They circulate through duct work and can affect your health. The filter in your furnace does not clean or purify your air. It simply keeps the furnace running. Pets in your home an cause odors and allergic reactions dues to skin flaking and dander. Second hand smoke can affect the health of everyone in your home.
Good ventilation is critical.
Since 1970, homes have been built much tighter. This can cause volatile organic compounds from chemicals used in the building materials to be trapped in your home and affect your indoor air quality. These compounds can make you feel ill, and cause harmful affects. Many homes have rooms that are too hot, or cold, compared to the rest of the house. Leaving your thermostat set at the same temperature consumes more energy and negatively impacts the environment.