In brief
Nitrogen and argon are not toxic, but they displace the oxygen in the air; without colour, smell or warning signs, they can cause loss of consciousness within one or two breaths. The risk arises most often when entering inerted tanks and reactors, when liquid nitrogen evaporates in an enclosed room and when welding with argon in confined spaces. The foundations of protection are continuous oxygen monitoring, a written entry permit, ventilation, correct storage and never carrying out an unplanned rescue.
Why does nitrogen asphyxiation risk go unnoticed?
A maintenance technician opens the manway of a tank that has been purged with nitrogen and leans in to retrieve a spanner that has fallen inside. There is no smell and no feeling of suffocation; a few seconds later he collapses. A colleague who sees him and rushes to help leans into the same tank. Nitrogen asphyxiation risk is a well-known yet still recurring scenario in confined space accidents.
The reason is physiological. What triggers our urge to breathe is not a lack of oxygen but a rise in carbon dioxide in the blood. A person breathing nitrogen or argon can exhale carbon dioxide normally, so they do not feel short of breath. The US Chemical Safety Board's (CSB) bulletin on nitrogen asphyxiation stresses that in an atmosphere very low in oxygen, loss of consciousness can occur within just one or two breaths, and the person receives no warning.
Nitrogen makes up about 78 per cent of the air we breathe; the danger starts when its share increases and it displaces oxygen. That is why the inert gas hazard should be questioned separately in every area where people say "there's no toxic gas".
Where will you encounter the inert gas hazard?
| Use | Example | When the risk arises |
|---|---|---|
| Inerting and purging | Nitrogen in reactors, tanks and silos to prevent explosions | Manned entry for maintenance, opening hatches |
| Liquid nitrogen | Laboratories, sample storage, cooling, blast freezing | Spills or tank venting in a small, unventilated room |
| Welding shielding gases | TIG and MIG welding with argon and argon mixtures | Welding inside a tank, pipe or pit |
| Laser cutting | High-pressure nitrogen assist gas | Tank room, leaks, enclosed machine cabin |
| Packaging | Modified atmosphere packaging in food, nitrogen generators | Generator room, leak points |
| Gas suppression systems | Inert gas systems in server and archive rooms | Entering the room after the system has discharged |
We cover the process side of inerting in reactors and tanks in our article on reactor safety in chemical plants, and the other risks of laser cutting machines in our article on laser cutting machine safety.
What does oxygen deficiency do to the body?
In normal air, the oxygen content is about 20.9 per cent. The table given in the CSB bulletin, based on the Compressed Gas Association (CGA), summarises the effects that may be seen as oxygen falls:
| Oxygen (%) | Possible effect |
|---|---|
| 20.9 | Normal |
| 19.0 | Some unnoticed adverse physiological effects |
| 16.0 | Increased pulse and breathing rate, impaired thinking and attention, reduced coordination |
| 14.0 | Abnormal fatigue on exertion, emotional upset, faulty coordination, poor judgement |
| 12.5 | Very poor judgement and coordination, impaired breathing that may cause permanent heart damage, nausea, vomiting |
| Below 10 | Inability to move, loss of consciousness, convulsions, death |
The same bulletin states that in an atmosphere with 4–6 per cent oxygen, a person falls into a coma in less than 40 seconds. In the US, OSHA requires oxygen in workplace air to be kept between 19.5 and 23.5 per cent; below 19.5 per cent is considered oxygen deficiency, and in practice this is also the entry criterion for many businesses.
The real message of the table is this: when oxygen falls to around 16 per cent, a person's ability to make decisions is impaired. A plan of "I'll get out if I feel dizzy" does not work, precisely because the brain that has to make the decision to leave is affected.
Oxygen detectors: choosing and using them correctly
Oxygen deficiency can only be detected by measurement. Two types of detector are used:
- Personal detector: worn on the worker's collar, close to the breathing zone. Most multi-gas detectors include an oxygen sensor; everyone entering a confined space, and the attendant, carries their own detector.
- Fixed oxygen alarm system: measures continuously in rooms where liquid nitrogen is used, nitrogen generator rooms and spaces with gas suppression; it also gives an audible and visual alarm outside the door.
For correct use, observe these rules:
- Place the sensor where the gas collects. Argon and carbon dioxide are heavier than air and collect in pits and at floor level. Nitrogen has a density close to air, but cold gas freshly evaporated from liquid nitrogen also sinks to the floor first.
- Measure from outside before entering. Measure different heights of the tank or pit before entering with a pumped detector fitted with an extension hose. We describe the method in our article on confined space gas testing.
