Nitrogen: The Silent Killer

Sept. 21, 2006
Nitrogen is an invisible, tasteless and odorless gas that comprises about 78 percent of the air we breathe. But its potential to kill workers in or near confined spaces should never be underestimated.

On Nov. 5, 2005, two contractors working at Valero Energy Corp.'s Delaware City, Del., oil refinery died from nitrogen asphyxiation. Interviews conducted by the Chemical Safety and Hazard Investigation Board (CSB) indicate the two men were assigned to re-attach piping to a vessel as part of preparations to bring the vessel back online.

Based on the results of CSB's investigation at press time the agency is expected to issue its final investigation report in October one possible scenario is the first victim may have inhaled concentrated nitrogen while working outside the confined space, directly above the access opening on top of the reactor, and then passed out and fell into the vessel.

After the first contract worker collapsed inside the vessel, witnesses told CSB the second contract worker entered the vessel, likely in an attempt to rescue his fallen colleague. The two workers "were quickly overcome" by the high-purity nitrogen gas, explains John Vorderbrueggen, PE, CSB's lead investigator for the Valero incident.

"They were in an environment that had probably less than 1 percent oxygen," Vorderbrueggen says. An oxygen concentration below 19 1/2 percent is considered unsafe for workers; when the oxygen content drops to about 8 or 10 percent, Vorderbrueggen adds, "you don't have much of a chance."

The Valero incident prompted CSB member John Bresland to remind stakeholders: "Nitrogen is a silent killer."

"You will not recognize you're in trouble in time to take action to save yourself," CSB Investigation Manager Bill Hoyle concurs. "That makes it an extremely hazardous situation, despite the fact that [nitrogen] is the largest constituent of air we breathe."

Common Causes

CSB compiled data from federal agencies, media reports and other sources to track workplace deaths and injuries between 1992 and 2002 that were caused by nitrogen asphyxiation. According to CSB, during that decade there were 85 nitrogen asphyxiation incidents, resulting in 80 fatalities and 50 injuries.

Of those 85 incidents, 67 incidents involved situations in which workers were in or near a confined space.

CSB reviewed the 85 total incidents and identified several major categories of causes:

  • Failure to detect an oxygen-deficient atmosphere In each of the 67 incidents involving confined spaces, personnel failed to detect elevated levels of nitrogen and take appropriate precautions. In the 2005 Valero incident, CSB noted the work permit issued to the contractors did not mention a nitrogen hazard, nor did it require the use of special breathing apparatus.
  • Fatalities and injuries during attempted rescue Approximately 10 percent of fatalities from the CSB data were co-workers attempting to rescue fallen colleagues in confined spaces, as appears to be the case in the Valero incident.
  • Mix-up of nitrogen and breathing air Confusing nitrogen gas with air, and problems with breathing-air delivery systems, accounted for 12 of the 85 total incidents. In one case, workers inadvertently connected the hose for their breathing-air respirator to a pure nitrogen line.

Good Practices

CSB, in a 2003 bulletin titled Hazards of Nitrogen Asphyxiation, discussed a number of "good practices for safe handling of nitrogen." They are detailed here.

Warning Systems

OSHA 29 CFR 1910.146 Permit-Required Confined Spaces for general industry requires employers to identify all confined spaces in their workplace and then to determine if any of those are permit-required confined spaces. A permit-required confined space is a confined space containing a hazardous atmosphere, an engulfment hazard, an entrapment or asphyxiation hazard or some other serious safety and health hazard.

As OSHA occupational safety and health specialist Patrick Kapust explains, employers need to clearly understand that confined spaces with elevated nitrogen concentrations (i.e., reduced oxygen concentrations below safe levels) qualify as a permit-required confined space.

"If the confined space contains an actual or potential atmospheric hazard, it's a permit-required confined space," Kapust says.

To warn workers of nitrogen-enriched atmospheres and other permit-required confined spaces, OSHA 29 CFR 1910.146(c)(2) requires the posting of a warning sign for example, "Danger: Permit-Required Confined Space, Do Not Enter" or "any other equally effective means."

CSB, in addition, recommends installing devices such as flashing lights, audible alarms and auto-locking entryways to prevent access. Personal monitors can warn workers via an audible or vibration alarm of low oxygen concentrations.

Continuous Atmospheric Monitoring

Because the atmosphere in a confined space may be unfit for breathing prior to entry, or it may change over time, CSB says the atmosphere in the entire confined space should be tested and confirmed safe before workers enter the space and should be monitored continuously while workers are in the space.

OSHA 29 CFR 1910.146(d)(5)(ii) and (iii) explain: "Test or monitor the permit space as necessary to determine if acceptable entry conditions are being maintained during the course of operations; and [w]hen testing for atmospheric hazards, test first for oxygen, then for combustible gases and vapors and then for toxic gases and vapors."

OSHA 1910.146(d)(6) also requires employers to have an attendant outside the permit-required confined space at all times while a worker is inside. The attendant's job is to use instruments to monitor the conditions within the space, to remain in contact with the entrant in case of emergency (and to alert rescuers, if necessary, or perform a non-entry rescue) and to know the hazards of the space and the signs or symptoms of exposure to the space's hazards, among other duties. The attendant should never enter the confined space unless the attendant is part of a rescue team and has been relieved by another attendant.

