In the search for clean indoor air, a three-tiered system is used:

• Control the source of pollutants at their source
• Provide adequate ventilation
• Clean the air

If all sources of indoor air pollutants are controlled, then there is no need to have air cleaners; controlling the source of the pollutants is obviously the first thing you want to do. However, not all sources can be controlled. Sometimes pollutants get in no matter how hard you try to prevent them. Therefore, the last line of defense is the air cleaner. Because of that, it should always be used in addition to, not in place of, the other steps!

One study found that air cleaners could be beneficial for asthmatic patients with animal allergies who want to keep their pets when air cleaning is used in combination with other measures. This was after a three-month intervention with 20 asthmatic children allergic to animals who had pets at home. They found that the airways of those children who had air cleaners in their living rooms and bedrooms were not as reactive as the airways of those children who had fake air cleaners in their homes.[2]