Nitrogen Purging and the Air You Cannot Smell

Why this matters

Nitrogen is the safest-feeling gas on the truck. It does not burn, it is not toxic, it is 78 percent of the air you are breathing right now, and the cylinder came from the same supply house as the brazing rod. That is exactly why it kills people. In a small enclosed space it pushes oxygen out without any color, smell, taste or sensation, and the human body has no alarm for low oxygen. The U.S. Chemical Safety Board's bulletin on nitrogen asphyxiation documented dozens of deaths over a single decade, some of them would-be rescuers who went in after a coworker.

For an HVAC tech purging a braze joint in an air handler closet, a plumber pressure testing a gas line in a crawlspace, or anyone venting a test charge in a small room, the hazard is real at volumes a single cylinder easily delivers. Refrigerant leaks displace oxygen too, and carry their own toxic and flammable hazards; see related: Refrigerant Decomposition and Oxygen Displacement. This card owns the oxygen line itself and what happens to a person below it.

Why there is no warning

The urge to breathe comes from carbon dioxide building up in the blood, not from oxygen running low. When you hold your breath, the burning feeling is carbon dioxide. Breathing nitrogen-diluted air, you exhale carbon dioxide normally, so nothing builds up and nothing feels wrong. Oxygen falls in the blood while breathing feels completely normal.

At moderately low oxygen, the first things to go are judgment and coordination, which are the faculties a person would need to notice the problem and leave. At very low oxygen, a person can lose consciousness within one or two breaths, with no stage in between where they feel short of air. Someone who walks into an atmosphere of nearly pure nitrogen, for example a space purged fully, can collapse before they have registered that anything is unusual.

Smell is useless. Nitrogen has none. A tech who "checks" a space by sniffing for refrigerant oil or flux smoke is checking for a different thing.

The oxygen line and what is below it

Normal air is 20.9 percent oxygen by volume. OSHA defines an oxygen-deficient atmosphere as less than 19.5 percent oxygen, at 29 CFR 1910.146(b) for permit-required confined spaces in general industry and at 29 CFR 1926.1202 for confined spaces in construction, and the respiratory protection standard at 29 CFR 1910.134(b) uses the same 19.5 line. An atmosphere below 19.5 percent calls for supplied air, not a cartridge respirator: an air-purifying respirator filters what is there and cannot add oxygen that is missing.

The effects below are the ranges commonly published by safety agencies and gas suppliers. They are approximate, they vary with the person, exertion and altitude, and they are not a scale to work by. The only working number is 19.5.

Notice the overlap of the middle rows. That is honest: published tables disagree by a point or two, and the person in the room is the worst judge of where they are on it.

How much nitrogen it takes. Getting from 20.9 percent to 19.5 percent means about 6.7 percent of the air has been replaced (1 minus 19.5 divided by 20.9). In a small closet, that is a few cubic feet of gas. A full tall nitrogen cylinder holds on the order of a couple of hundred cubic feet of gas at atmospheric pressure; read the cylinder's own label for its capacity.

Where field nitrogen goes wrong

• The purge left running. A brazing purge needs only a low flow to keep oxygen out of the pipe, but the regulator gets cranked up, or left open while the tech goes to the truck, and the gas exits the open end of the line into the room for the whole job.
• The pressure test vented indoors. Releasing a test charge from a system or a gas line inside the closet, crawlspace or utility room puts the whole charge into that room at once. A system's internal volume at test pressure becomes many times that volume at atmospheric pressure.
• The hose or fitting that leaks. A damaged regulator diaphragm, a cracked hose or a loose fitting at cylinder pressure, which is commonly over 2,000 psi when full, can release gas fast.
• The cylinder stored in the cab. A slow valve leak in a closed truck cab overnight.
• The space that was purged on purpose. A tank, vessel or pipe that has been inerted with nitrogen is a near-pure atmosphere. Nobody puts a head in.

Nitrogen's density is close to air's, a little lighter, so unlike heavier refrigerants it does not reliably pool at the floor. It mixes through the space, which means the low-oxygen air is at head height too. Gas released cold from a cylinder can briefly sink before it warms and mixes. Do not count on the high or low point of a room being safe.

A worked closet

An air handler closet in a hallway, 4 feet by 6 feet with an 8-foot ceiling: 192 cubic feet. The door is closed to keep brazing smoke out of the house. A tech brazes a new coil with a nitrogen purge set, by mistake, at 20 standard cubic feet per hour (SCFH), and leaves it running for one hour. At the end of the job, the tech vents a pressure test whose released gas comes to about 20 cubic feet (an illustrative figure; it depends on the system's internal volume and the test pressure).

