Activated Carbon Air Filter
Activated carbon air filters are the most common type of gas phase air filter in common use. A gas phase air filter is so named simply becuase it tackles the problem of gaseous air pollution, which is not effectively removed by HEPA filtration (which is more effective at removing solid particles). Activated carbon is not suitable for removing dust from the air and so is often used together with HEPA filtration for an overall combined air filtering effect.
Also known as air scrubbers, activated carbon air filters are used to remove volatile organic compounds (VOCs), solvents, tobacco smoke, gases and odors from air.
Activated carbon is a type of carbon that has been specially treated with Oxygen in order to open up pores between the carbon atoms and create a form of carbon with an enormous surface area - hundreds of square metres of "bonding sites" per gram! This means that chemicals which are "attracted" to carbon (such as chlorine and many VOCs) will have a great opportunity to be "adsorbed" by it.
It stands to reason that the most effective activated carbon filters will be ones which contain the greatest mass of activated carbon! These however are thicker and are therefore more resistive to airflow. Some ACFs currently on the market contain many pounds of activated carbon and it's said that activated carbon in granular form is much more effective than "pads impregnated with carbon fiber dust".
Activated carbon filters / air scrubbers are now a common technology and can be purchased for use with HVAC systems in several forms, including carbon-treated "filtrete" filters and cylinder filters.
It is important to note that not all gases react with activated carbon and some will pass right through. (source: http://science.howstuffworks.com/question209.htm) In particular, gaseous contaminants with low molecular weight may be left behind - and according to Frank Hames, president of IQAir, these include ammonia, formaldehyde and hydrogen sulphide.
For this reason, activated carbon in the filter is often mixed with other substances which remove other types of gaseous contaminants from the air - such as Potassium Permanganate (which is intended to remove formaldehyde, hydrogen sulfide, and sulfur dioxide). Products would appear to be able to address a wider range of gaseous pollutants. However, some say that potassium permanganate filters can possibly release Manganese into the air. (source: http://www.allerair.net/buyers_beware.html )
Zeolite (intended to remove ammonia) is also sometimes included in activated carbon filters but its effectiveness has been strongly challenged and it has been said that zeolite has not been proven scientifically to be any more effective than impregnated carbon or impregnated alumina. (source: http://www.achooallergy.com/air-filter-manufacturers.asp
