Whatever air filter system you decide to install, you will have to decide what type of filter to put in. Air filters come in three types: gas, particulate, and hybrid.

Gas filters

Used in homes suffering from any sort of gaseous air pollution, these filters remove all sorts of volatile organic compounds outgassed by carpets, paint, engineered wood, and furniture.

Gas filters usually contain an absorbent material which is porous. They have a huge surface area which grabs gas molecules as they pass. The most common material is activated carbon, though activated alumina is also available. Basic activated carbon is effective against large gas molecules, but it won't stop formaldehyde, so you should look for either a "treated" activated carbon filter or one made of activated alumina impregnated with potassium permanganate. Some manufacturers sell filters which use both materials.

Gas filters are extremely fine and will also block particulate pollution. However, using them in this way is a bad idea as they will clog up very quickly and need replacing. It is much better to place a particulate filter upstream to block the bigger particles, allowing the gas filter to concentrate on the tiny gas molecules.

Particulate filters

Particulate filters come in two categories: mechanical and electrical. The mechanical filters are usually one of three types:

• Fiberglass or some other synthetic fiber in a cardboard or plastic frame.
• Permanently charged plastic film or fiber material.
• Pleated fabric.

Mechanical filters are cheap (a few dollars each) and last about a month. They trap up to 40% of the particulate pollution in the air, but they have one major drawback – they grab the biggest particles and let the medium and small ones through. In health terms, this is a disaster. The medium-sized particles are the ones that trigger allergies and asthma attacks; the small particles are the ones that get breathed in, avoid the body's own filter system and end up in the lungs. The small particles can also carry heavy metals and toxic pollutants along with them, planting them deep in your lungs where they can do terrible damage.

Electrical filters work by charging the particles in the air, causing them to be attracted to oppositely-charged surfaces in the room. In other words, they force the air pollution to settle on your furnishings. They are very good at their job, affecting up to 95% of large particles, but their drawback is obvious: you end up with gunk all over your walls and furniture, any of which can be disturbed and flung back into the air again. Some come with a fan and a mechanical filter to trap the particles, though they are mostly portable models.

A better electrical option is the "electrostatic precipitator", which comes in portable and whole-house models. This works in the same way as the basic electrical model, but instead of leaving the particles wandering around the house, it captures them itself. They can filter out up to 95% of large particles and 10% of the smallest. They have a significant advantage in that the filters can be washed