FAQ on filtration of gaseous pollutants and odors Certain air filters are better for control of specific odors, mold, and gases by Frank Hammes, President IQAir North America

Editor's note. We posed these questions to the President of IQAir since our members often ask the same questions and the answers are complex.

Answer: The air we breathe contains apart from oxygen, carbon dioxide, carbon monoxide, a few precious gases and a percentage of humidity, a range of pollutants. These pollutants take three physical states: solid (e.g. dust, allergens, bacteria, viruses), liquid (e.g. mist, fog, aerosol-sprays) and gaseous (e.g. VOCs, formaldehyde). The technology needed in an air cleaner to remove gaseous pollutants and odors is entirely different to that need to filter dust particles and aerosols.
While solid and liquid particles can be effectively removed by media filters, such as a good HEPA filter, gaseous pollutants and odors, due to their physical state and extremely small size (< 0.001 microns) cannot. In fact HEPA filters have 0% efficiency for gaseous chemical compounds. These compounds simply pass through the HEPA filter.
In order to filter gaseous pollutants and odors effectively a number of filter technologies exist. The most common one found in air cleaners is activated carbon.
Activated carbon consists of a carbon structure, which has been cleaned with hot steam to create an extremely porous structure. Like a miniscule sponge, activated carbon contains thousands of little cracks and pores. These are responsible for a very large internal surface area. This surface area exerts a physical force, which attracts many gas molecules and binds them to the surface of the carbon. This process is called adsorption. In order for an activated carbon filter to work effectively, there needs to be a sufficient amount of carbon, so that the air, which is passing through the filter is able to deposit its pollutant molecules with the activated carbon.
Coconut shell carbon. Activated carbon can be used in many different forms in an air cleaner. Most air cleaners only contain a fiber pad, which is impregnated with a few ounces of carbon dust. This is the least effective way to use activated carbon, because of the small amount of carbon. Some air cleaners contain more substantial granular activated carbon filters. Most of these air cleaners use activated carbon made from coconut shell. While this carbon is inexpensive and widely available some allergy sufferers have reported to be allergic to dust from coconut shell carbon. Coconut carbon is also very soft and tends to generate dust during transport and sometimes even during usage. Most systems for professional gas and odor removal utilize activated carbon in the shape of small pellets made from coal or peat. This carbon is significantly harder and gives off virtually no dust.
Another aspect which affects the effectiveness of activated carbon for indoor air quality issues is its degree of activation. Most activated carbon available today is manufactured for industrial applications, such as solvent recovery. For this application, the carbon is activated for the maximum amount of pores. While this increases the capacity of the activated carbon at very high pollution concentrations, it actually decreases its effectiveness to remove odors and chemicals at the typical concentrations in home and office environments. This is explained by the fact, that the higher the carbon is activated the larger the pores become. It is, however, only the very smallest pores in the activated carbon, called micropores, which are capable for removing odors and chemicals under home and office conditions. IQAir uses activated carbon in its IQAir HealthPro Plus and its IQAir GC Series air cleaners which is activated in a way to preserve a maximum amount of micropores for best possible gas and odor control in homes and offices.
When one looks at controlling gaseous pollutants and odors in indoor environments, it is important to realize that the right activated carbon filters is able to deal with many gaseous pollutants and odors, it is not suitable for all. While activated carbon is particularly suitable for volatile organic compounds (VOCs), such as benzene, toluene, styrene, it is not effective for less volatile compounds, such as formaldehyde or many inorganic gases such as hydrogen sulfide and sulfur dioxide. These compounds require a chemical reaction to break them down. The best results, for removing these compounds, are granular chemically active sorbant materials, called chemisorbers. Chemisorbers consist of aluminoxyde impregnated with potassium permanganate. When the potassium permanganate gets in contact with humidity and a suitable gas molecule it breaks it down to harmless CO2. This is particularly effective for controlling formaldehyde, hydrogen sulfide, and sulfur dioxide.
Question: How about carbon filters mixed with Zeolite? Are they better or worse?
Answer: A number of manufacturers use a granular material called zeolite in combination with activated carbon and claim that this is a more effective gas filtration mix. We have found no independent scientific evidence for this claim. In fact, zeolite is not used in industrial or commercial air filtration, because chemisorbants and impregnated carbons have been found to work best. The best explanation as to why zeolite is used in residential air cleaners is that it is even cheaper than coconut shell activated carbon and the use of another gas phase media sounds good to unsuspecting residential consumers.
As important as the gas and odor filtration media chosen, is the way in which it is used in an air cleaner to ensure best possible gas and odor removal. It is particularly important that gas filtration media is incorporated in a way in the air cleaner that insures maximum contact with air being cleansed, while ensuring that the air is restricted as little as possible. The best way this can be done is with a V-shaped granular media element, such as is featured in IQAir HealthPro Plus (shipping from December 1, 2000) or by using cylindrical shaped cartridges as in the IQAir GC Series.
