A worker who became unconscious Wednesday after inhaling toxic gas at a wastewater treatment plant in Busan was pronounced dead Sunday afternoon, police said Monday.

The 52-year-old worker, surnamed Lee, is believed to have died from hydrogen sulfide poisoning, according to the hospital where he was being treated. Police have asked the National Forensic Service to carry out an autopsy to determine the exact cause of death. 

The poisonous gas, believed to be hydrogen sulfide, was leaked at 1:08 p.m Wednesday at the wastewater treatment plant. Four of the 10 workers who inhaled it fell unconscious. The other three remain unconscious, according to local media reports.

As of 2:21 p.m. Wednesday, the hydrogen sulfide level at the site of the leak was measured at 150 parts per million -- 10 times higher than the permissible level under the Occupational Safety and Health Act.

The toxic gas is believed to have been produced when the workers poured alkaline wastewater into a well, the Busan Fire Department said. The police collected samples from the well and requested an investigation by the National Forensic Service.

By Park Ju-young (jupark@heraldcorp.com)