Gas supply unit, gas supply method and exposure system

An exposure system comprising:

• an exposure apparatus for exposing a wafer to ultraviolet light, said exposure apparatus including an optical system which has a path of the light;
• and
• a gas supply unit including a first channel connecting a plant facility and a first space including the path of the light without any intervening spaces including the path of the light and configured to supply gas from the plant facility to the first space, wherein said gas supply unit includes a first detector configured to detect an impurity concentration in said first channel, a second channel connected to said first channel downstream of said first detector, and a switch mechanism configured to switch said first and second channels based on the impurity concentration detected by said first detector.