The watchman at the pump house at the plant at Gendehalli village near Belur noticed the leakage in the wee hours and informed senior officers of the Town Municipal Council (TMC), who rushed to the spot.
Several residents nearby complained of breathlessness, nausea and burning sensation in the throat and they were asked to move to a safer place, the officials said.
The fire staff and municipal staff of Belur rushed to the spot and evacuated the villagers within 500 metre radius, they said.
The fire and emergency services stopped the leakage, he said.
Four of the TMC staff were who suffered from breathlessness after inhaling the gas were treated at Hassan Institute of Medical sciences, the officer said.