The leak was caused by cracks in the water-heating pipes of a shower inside an apartment in Jinshan District on Saturday afternoon.

Three men and two women perished. The deceased were identified by city police as three Shanghai residents and two migrant workers.

Police said the sole survivor, a woman, is a migrant worker, and is off the danger list.

The incident occurred in a residential home of about 40 to 50 square meters on Fuchuan Road, which was rented to the six people.

The tragedy did not unfold until Saturday afternoon, when a friend of the tenants felt uneasy after failing to hear from them for the whole day and called authorities.

City police officers, fire officials and the gas company arrived at the scene immediately after receiving an emergency alarm at 4pm. They found five corpses. The injured woman was taken to Shihua Hospital in Jinshan District.

After investigation, the gas was identified as carbon monoxide that was leaking from the cracked water-heating pipes of the old shower, police said.

A doctor at the Shihua Hospital said the survivor was receiving oxygen in the intensive care unit and was lucky to escape.

The accident did not surprise Yang Yu, director of the neighborhood committee of the residential complex, who put forward her concerns over safety problems to the upper-level leaders just a day before the accident.

She said the apartments are 30 years old and showing their age. Pipe breaks have been frequent but the property management company has refused to act, Yang said.

In addition, the rental of a divided room also raises safety concerns. In divided rooms, Yang said, it's common to see tenants improperly using facilities due to their lack of safety awareness.