What is the main point of using an exhaust fan?

Greenheck exhaust fans are used to ventilate rooms, buildings, attic spaces and more. Many of their fans are "green" which means that they meet certain standardized specifications for energy consumption. For example Greenheck's Cube model roof mounted exhaust fans were used in the Solaire apartment complex in New York contributing to a 30% greater energy efficient requirement than was mandated by the New York State Energy Code.