Ensure Ventilation with Fresh Air

Any time workers are entering a confined space or a small or enclosed area without wearing a supplied-air breathing apparatus, it is critical to provide continuous ventilation with forced-draft fresh air, CSB says. While fresh-air ventilation is not an option when workers are entering a pure nitrogen environment such as when workers are changing a catalyst in a reactor (nitrogen, in such a case, likely would be used to protect the catalyst from being damaged or contaminated by oxygen or moisture) it is particularly applicable when an area recently has been purged with nitrogen or carbon dioxide or some other gas and the area has been brought to the minimum safe breathing level of 19 1/2 percent oxygen.

Ventilation also is needed in rooms and chambers near confined spaces.

CSB, in its 2003 safety bulletin, notes a few of the nitrogen asphyxiation cases between 1992 and 2002 involved people who were not working in the nitrogen-enriched space, room or enclosure but were working close by.

"One aspect of our investigation in the Valero case is the possibility that nitrogen escaping from the open manway may have created a hazardous atmosphere just outside of the confined space," Vorderbrueggen explains. "The first victim may have been overcome while working outside the confine space, above the opening."

Implement a Rescue System

The issue of rescue presents a particularly vexing problem when it comes to workers in nitrogen-enriched confined spaces. Because nitrogen as well as other odorless, colorless gasses is a silent killer, a worker who sees his co-worker lying on the floor of a nitrogen-enriched confined space might think the co-worker was the victim of a fall, a heart attack or some event unrelated to nitrogen asphyxiation. When human instinct kicks in and the worker attempts to save his fallen co-worker, the rescuer often becomes the second victim.

Indeed, Vorderbrueggen points out fatalities in confined spaces "often come in multiples."

The answer, according to CSB, is training, training, training. In addition to training workers on proper rescue procedures, employers need to hammer into workers the awareness that, in the words of Hoyle, "just a few breaths can render them unconscious and unable to safely exit the confined space."

A rescue plan might involve attaching a body harness and lifeline to workers entering confined spaces (although this might not work in narrow-diameter spaces such as furnaces or ducts) or attaching wristlets or anklets to a lifeline and retrieval mechanism to allow the confined space attendant to pull the person out by the arms or legs, according to the CSB bulletin.

Whatever the method of rescue, the hallmarks of good rescue programs include effective means of communicating with personnel inside confined spaces and having the attendant and rescue personnel available at all times, according to the CSB bulletin. And, of course, no one ever should enter a hazardous atmosphere without proper PPE, even to aid a fallen colleague.

According to OSHA 29 CFR 1910.146, employers have the option to provide rescue with in-house personnel or by calling an outside emergency service, but either way the onus is on the employer to make sure the worker is rescued "before any long-term harm comes to that person," OSHA's Kapust explains.

If an outside emergency service is used, employers need to evaluate whether the service has the personnel and resources to respond to the types of confined space hazards at their facility, would be available to respond to a confined space emergency and can get there in a timely manner.

"It wouldn't be adequate, if you had workers going into a nitrogen-enriched atmosphere, to say: 'We're just going to call 9-1-1 if someone goes down,'" Kapust says.

Integrity of Breathing Air

In situations when workers must enter a confined space with an oxygen concentration of less than 19 1/2 percent (or a confined space where the oxygen level might dip below that level), workers must be supplied with breathing air, either through a self-contained breathing apparatus or an airline respirator.

Employers need to have a system in place to protect against any interruption of airflow (such as from a power failure). The system should include an alternate source of power for the air compressors; continuous monitoring of air supply; routine inspection and replacement of supplied-air hoses; and restriction of vehicular traffic in the area of supply hoses (vehicles can inadvertently cause a supply hose to become twisted or obstructed).

Prevent Mix-Ups of Nitrogen and Breathing Air

To prevent interchanging compressed nitrogen with compressed industrial-grade air or compressed breathing-quality air, CSB recommends that cylinders for nitrogen, industrial-grade air and breathing-quality air have distinct, incompatible fittings that cannot be cross-connected. Cylinders should be clearly labeled; placing labels on piping systems, compressors and fittings provides additional reminders of which gas is contained inside, according to CSB.


Training is the glue that is necessary to bring these good practices together and make them part of an effective nitrogen-enriched confined space safety program. According to CSB, workers should be trained on the use of ventilation systems, retrieval systems and atmospheric monitoring systems; hazard communication; mandatory safety practices for entry into confined spaces (such as providing ventilation and an attendant); precautions when working around equipment that may contain elevated levels of nitrogen; the reason for special fittings on compressed gas cylinders; proper use of air supply equipment; and the hazards of nitrogen-enriched atmospheres.

It is on this last point continuously communicating the seriousness of the hazards associated with nitrogen-enriched environments that employers sometimes fall short.

"The challenge is to get workers to recognize that there are no warning associated with it," Vorderbrueggen says. "That's probably where some employers are coming up short. They don't emphasize how risky oxygen-depleted environments can be and how quickly you are taken down.

"There's no fear factor thrown into training for nitrogen awareness."

Such a fear factor might help employers and employees follow the advice of Garvin Branch, an OSHA occupational safety and health specialist in construction: "You treat all confined spaces as if they do have hazardous atmospheres until you've done testing to prove that they don't."

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