One reason is increased energy efficiency. To cut energy costs, homes are better insulated and sealed. This decreases the amount of air that gets exchanged and refreshed.

But more significantly, pollution can come from what is inside your home, not outside.

A wide range of pollutants
Houses are full of objects and materials that may create pollutants. Carpeting, furniture, paint, cleaning agents - all can emit chemical vapors. Building materials, paneling and glues can give off chemicals.

Dust, pet dander, pollen and odors are typically in household air. Mold, mildew, and dust mites may be present. Even gases such as radon and carbon monoxide are found in many homes in measurable quantities.