Fatality Assessment and Control Evaluation (FACE) Program
Two Sewer Pipe Relining Workers Killed by Toxic Sewer Gas
Midsummer 2002, two male workers age 25 and 19 were overcome by toxic sewer gas - probably hydrogen sulfide - while walking through a 600 ft (183 m) length of underground sewer pipe. The men were clearing debris from the pipe in preparation for installing a Cured-In-Place-Pipe (CIPP) liner. The sewer pipe was 15 ft (4.5 m) under ground, measured 2 ft (0.6 m) wide by 5 ft (1.5 m) high, and had 1 to 2 ft (0.3 to 0.6 m) of untreated sewage standing in it. The worksite had been inactive for about 10 days prior to the two workers entering the pipe through a manhole at the downstream end of the section. They walked toward an access pit (Photo 1) at the upstream end of the section. There other workers were preparing for installation of the liner when the workers walking through the duct called for help. Five co-workers attempted to rescue them and were also overcome. Emergency personnel from the local fire department responded and entered the access pit after donning air-supplied respirators. They lifted the seven workers out of the pit with the aid of a nearby mobile crane. The two workers walking in the pipe were found face down in the sewage and had drowned. Neither of them was wearing a respirator nor were several key permit-required confined space precautions followed, such as monitoring before and during the workers' entry. The other five men recovered after several days in the hospital.
RECOMMENDATIONS based on our investigation are as follows:
Midsummer of 2002, two male workers, ages 25 and 19, were overcome by toxic sewer gas - probably hydrogen sulfide - and drowned while walking through an approximately 600 ft (183 m) section of underground sewer duct. The Iowa FACE program became aware of the incident the same day through local news reports and immediately began an investigation. An Iowa FACE investigator and a Deputy Medical Examiner visited the worksite and interviewed investigators and employees involved in the incident. Among other sources of information were reports from the Iowa State Medical Examiner's Office, the Iowa Division of Labor Services Occupational Safety and Health Bureau, and courts of appeal. In addition, information associated with the incident was obtained from local news reports, interviews by local police, eyewitness accounts, and photographs of the scene.
The employer specialized in lining sewer and water mains and had been in business for over 25
years, employing 1500 workers on multiple work crews. The company had been hired to reline a 5
mile (8 km) section of municipal sewer
pipe which was leaking into a storm
sewer that ran alongside it. The sewer
pipe was buried 15 ft (4.6 m) below
ground level and measured roughly 2 ft
(0.6 m) wide by 5 ft (1.5 m) high with a
flat bottom and a rounded top. The
company's specialty was installing
Cured-In-Place-Pipe (CIPP) linings. The
installation process involved application
of flexible fiberglass (Photo 2) sections
to the inside of existing underground
pipes. The fiberglass sections are held
in place by fillets - materials anchored
to the walls of the sewer duct - and
resins with the sections are cured in
place when hot water is pumped through the CIPP lining. Once the process is completed, the resin-fiberglass matrix forms a hard, durable
liner on the inside of the pipe. Major advantages of this approach are that above ground structures
are not disturbed and most of the work can be done through existing manholes and other access
The company dug 15 ft (4.5 m) deep access pits periodically along the length of sewer line to be relined. A temporary steel tower was erected at each pit to assist with insertion of lining and curing materials (Photo 3). The crew had been working on relining the five mile run for approximately a year. However, work on this section had been suspended for 10 days before the two victims entered the pipe through a manhole approximately 600 ft (183 m) down-stream from an access pit and proceeded to walk upstream removing debris and old resin in preparation for installing the liner.
The flow of sewage had been blocked upstream from the access pit and a 12 in (300 mm) diameter sewage bypass pipe had been installed on top of the ground. However, the underground sewer duct still had 1 to 2 ft (0.3 to 0.6 m) of stagnant, untreated sewage in it.
One of the workers was nearing the
access pit where other workers were
preparing for installation of the liner when
he called for help. Five co-workers
attempted to rescue the workers from the
pipe but were subsequently overcome.
Emergency personnel were summoned
and the local fire department crew
responded. They entered the access pit
after donning supplied-air respirators and
extricated all seven workers with the help of a nearby mobile crane (Photo 3). The two workers
walking through the pipe were found face down in sewer water, had drowned, and were
pronounced dead at the scene. Their five co-workers were taken to local hospitals: three in critical
condition. After several days of treatment in hyperbaric conditions they all recovered. Five of the
responders were also taken to local hospitals and treated for symptoms of heat exhaustion.
The workers were wearing hardhats and were equipped with flashlights, but they did not have respirators or two-way radios for communication with other workers. The workers had also not employed confined space entry protocols or procedures. Toxic gas levels were not monitored before the workers entered the manhole or as they walked through the sewer pipe.
The probable cause of the incident was the release of dangerous levels of hydrogen sulfide gas from the sewer water which had been stagnant for several days before the workers entered the sewer pipe. Hydrogen sulfide is formed by microbes in the anaerobic breakdown of organic wastes. It is heavier than air, only slightly soluble in water, and is readily released when waste water containing it is disturbed. As the two victims walked through the sewer pipe, hydrogen sulfide gas was probably released increasing its concentration in the duct.
It is not known if either of the victims noticed the odor associated with low concentrations of hydrogen sulfide gas as they walked through the sewer pipe. Hydrogen sulfide has a "rotten egg" odor at low concentrations. However, odor perception is unreliable as a forewarning because rapid olfactory fatigue may develop when concentrations are high, above 100 ppm (parts per million). Concentrations above 1000 ppm may cause rapid unconsciousness and death. Consequently, both men could have unknowingly been breathing increasing concentrations of hydrogen sulfide as they approached the pit area.
Cause of Death
The official cause of death from the autopsy report was drowning. Acute hydrogen sulfide intoxication was a probable contributing factor.
Recommendation #1 Formal communication between the hosts of a construction or maintenance site and contractors or employers regarding hazardous conditions and comprehensive safety procedures must be established before any work is to begin.
Discussion: The hosts (owners or operators of premises, city, county, state, or federal agencies) of construction or maintenance sites must inform contractors of hazardous conditions which may exist in their facilities as well as any procedures they must have in place to protect their employees from hazards that might exist in their facilities.
Consequently, all contractors and employers must obtain available information regarding hazards in permit-required confined spaces from the host before entry processes begin. All employers must inform the host of any permit-required confined space entry process in use and the hazards which may be confronted or created in the permit-required confined space (OSHA 29 CFR 1910.146 (c) (8-9)). The presence of toxic gases is a well-known hazard in sewer work and comprehensive safety procedures, which had been established by the primary contractor, should have been strictly followed.