Daegu, Jul 20 (IANS): A toxic gas leak at a water purification plant in South Korea's Daegu city on Wednesday killed one person and seriously injured two others, police said.

The accident occurred at the city-operated Jukgok Water Purification Plant just before 10 a.m., when one of two private service workers hired to clean its water storage tank collapsed on the tank floor, Yonhap news agency reported.

The worker, identified only as a man in his 60s, was transferred to a university hospital but died apparently from poisoning with hydrogen cyanide, a highly toxic and colourless gas or liquid, police and firefighters said.

Two civil servants in their 30s and 50s also collapsed inside the water storage tank while trying to rescue the ill-fated worker, police said, adding they were now in critical condition.

The surviving private service worker in his 50s, who barely escaped from the water storage tank after his co-worker's collapse, told police that they smelled hydrogen cyanide gas on their way down to the second basement floor of the tank.

According to firefighters, 47 ppm of hydrogen cyanide gas was measured inside the tank at the time of their rescue operations. The lethal dose of hydrogen cyanide is known to be 50 ppm.

Water plant officials had reportedly ventilated the water storage tank naturally before the cleaning work began, but it was not known whether hydrogen cyanide was measured there in advance.