- Measure continuously throughout the work. A one-off measurement is not enough; welding gas, a leaking valve or a line backing up can change the atmosphere within minutes.
- Carry out bump tests and calibration. Test that the detector alarms before use, have it calibrated at the interval specified by the manufacturer, and track sensor life.
- Set alarm thresholds in writing. Decide the thresholds with the recommendations of the device manufacturer and your gas supplier; write what to do on alarm into the instructions.
Nitrogen tank safety: storage and transfer
Nitrogen tank safety is considered in three parts: fixed cryogenic tanks, portable liquid nitrogen containers and gas cylinders.
- Cryogenic tank: should be outdoors, in a well-ventilated area closed to unauthorised access. Relief valve outlets are not directed towards places where people are present or towards building openings. Excessive icing on the vaporiser may indicate that capacity has been exceeded.
- Portable containers: when one litre of liquid nitrogen evaporates, about 700 litres of gas are formed. If a liquid nitrogen container is to be moved in a lift, it does not travel with a person; if the lift breaks down, the car can quickly become oxygen-depleted. A face shield, cryogenic gloves and closed shoes are worn for filling and carrying.
- Gas cylinders: stand upright and secured with a chain, are carried with the valve protection cap fitted, and the cylinder store is ventilated. For details, see our article on using oxygen and acetylene cylinders.
- Wrong connections: connecting a nitrogen line to a breathing air or oxygen line has caused fatal accidents. Label the lines and use connectors for different gases that cannot be fitted to each other.
If the nitrogen tank and buffers fall within the scope of Annex III of the Work Equipment Regulation as pressure vessels, a periodic inspection is required at least once a year. If the tank belongs to the gas supplier, maintenance may be their responsibility; however, the person who maintains a piece of equipment may not carry out the periodic inspection of the same equipment. Bursa Sistemer OSGB does not carry out the inspection itself; it plans the periodic inspection schedule and follows it up with authorised inspectors. For the general rules on pressure vessels, see our article on pressure vessels and compressor tanks.
Argon welding in confined spaces and entry into inerted equipment
In argon welding in confined spaces, the hazard is the shielding gas building up in the space where the welder is working. TIG welding inside a tank, root welding with gas fed into the pipe, and joints made in a pit are the riskiest jobs. In this work:
- The welding gas cylinder and hose are kept outside the confined space where possible; the gas is shut off during breaks and at the end of work.
- Forced ventilation is supplied to the space; oxygen is never used for ventilation.
- The welder carries a detector that monitors oxygen continuously.
Entry into an inerted tank or reactor requires a separate discipline: isolating lines with blank flanges, gas freeing and ventilation, measurement, a written permit, an attendant waiting outside at all times and a rescue plan ready to go. You will find the framework in our articles on working in confined spaces and the permit-to-work system, and other risks specific to welders in our article on welders' occupational safety risks.
In chemicals and plastics plants, inventorying the points where inert gas is used, writing entry procedures and testing them with drills is part of our OHS specialist service; findings are entered into the risk assessment document. You can track detector calibration dates with the OHS calendar.
Frequently Asked Questions
Is nitrogen a toxic gas?
No, nitrogen is not toxic; it already makes up most of the air. Its danger is that it displaces oxygen in an enclosed or poorly ventilated space. As oxygen falls, judgement is impaired and loss of consciousness and death can follow; the person does not notice this.
At what level should an oxygen detector alarm?
A frequently used reference in practice is 19.5 per cent, the US OSHA limit for oxygen deficiency. Set alarm thresholds according to the recommendations of the device manufacturer and your gas supplier, write them into your instructions and state clearly that entry is prohibited when the alarm sounds.
Does a gas mask protect against nitrogen and argon?
No. Filter masks capture contaminants in the air; they do not make up for missing oxygen. An oxygen-deficient atmosphere may only be entered with supplied-air breathing apparatus or an airline mask, with a written permit and an attendant.
Can we move a liquid nitrogen container in a lift?
The container can be moved in a lift, but not together with a person. If the lift gets stuck, evaporating nitrogen can lower the oxygen in the car. Place the container in the car and send the lift up, have another person collect it on the arrival floor, and put up a warning so that nobody gets in on the way.
A certified specialist matched to your hazard class: site inspections, approved logbook, training plan, annual plan and report follow-up.
Bursa SİSTEMER OSGB Technical Team — written from the field experience of our OHS specialists and workplace physicians. Legal information is valid as of the publication/update date; contact us for an assessment specific to your business.