Two Hong Kong sewer workers have died after suspected exposure to hydrogen sulphide while underground, prompting authorities to temporarily suspend related cleaning operations.

Chief Executive John Lee Ka-chiu said on Tuesday morning a full investigation was needed to understand why the workers, employed by a subcontractor of the Drainage Services Department, had ventured underground and whether safety regulations were followed.

“Everyone has a responsibility to improve industrial safety, including site owners and workers, [we also need] an effective management system,” he said ahead of his weekly Executive Council meeting. “I hope a safety culture can become part of the norm, and everyone will work according to safety guidelines.”

The two men, aged 26 and 34, were found unconscious late on Monday night inside a four-metre-deep (13 feet) manhole chamber.

Development minister Bernadette Linn Hon-ho told lawmakers on Tuesday morning that workers did not need to enter the sewers according to standard procedures, and were only required to use a high-pressure hosepipe to clean the drains from outside.

The workers were cleaning the sewage drains at Yuen Wo Playground in Sha Tin on Monday night, along with two other colleagues.

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Another colleague alerted police just before midnight that the pair had lost consciousness underground.

Firefighters rescued the unconscious pair, who were taken to Prince of Wales Hospital in Sha Tin but certified dead soon after arrival.

The cause of death would be determined by a postmortem, a police spokesman said.

Initial investigation suggested the pair had passed out after breathing in hydrogen sulphide, a toxic gas.

The other two workers, who had also entered the manhole chamber, were able to climb out on their own and were taken to hospital.

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Development chief Linn said a key focus of the investigation was why the men had gone underground, as she pledged to follow up on the case.

All underground drainage cleaning works were temporarily suspended following the deaths to ensure all contractors and workers were well aware of standard procedures, she said.

Director of drainage services Mok Wing-cheong also expressed his condolences, pledging to provide immediate support to the families affected and the contractor.

An investigation by the Drainage Services Department, Labour Department and police is under way.