Because most Americans spend up to 90% of their time indoors, the air in their home is a significant component to their overall health and wellbeing. Occasionally, indoor pollutants can accumulate to levels that pose health and comfort issues when too little outdoor air enters a home. These pollutants may include mold, bacteria, tobacco smoke, pesticides, volatiles organic compounds (VOCs), radon, various allergens, elevated level of carbon dioxide and carbon monoxide, and other potentially hazardous substances.
One approach to lowering the concentration of indoor air pollutants in a home is to increase the amount of outdoor air coming in. Outdoor air enters and leaves a house by infiltration, natural ventilation and mechanical ventilation.
With infiltration, outdoor air lows into the house through openings, hints and cracks in walls, floors and ceilings, as well as around windows and doors. Air may also move out of the house in this manner and this is known as exfiltration.
During natural ventilation, air moves through opened windows and doors. Air movement associated with infiltration and natural ventilation is caused by air temperature difference between indoors and outdoors and by the wind.
Finally, there are a number of mechanical ventilation devices, from exhaust fans that remove air from a single room, such as bathrooms and kitchens, to air handling systems that use fans and duct work to continuously remove indoor air and distribute filtered and conditioned outdoor air throughout the house. The rate at which outdoor air replaces indoor air is described as the air exchange rate. When there is little infiltration, natural ventilations, or mechanical ventilation, the air exchange rate is low and pollutant levels can rise.