NIOSH warns of CO fumes after using explosives

Safety officials are warning occupational safety and health professionals about a deadly hazard that can occur when explosives are used in conjunction with sewer construction projects. In some circumstances, deadly levels of carbon monoxide can be produced and threaten workers.

Carbon monoxide (CO) is an odorless, colorless gas that can cause illness and death by asphyxiation. Although many workers understand the dangers of CO toxicity, occupational CO exposure can occur from unrecognized sources, according to the recent warning from the National Institute for Occupational Safety and Health (NIOSH) in Cincinnati. In a recent incident, three cases of CO poisoning in a confined space, including one fatality, were caused by CO migrating through soil after nearby use of explosives.

A municipal sewer project involved the installation of new pipes and manholes. Explosive blasts were used to break up rock layers 6 feet below the surface before excavating pipeline trenches and manhole pits.

On the day of the fatality, a construction crew installed a 12-foot-deep manhole without incident. After the crew left the area, 265 pounds of nitroglycerin-based explosive in 20 boreholes, each 18 feet deep, were detonated 40 feet to 60 feet from the manhole. A worker who entered the manhole 45 minutes after the explosion collapsed within minutes, and two co-workers descended into the manhole to rescue him. One rescuer retrieved the unconscious worker before collapsing on the surface, and the other rescuer died in the manhole. All involved construction workers had elevated blood levels of carboxyhemoglobin, indicating they had inhaled air containing high CO concentrations.

An investigation determined that carbon monoxide released from the explosion had migrated through the soil into the manhole. CO concentrations in the bottom of the manhole two days after the incident were 1,905 parts per million (ppm), well above the immediately dangerous to life and health (IDLH) concentration of 1,200 ppm. Tests following ventilation of the manhole showed that high levels of CO reappeared as a result of continued diffusion from the surrounding soil. Subsequent monitoring of the manhole showed a decline in CO levels over the next eight days.

NIOSH says this report is apparently the first occupational fatality from this type of CO exposure, though nonfatal CO poisonings have been reported in residential basements following nearby use of subsurface explosives.

Also, NIOSH notes this incident involved a "chain-reaction" death, a well-known danger associated with confined space rescues. Chain-reaction deaths are so named because after the first victim is found in a confined space, a rescuer enters without proper precautions and is overcome, a subsequent rescuer enters and is likewise overcome, and so on. Chain-reaction rescuer fatalities have accounted for 36% of the deaths in confined spaces.

NIOSH advises that all manholes should be considered confined spaces with potentially hazardous atmospheres, and appropriate air monitoring should be conducted before each entry into a manhole, as well as during worker occupancy. Even if appropriate monitoring had been conducted earlier in the day for this incident, the fatality might have occurred if the manhole had not been monitored for CO after the blasting.

[For free information on CO generation by explosives, call NIOSH Pittsburgh Research Laboratory at (412) 892-4213. For free information about the hazard of confined spaces and procedures to protect workers, call NIOSH at (800) 356-4674, or visit the Web site at]