Concentration of Hydrogen Sulfide in Manhole Exceeded Legal Level; Police Probe Death of Workers Inspecting Pipe in Japan’s Saitama Pref.

The Yomiuri Shimbun

13:44 JST, August 4, 2025

SAITAMA — A sewer manhole where four workers died during an inspection had a concentration of hydrogen sulfide at least 15 times the legal level, according to the contractor assigned to the work.

The Saitama prefectural police believe that the workers may have inhaled hydrogen sulfide and are working to determine the exact cause of death. An investigation has been opened for suspected professional negligence resulting in death.

Police on Sunday announced that the deceased were employees of a Saitama City–based civil engineering company hired by Gyoda city government to perform the inspection.

According to the police, one of the workers was trying to climb down a ladder to the bottom of the sewer pipe, which had stopped flowing, when he lost consciousness and fell on Saturday morning. The other three workers apparently fell while attempting to rescue him.

All four were pulled from the pipe, where wastewater was about 1.8 meters deep, and were later pronounced dead at a hospital. None of the workers had any visible injuries.

The Industrial Safety and Health Law requires that in locations where hydrogen sulfide may be generated, ventilation or other measures must keep concentrations at 10 ppm or less. According to the company, a reading of 30 ppm was recorded when work began. After the four men fell, a concentration of 150 ppm or higher was measured. None of the workers were wearing safety harnesses to prevent falls, nor masks to guard against oxygen deficiency.

Police are examining whether there were any problems with the company’s safety management.

4 Workers Die After Falling Down Saitama Sewer Manhole During Inspection; Hydrogen Sulfide Suspected as Cause of Fall 

4 Workers Fall into Manhole; All Confirmed Dead