The rate at which the internal air in a building is exchanged with the exterior air, whether naturally (see infiltration and ventilation) or artificially (see exhaust fan). In typical Australian domestic construction the air change rate has been measured at up to 12 air changes per hour. This clearly has implications, especially for winter comfort and for the cost of heating, as the air takes heat with it! A level of 1/2 to 1 air change per hour is considered acceptable; a very low rate can be dangerous, due to the accumulation of bacteria, radon gas and other noxious factors. (See sick building syndrome)