The Definition of Nitrogen Asphyxiation

Definition of Nitrogen Asphyxiation

Nitrogen asphyxiation occurs when nitrogen gas displaces oxygen in the air to the point where a person can no longer get enough oxygen to stay conscious and alive. Nitrogen itself is not toxic. It makes up about 78 percent of normal air. The danger comes when extra nitrogen is released into an enclosed or poorly ventilated space, lowering the oxygen level below what the body needs.

Nitrogen is a simple asphyxiant that is colorless, odorless and tasteless. People cannot sense it, and the body's urge to breathe is driven mainly by carbon dioxide build-up rather than lack of oxygen. As a result, a worker breathing a nitrogen-rich atmosphere usually feels no shortness of breath or warning before collapsing. In very low-oxygen atmospheres, unconsciousness can follow within a few breaths, and death within minutes.

Nitrogen asphyxiation has caused many deaths across industry, including would-be rescuers who entered a space to help a collapsed coworker. The U.S. Chemical Safety Board has issued a safety bulletin on the hazards of nitrogen asphyxiation because of these repeated incidents.

Where Nitrogen Hazards Occur

• Inerting and purging: nitrogen is widely used to purge tanks, reactors, pipelines and vessels to remove flammable vapors or oxygen before maintenance or start-up.
• Confined spaces: tanks, vessels and pits that have been purged with nitrogen and not properly ventilated.
• Liquid nitrogen: used in laboratories, healthcare, food freezing and manufacturing. A small amount of liquid expands to roughly 700 times its volume as gas, which can quickly lower oxygen in small rooms, cold rooms or elevators.
• Near vents and openings: nitrogen leaking from vessel openings, manways or vents can create low-oxygen pockets just outside a space.
• Mistaken connections: nitrogen supplied in place of breathing air through incorrectly connected hoses or fittings.

Oxygen Levels and Their Effects

Normal air contains about 20.9 percent oxygen. OSHA defines an oxygen-deficient atmosphere as one with less than 19.5 percent oxygen. As oxygen drops further, workers may experience impaired judgment and coordination, then rapid loss of consciousness. Because judgment is affected early, a worker may not realize anything is wrong or be able to escape on their own.

Preventing Nitrogen Asphyxiation

• Test the atmosphere: use a calibrated oxygen monitor before and during entry into any space that may contain nitrogen. See gas detection and air monitoring.
• Ventilate: use forced-air ventilation to restore normal oxygen levels after purging, and keep it running during work.
• Treat inerted spaces as deadly: entry into a space that must stay inerted requires supplied-air respiratory protection and a strict permit system. An N95 or cartridge respirator gives no protection.
• Isolate nitrogen lines: blank or disconnect nitrogen supply lines before entry, using lockout/tagout.
• Warn workers: post signs at openings of vessels being purged and at rooms where liquid nitrogen is stored or used.
• Prevent cross-connections: use incompatible fittings so nitrogen lines cannot be connected to breathing air systems.
• Plan rescues: never enter to rescue a collapsed worker without proper respiratory protection. Use non-entry rescue systems where possible.

Example of Nitrogen Asphyxiation

At a chemical plant, a contractor was asked to retrieve a tool he had dropped inside a reactor that had been purged with nitrogen the day before. The reactor was not on the confined space entry permit for that day, and the nitrogen supply valve had been closed but not isolated. He leaned into the manway and collapsed. A coworker who climbed in to help also collapsed. Both were pulled out by a rescue team wearing self-contained breathing apparatus, and one did not survive.

The investigation found that oxygen levels inside the reactor were below 5 percent and that there was no warning sign at the manway. The plant added oxygen monitoring at all nitrogen-purged vessels, required blinding of nitrogen lines before any entry, posted asphyxiation hazard signs at every manway and retrained all employees and contractors on rescue procedures.

Frequently Asked Questions: Nitrogen Asphyxiation

Why don't people notice when they are breathing nitrogen?

Nitrogen has no color or smell, and the body's breathing reflex responds mainly to carbon dioxide, not low oxygen. Workers often feel nothing before losing consciousness.

What oxygen level is dangerous?

OSHA considers any atmosphere below 19.5 percent oxygen to be oxygen deficient. Much lower levels can cause unconsciousness within seconds.

Can a dust mask or cartridge respirator protect against nitrogen?

No. These respirators filter air but do not add oxygen. Only supplied-air respirators or self-contained breathing apparatus protect workers in low-oxygen atmospheres.