Some 10 workers were hospitalized Monday after falling unconscious or ill at a sewer works site in Nagoya's Mizuho Ward, police and fire department officials said.
The workers, six of whom were inside a sewer pipe some 10 meters underground, appeared to be suffering from lack of oxygen, the officials said.
Two of them, identified as Takashi Kusano, 32, and Koichi Sekiguchi, 60, were in serious condition but were conscious, police said.
Three workers entered the pipe at about 9 a.m. and proceeded some 700 meters inside on a gasoline-powered vehicle to reinforce the pipe. Around two hours later, two of them noticed an unusual odor that made them ill and returned above ground.
Three other workers went into the pipe to help the one remaining there and they all fell unconscious. Firefighters had helped everyone out of the pipe by around 1 p.m., the officials said.
Workers had been using an electric-powered vehicle inside the pipe, but they switched to a gasoline-powered vehicle Monday because the electric one was not very efficient, according to police.
Police said they suspect the exhaust fumes from the vehicle may have caused the workers to fall ill.
Some of the workers said they started feeling sick almost immediately after an alarm to warn of oxygen deficiency inside the pipe sounded.