In a process known as infiltration, outdoor air flows into the building through openings, joints, cracks in walls, floors, ceilings, and around windows and doors. In natural ventilation, air moves through opened windows and doors. Air movement associated with infiltration and natural ventilation is caused by air temperature differences between indoors and outdoors and by wind. The rate at which outdoor air replaces indoor air is known as the air exchange rate. When there is little infiltration, natural ventilation, or mechanical ventilation, the air exchange rate is low and indoor pollutant levels can accumulate to levels that can pose health and comfort problems.