The Environmental Protection Agency lists Indoor Air Quality among the top 5 risks to human health, probably because indoor air can be 2-5 times more polluted than outdoor air.
While homeowners can't see the majority of indoor air contaminants, every cubic foot of air breathed carries a mixture of millions of microscopic particles such as pollen, mold spores and dust mite debris. In small concentrations, these particles and gasses may cause discomfort in the home. In significant concentrations, they can cause sickness as these are among the most troublesome triggers of such ailments as asthma and allergies.