According to the police, Raj Kumar, Rinku and Nanhe had gone inside the tank to rescue Binesh Kumar, who later regained consciousness during treatment at a hospital.
The incident took place at 10 am in JNJ Electronics Ltd in Sector 37 Industrial Area when maintenance staffers were removing sludge from the tank.
The company had announced holiday for employees on Dussehra today and only the maintenance staffers were called for cleaning the tank.
"I was asked to go inside the tank without any safety gears such as oxygen cylinder and mask. I got suffocated within two minutes after inhaling poisonous gas and became unconscious. When I gathered consciousness, I found myself on a hospital bed," Binesh Kumar said.
He did not know what happened after he fell unconscious, but his three colleagues waiting on the ground quickly went down to rescue him. They too fell unconscious after inhaling poisonous gas.
"All four maintenance staffers remained stuck inside the tank for over half-an-hour till fire fighters rescued them," said Vijay, another staffer.
Gurgaon Police spokesperson Ravinder Kumar said a complaint has been filed against company officials. No one has been arrested so far, he said.