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Dust, hair, and insect feces are just a few of the particles that swirl through the air inside most homes. The small size of these particles allows them to slip past the body's protective cilia and mucous, and to penetrate deeply into the lungs. The health problems they cause range from minor irritation to severe allergies. You can remove these particles by installing a filter in the forced-air heating system. But if you think a standard furnace filter is up to this job, think again. Any particle that's small enough to inhale will easily pass through a standard filter. To have any success at removing such particles, you'll have to look at better, and more expensive, filters. Types of