Two Vietnamese workers reported missing on Sunday were later found dead at the bottom of a sewer – with a high concentration of hydrogen sulphide – at a manufacturing plant in Tainan in southwest Taiwan.

The environmental company, which makes products from recyclable materials in Guantian Industrial Park, did a roll call at 7am on October 15 for all its migrant workers, who were due to undergo check-ups at hospital, China Times reported.

However, the two Vietnamese workers, aged 25 and 27, were absent. Closed-circuit television footage showed that they were last seen walking past the factory’s sewerage treatment unit the previous evening.

Firefighters arrived at the scene, only to find the gas sensor gave an alarming reading of 14.4ppm of hydrogen sulphide inside the three-meter-deep well. It required around three hours of ventilation to make the place safe to enter.

The two missing workers were pulled out of the sewer and pronounced dead at the scene. They only had rubber boots and no other protective clothing.

Police are investigating the cause of the workers’ death. Meanwhile, the factory was order to suspend production for a month by Tainan City administration’s Labor Affairs Bureau.

Tainan District Prosecutors Office is also following up the case and gathering evidence – to see if the factory violated any sections of the Occupational Safety and Health Act.