Treat the room as well mixed, with each cubic foot of nitrogen pushing a cubic foot of mixed room air out under the door. Oxygen then falls as 20.9 times e to the power of minus (nitrogen added divided by room volume).

Closet 4 x 6 x 8 ft = 192 cu ft, door closed
Nitrogen to reach a given oxygen level:
  19.5 %  ->  192 x ln(20.9 / 19.5) = 13.3 cu ft
  16.0 %  ->  192 x ln(20.9 / 16.0) = 51.3 cu ft
This job:
  purge 20 SCFH x 1 h         = 20 cu ft
  -> 20.9 x e^(-20/192)       = 18.8 % O2
  plus vented test, 20 cu ft  = 40 cu ft total
  -> 20.9 x e^(-40/192)       = 17.0 % O2
With a fan pulling 100 cfm through the closet:
  purge 0.33 cfm into 100 cfm -> about 20.8 % O2

Read it. The purge alone took the closet below 19.5 percent in under 45 minutes: 13.3 cubic feet at 20 SCFH is 0.67 hours, about 40 minutes. A tech kneeling in that closet for the last 20 minutes of the braze was in an oxygen-deficient atmosphere and felt nothing. Venting the test inside then took it to about 17 percent, into the range where coordination and judgment start to fail, at exactly the moment the tech stands up to pack the tools. A second person opening the door to check on the first walks into the same air.

The fan row is the fix. A box fan in the doorway exhausting to the hall, or better, to outdoors, moving 100 cubic feet per minute of fresh air through the closet, dilutes a 20 SCFH purge (one third of a cubic foot per minute) roughly 300 to 1, holding the closet at about 20.8 percent. That is the difference ventilation makes, and it is why "door open, air moving" is the default for any purge in a small space.

What changes the answer. Set the purge flow correctly low and the closet stays above 19.5 percent for hours even with the door shut, though the vented test can still drop it. Vent the test through a hose to outdoors and that half disappears entirely. A larger, open basement mechanical room dilutes the same 40 cubic feet into thousands, and the drop becomes a rounding error. A crawlspace or attic with poor exchange behaves like the closet, and a crawlspace may be a confined space with its own rules; see related: Confined Space Entry (OSHA 1910.146).

The controls, in order

• Vent test gas outdoors. Run a hose from the release valve out a door or window, or release the test at the outdoor service valve.
• Set the purge flow on a flowmeter, not by feel, at the low flow the braze actually needs, and close the cylinder whenever the torch is off for more than a moment.
• Ventilate the space for the whole job. Door open, a fan moving fresh air through, discharge to outdoors where you can.
• Wear a personal oxygen monitor in any small or closed space where nitrogen is flowing. A single-gas oxygen monitor clipped near the breathing zone, bump tested before use per its manufacturer and fresh-air calibrated at the altitude you are working at, alarms at 19.5 percent. A multi-gas monitor with an oxygen sensor covers this too. When it alarms, everyone leaves the space first and reads the meter outside.
• Transport and store cylinders upright, secured, valve closed, cap on, outside the cab.

If someone goes down

This is the response the CSB's record says people get wrong, and it is the one to rehearse in a toolbox talk.

• Do not go in after them. The air that dropped them will drop you in the same breaths. When an asphyxiation incident has more than one victim, the second is commonly the rescuer.
• Call 911 immediately and say a person is down in a possible low-oxygen atmosphere, so responders arrive with supplied air.
• Stop the gas and ventilate from outside: close the cylinder valve if you can reach it without entering, open doors and windows from outside, put a fan blowing fresh air in.
• Reach without entering only if you can pull the person out from the doorway without putting your own head into the space.
• Once the person is in fresh air, follow mainstream first aid guidance (American Red Cross, American Heart Association): check breathing, start CPR if they are not breathing normally and you are trained, and stay with them until responders take over. Hands-on certification is what makes a responder; this card is not training.

See related: The Dizzy Tech After the Nitrogen Test, a case worked from the record.

References

• 29 CFR 1910.146(b) and 29 CFR 1926.1202, oxygen-deficient atmosphere defined as less than 19.5 percent oxygen; 29 CFR 1910.134, Respiratory Protection, supplied air for oxygen-deficient atmospheres
• U.S. Chemical Safety Board, Safety Bulletin: Hazards of Nitrogen Asphyxiation
• Compressed Gas Association guidance on oxygen-deficient atmospheres and cylinder handling
• See related: Refrigerant Decomposition and Oxygen Displacement; Confined Space Entry (OSHA 1910.146); How to Ventilate a Small Space You Are Working In; The Dizzy Tech After the Nitrogen Test