Another issue important issue to consider in how an air cleaner uses gas phase and odor filtration media, is how well it is protected against clogging. Since both activated carbon and chemisorbant media relies on its fine pore structure for the removal of gas molecules it is essential for these pores not to be clogged by dust particles. Yet most manufacturers use carbon filters as dust pre-filters or use low efficiency pre-filters. This means that that gas and odor removal effectiveness decreases rapidly with use. Some manufacturers try to work around this by placing the activated carbon filter after the HEPA filter. While this indeed protects the activated carbon filter it, creates a problem for users, since any carbon dust released by the carbon filter is not retained. IQAir has solved this issue the same way large-scale commercial air cleaning systems have solved it. We place a high-efficiency particulate pre-filter ahead of the activated carbon filter and places a HEPA filter after the activated carbon filter.
The last item to look out for in an air cleaner with gas and odor filters is that these can be replaced independently of HEPA and pre- filters. HEPA filters generally last several times longer than activated carbon filters and pre-filters have a shorter life. If they are combined in one filter element both need to be replaced even if only one is used up. The IQAir HealthPro Plus contains the pre-filter, the gas phase filter and the HEPA filter in three separately replaceable filters. Only filters which are used up need to be replaced.
Question: We get a number of people who are looking for a filter for their basement. It appears that although basements are inherently unhealthy living spaces, people want a filter to eliminate mold, odors, and other unhealthy aspects of basement air. Your comments?
Answer: The reason that basement air is considered "bad" has a number of reasons. Basements are many times cooler and more humid that other parts of the house. This can lead to condensation of humidity on walls and windows, which may lead to mold growth.These molds are not only responsible for producing mold spores, but also a range of complex chemicals, called mycotoxins, some of which are toxic.
Another reason that the air in basement is "bad" is that there tends to be less ventilation in basements as in other parts of a house.
The above issues of humidity and lack of ventilation should be address by inhabitants as a first part of a strategy to improve air quality. This can be done with dehumidification, insulation and ventilation. An air cleaner should be used only after the underlying sources of the problem have been addressed.
Question: What about odors from building materials and furnishings?
Answer: While all of the above odors are emitted by specific sources, e.g. a burning cigarette, it needs to be realized that over time secondary sources for odors may develop. These secondary sources are caused by the deposition of odor causing substances on furnishings or room surfaces. These act as sinks, which accumulate odor-causing substances. An example of how this works, is when you come home from spending an hour in a smoky bar, your clothes will start emitting cigarette odor. While you were in the bar particles and gas molecules will have deposited on your cloth. Back in a clean environment odor molecules which have attached to the cloth and the particles on your cloth will come off.
Question: Which filters do best in a damp environment?
Answer: Activated carbon filters do badly in damp environments because the miniscule pores in the carbon get filled with humidity molecules which don't allow gaseous pollutant molecules to be taken up. Filters with chemisorbant do very well in damp environments because their chemical breakdown process of air pollutants accelerates with increasing humidity.
Question: I am hearing more about people who live in smog or polluted areas, even just car and highway fumes wafting into their apartments.
Answer: In order to help prevent smog and highway fumes penetrating indoor environments; the most effective solution is based on a two-stage strategy. The best solution is to prevent smog and fumes from entering in the first place. This can be done by creating a slight overpressure (using the IQAir MultiGas GC and Inflow W125) in the apartment by installing an air cleaning system which is optimized for smog and traffic fumes which draws in air from outside and filters it before releasing it into the indoor environment. It is only the second best strategy, to have an air cleaner that filters the air within the room by recirculation.
Question: What about those people who get forest fires in their area?
Answer: The same as for smog and highway fumes goes for forest fires. It is essential to make sure that only cleaned air enters the apartment or home, to ensure the best possible improvement in indoor air quality. Due to the high concentrations of sulfur dioxide and other combustion gases, we recommend the IQAir MultiGas GC, which contains a wide spectrum gas phase control filter.
Question: We have club members with Multiple Chemical Sensitivity (MCS). What kind of machine do these people need?
Answer: Individuals with MCS should select an air cleaner which is capable of effectively filtering a wide range of chemical compounds. A good choice is an air cleaner with substantial amounts of granular activated carbon and chemisorbant granulate. This covers not only volatile organic compounds, but also semi-volatile organic compounds and some inorganic compounds. Ideally, the unit of choice should have at least 5 pounds of granular activated carbon and chemisorbers (IQAir MultiGas GC). Make sure that it also contains an effective particulate filter after the granular gas filtration media, to prevent from carbon or chemisorbant dust being emitted by the air cleaner.Some manufacturers have claimed that air cleaners with a metal housing are preferable to air cleaners with a plastic housing, due to less off-gassing. We have found that high-quality plastic housings made from ABS (used for making personal computers), PP or PC plastic poses no problem of off-gassing. The plastics that pose a threat of off- gassing are plastics which contains softening agents, such as PVC. In fact many air cleaners with metal housing may be responsible for significant off-gassing due to internal soft gaskets and sealing compounds that need to be used between metal parts and filters. The unit of choice for MCS sufferers from the IQAir product line is the IQAir MultiGas GC due to its wide spectrum gas phase control.