Air quality is a serious concern, especially in retrofits and buildings where air pollution is particularly bad. The answer to the question of whether you need air filters depends on a variety of positive and negative influencers. Let's start with the positives:

• Healthy building materials were used in construction.
• Unhealthy building materials have been isolated from the interior.
• The home is adequately ventilated, replacing between 35% and 50% of its air every hour.

Those are simple enough. The negative affecters are far more numerous and complicated, depending much more on the home's inhabitants, their habits, and similar factors. Here's a list, which is by no means complete:

• The home's inhabitants smoke.
• The home's combustion appliances use internal air or vent indoors.
• The inhabitants use certain beauty products regularly – hair spray, nail polish, and others with similar chemical compositions.
• Chemical detergents, cleaning products, and air fresheners are frequently used.
• Chemical pesticides are required to keep invading insects at bay.
• The inhabitants have craft hobbies involving solvents.
• The home houses pets which shed a great deal of dander.
• The house smells stuffy and stale.
• Excessive condensation is visible on windows in winter.
• Mold and mildew grow on window sills.
• Someone living in the home suffers from hay fever or is allergic to (or very sensitive to) chemicals.

If you've tried eliminating and isolating as many sources of air pollution as possible, and have improved ventilation, but you still have problems, you may need to consider air filters. It is important to note that air filtration systems are a last resort, after you have tried everything else.

Many different air filters are available, but most fall into one of two types: portable or whole-house. They range widely in price, from a few dollars (for either type) up to four figures for complex systems.

Portable filters are designed to purify the air in a single room. The room must be sealed off from the rest of the house – doors closed, windows closed, any air ducts covered. This gives rise to a problem in most passive solar homes, as a room closed off in this way won't get any heat from the rest of the house.

Whole-house systems clean the air for a whole building. In their simplest form, they are special filters installed in the ducting of the home's air conditioning, whole-house ventilation or forced-air heating system. As the air is pushed around the home, it gets cleaned automatically. These filters are not standard spun-glass models used in furnaces and air con systems: they need to be a lot finer to trap gases and particulate matter.

The down-side is that the system needs to run for several hours a day, or longer if there's a smoker, lots of carpeting or pets around. As well as the additional electrical cost (which is admittedly minor for a few fans), there's the extra noise.

An alternative setup is to use heat-recovery ventilators with additional air filters: these are expensive (often thousands of dollars) but they work very well and retain extra heat in the winter.