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 filters not suitable for removing dust from the air and so is often used together pre-filtering that uses HEPA filtration for an overall combined air filtering effect.
Activated carbon is also extensively used in water filtration owing to its ability to removed a wide array of pollutants - including VOCs - from water.
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 a massive 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/a> - which are popular solutions and regarded as effective.
It is important to note that not all gases will 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 a gas phase filter is sometimes mixed with other substances which are able to remove other types of gaseous contaminants from the air - such as Potassium Permanganate (which is intended to remove formaldehyde, hydrogen sulfide, and sulfur dioxide). However, potassium permanganate filters are a little controversial as some say that they can possibly release Manganese into the air. It should also be noted that some activated carbon filters - especially cheap ones, can release carbon dust into the air - so an all round effective solution would be to have a triple layer filter:
LAYER 1: - Pre filter to remove large particles, dander, lint, hair etc without obstructing airflow. Ideally removable and washable.
LAYER 2: - HEPA filter to remove solid particles.
LAYER 3: - Gas Phase - Activated carbon / permanganate / etc.
LAYER 4: - Final high grade HEPA filter to remove dust from activated carbon / gas phase.
Zeolite (intended to remove ammonia) has also sometimes been included in activated carbon filters but its effectiveness has been strongly challenged and it has been said that zeolite has not been scientifically demonstrated to be any more effective than impregnated carbon or impregnated alumina.