The incident occurred near a Tesco Lotus superstore on Sri Sena Road in Phuket municipality on Wednesday morning.
The four dead men were employees of Eastern Thai Consulting 1992 Co Ltd., a private contractor hired by Phuket municipality to oversee a wastewater treatment piping system, ASTV-Manager website reported. .
The four - Tawatch Chitchonlatarn, Sanan Buachum, Suchet Chitchonlatarn and Abhisit Tiprasong - were in a six-man team assigned to remove rubbish in sewer pipes in the area as well as clean and maintain a sewage pump which drains wastewater into a treatment system.
A worker in the team said they opened manhole covers in the area for 30 minutes to drain the stench and dangerous sewage gases before Tawatch started working in one of them.
After 10 minutes, hydrogen sulfide which had collected at the bottom of the 1mx3m pipe was stirred up, causing Tawatch to lose consciousness. Sanan went down to try carry him out but failed as Tawatch was too heavy for himใ He inhaled the gas and passed out.
Suchet and Abhisit, who struggled to rescue their colleagues, also passed out from exposure to the deadly gas.
Rescue workers faced difficulty in recovering the bodies as hydrogen sulfide was sporadically released and rose up to ground level. Four were affected and taken to hospital.
A rescue crew wearing oxygen tank and mask tied rope around each of the unconscious employees, who had been exposed to the gas for two hours, and used a crane truck to pull their bodies out of the manhole. The men were pronounced dead in hospital, the report said.
Hydrogen sulfide, also known as sewer gas, is heavier than air and builds up in pipes, escaping when it is stirred up by movement. .