A 40-year-old Indian national died on Thursday after inhaling a poisonous gas while carrying out routine tank cleaning at a waterwork site here.

The Indian national, who has not been named, was one of three men aged between 24 and 40 who had collapsed and were found unconscious at the waterworks agency’s Choa Chu Kang Waterworks at about 11.15 am, reported The Straits Times.

All three were taken to the hospital unconscious, where the worker died.

The two surviving workers are in the intensive care unit at Ng Teng Fong General Hospital, said the Public Utilities Board (PUB) in a statement.

The two men are Malaysians aged 24 and 39, and employed as general workers, said the Ministry of Manpower (MOM) in a statement.

The Indian national was employed as a cleaning operations manager by Supersonic Maintenance Services, according to a report by the Channel News Agency, citing a Ministry of Manpower spokesperson.

The PUB expressed its condolences to the family of the worker who died and said it was in touch with its contractors and would provide support to the workers’ family members.

MOM added that the man who died was a cleaning operations manager and, along with the older Malaysian, was employed by Supersonic Maintenance Services. The younger Malaysian works for Stargroup Est.

The ministry said it is investigating and has instructed PUB to stop all tank cleaning work.

PUB, in a statement, said early investigations suggest that the workers had inhaled hydrogen sulphide – a gas produced from sludge, which is a by-product of the water treatment process.

Cleaning works are carried out at the plant routinely once every three months.

The Singapore Civil Defence Force (SCDF) said it was alerted to a hazardous material (hazmat) incident at 51 Nanyang Drive at about 11.25 am.

Firefighters and hazmat specialists in personal protective equipment used two water jets to disperse the poisonous gas.

After about an hour of “vapour dispersion”, there was no more hydrogen sulphide detected in the plant, said SCDF. It added that cardiopulmonary resuscitation, or CPR, was performed on one of the workers on the way to the hospital.

The police have ruled out foul play.

Hydrogen sulphide is a colourless gas that gives off the odour of rotten eggs, according to an entry on the United States National Institute for Occupational Safety and Health (Niosh) website.

Exposure to the gas can cause irritation to the eyes and respiratory system and can lead to issues like falling into a coma.

Workers in wastewater treatment industries are a group at risk, the Niosh website states.

Choa Chu Kang Waterworks is a key plank in Singapore’s goal of achieving water security, and supplies 80 million gallons of safe drinking water to the western part of the island, said an environmental impact assessment published in 2022.

Upgrading of the plant began in 2022, and the works are slated to be completed by 2026.

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