energy. Of all the adsorbents known, activated carbon is one of the strongest physical adsorbents.


Filtration System Design

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 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, the HVAC system needs adequate fan capacity to maintain designed air flow rates capable of handling any increase in airflow resistance associated with these filters.


In more typical scenarios, gaseous contaminant levels are low to moderate, and single-stage pleated gas phase filters may be used. These may be dual-layer filters, with an upstream layer that provides particulate filtration and protects the carbon layer from particulate loading, to ensure maximum odor removal, and a downstream layer that absorbs and retains gaseous contaminants from the air stream.


While these filters have a higher price point than comparable MERV-rated particulate-only pleated filters, they do provide the added benefit of gaseous contaminant removal. Be sure to select a filter that has a particulate filtration level of at least MERV 8, and again, be sure that the HVAC system is capable of handling any changes in airflow resistance across the gas phase filter.




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