Ventilation is the intentional introduction of outside air into a space. The general purpose of ventilation in buildings is to provide healthy air for breathing by both diluting the pollutants originating in the building and removing the pollutants from it.
Building ventilation has three basic elements:
ventilation rate - the amount of outdoor air that is provided into the space, and the quality of the outdoor air;
airflow direction - the overall airflow direction in a building, which should be from clean zones to dirty zones; and
air distribution or airflow pattern - the external air should be delivered to each part of the space in an efficient manner and the airborne pollutants generated in each part of the space should also be removed in an efficient manner.
The intentional introduction of outside air can be categorized as either mechanical ventilation, or natural ventilation. Mechanical ventilation uses fans to drive the flow of outside air into a building. Natural ventilation is the intentional passive flow of outside air into a building through planned openings (such as louvers, doors, and windows).
Mixed mode ventilation systems use both mechanical and natural processes. Since the natural component can be affected by unpredictable environmental conditions, it may not always provide an appropriate amount of ventilation. In this case, mechanical systems may be used to supplement or to regulate the naturally driven flow. And with the air pollution becoming more and more serious, mechnical systems make it possible to maintain air quality.
When natural ventilation alone is not suitable, exhaust fans can be installed to increase ventilation rates in rooms. However, this simple type of mixed-mode ventilation needs to be used with care. The fans should be installed where room air can be exhausted directly to the outdoor environment through either a wall or the roof. The size and number of exhaust fans depends on the targeted ventilation rate, and must be measured and tested before use.