Guide
Nitrogen safety and asphyxiation: why the gas that makes up 78 % of air kills without any warning
Nitrogen is not flammable; the hazard is asphyxiation. Harmless in air at 78 %, it displaces oxygen without smell, taste or any warning. Below 19.5 % oxygen an atmosphere is deficient; below about 10 % a person can collapse within a few breaths. One litre of liquid nitrogen makes about 694 litres of gas.
- Transport identity
- UN 1066
- Oxygen-deficient atmosphere
- Below 19.5 % oxygen by volume
- Collapse threshold
- Below about 10 % oxygen
- Liquid expansion
- 1 litre of liquid nitrogen at −196 °C becomes about 694 litres of gas at 21 °C
- Room dilution
- Releasing nitrogen equal to just 7 % of a room's volume takes the room from 20.9 % to 19.5 % oxygen
Nitrogen is chemically one of the most harmless gases in industry and one of the most dangerous to breathe in a confined space. Both are true for the same reason: it is inert, colourless, odourless, tasteless and makes up 78 % of every breath you take, so the body has no way of noticing when the proportion climbs to 90 or 100 %. This page explains the numbers behind the nitrogen safety data sheet, which you will find in full at /sds/nitrogen, and why the only effective protection is measuring oxygen rather than trusting your senses.
What the SDS classification means
| Item | Value | Source |
|---|---|---|
| UN number | 1066 (compressed gas); 1977 (refrigerated liquid) | UN Model Regulations; IS 4379:2021 Table 1 |
| Transport class | 2.2 non-flammable, non-toxic gas | UN Model Regulations |
| GHS classification | Gas under pressure: compressed (H280) or refrigerated liquefied (H281); no health classification | UN GHS |
| Pictogram | GHS04 gas cylinder only | UN GHS |
| CAS number | 7727-37-9 | NIST Chemistry WebBook |
| Molar mass | 28.013 g/mol | NIST Chemistry WebBook |
| Boiling point | −195.8 °C at 1 atm | NIST Chemistry WebBook |
| Critical temperature and pressure | −147.0 °C; 33.9 bar | NIST Chemistry WebBook |
| Gas density | 1.250 kg/m³ at 0 °C and 1 atm; relative density 0.967 (air = 1) | NIST Chemistry WebBook |
| Liquid density at boiling point | about 806 kg/m³; roughly 694 volumes of gas per volume of liquid at 21 °C | NIST Chemistry WebBook |
| Nitrogen in normal air | 78.1 % by volume; oxygen 20.9 % | Standard atmosphere |
The absence of any health pictogram is the trap. A sheet that shows only the gas-cylinder symbol looks benign next to the acetylene or chlorine sheet, yet the hazard it carries in Section 2 under "simple asphyxiant" is the one industrial-gas workers most often die from. EIGA Doc 44, Hazards of inert gases and oxygen depletion, exists because inert-gas asphyxiation is a recurring cause of fatalities in industry.
How asphyxiation happens, and why there is no warning
Nitrogen does nothing to the body. It is breathed in and breathed out unchanged. What it does is take up space: every cubic metre of nitrogen that enters a room, a tank or a pit pushes out a cubic metre of air, and the oxygen fraction of what remains falls in proportion.
The reason the victim notices nothing is physiological. The urge to breathe, the feeling of suffocation that makes holding your breath unbearable, is triggered by carbon dioxide accumulating in the blood, not by oxygen falling. In a nitrogen atmosphere the lungs still clear carbon dioxide normally, so the breathing reflex is satisfied while the oxygen reaching the brain drops. The sequence commonly described is a slight light-headedness, a feeling of well-being, then unconsciousness, usually without the person ever understanding that anything was wrong. In high concentrations, one or two breaths of pure nitrogen are enough to cause collapse.
| Oxygen in air (% v/v) | Effect on a healthy adult at rest |
|---|---|
| 20.9 | Normal |
| 19.5 | Minimum for entry without breathing apparatus (OSHA 29 CFR 1910.146) |
| 15 to 19 | Reduced ability to work; impaired coordination; early symptoms in people with heart or lung disease |
| 12 to 15 | Breathing and pulse rate increase; poor coordination, perception and judgement |
| 10 to 12 | Breathing increases further; lips turn blue; judgement fails |
| 8 to 10 | Mental failure, fainting, unconsciousness, nausea and vomiting |
| 6 to 8 | Fatal after a few minutes; recovery possible only with immediate treatment |
| Below 6 | Convulsions, respiratory arrest, death within minutes |
These bands, in the form widely published by CGA SB-2, are for a person at rest; exertion shortens every interval. The practical reading is that anything below 19.5 % is forbidden territory, and anything below 10 % is a space from which a person will not walk out.
How much nitrogen does it take?
Far less than the size of the cylinder suggests. If a quantity V of nitrogen is released into a sealed room of volume R and mixes completely, the oxygen fraction becomes 20.9 × R / (R + V). Setting that equal to 19.5 % gives V = 0.072 R: nitrogen equal to 7 % of the room's volume takes it to the deficiency limit. For 10 % oxygen, V is 1.09 R, roughly the room's own volume again.
| Space | Volume | Nitrogen to reach 19.5 % O₂ | As liquid nitrogen |
|---|---|---|---|
| Cold room or lab | 50 m³ | 3.5 m³ | 5 litres |
| Vehicle cab, lift car | 5 m³ | 0.36 m³ | 0.5 litres |
| Cylinder store | 200 m³ | 14 m³ | 20 litres |
The liquid column is the sobering one. A 25-litre laboratory dewar holds enough nitrogen to turn a 50 m³ room into a 15 % atmosphere, and a dewar left in a lift, which is the classic fatal scenario, needs to boil off only half a litre. Real rooms exchange air with the outside, which is why these figures are worst cases; pits, tanks, cold rooms and vehicle cabs are the places where the worst case is the real case.
Rules that matter
At room temperature nitrogen is almost exactly as dense as air (relative density 0.967), so it neither pools nor rises reliably; it mixes and stays. Cold nitrogen vapour from a liquid spill or a boil-off vent is denser than air and flows downhill like water until it warms, filling pits, trenches, sumps and the bottom of tanks first. The spaces to treat as suspect are:
Where nitrogen accumulates
- Any vessel, tank or pipeline that has been purged, blanketed or pressure-tested with nitrogen, including the acetylene generators and gasholders that are purged under the procedure in acetylene plant start-up, shutdown and purging.
- Rooms that house a nitrogen generator or a liquid tank's vaporiser; a PSA plant's waste stream is oxygen-enriched, but its product leaks are nitrogen.
- Lifts, vans and cold stores carrying dewars or dry-shippers.
- Pits and trenches below a nitrogen pipeline joint.
EIGA Doc 237, the acetylene-generator operating code, makes the point in its maintenance clause 8.3: before any person enters a generator the plant must test for acetylene in nitrogen, then for acetylene in air below the lower explosive limit, and then, as the final test, for oxygen in nitrogen. The third test is the asphyxiation check.
Rules that matter
Where nitrogen is delivered as a cryogenic liquid, the SDS adds H281 and three further hazards:
Liquid nitrogen: cold, brittle, expanding
- Cold burns. Liquid nitrogen is at −196 °C; contact with skin causes instant frostbite, and the splash into an eye is an emergency. Cryogenic gloves, face shield and closed footwear are not optional.
- Embrittlement. Carbon steel, most plastics and rubber become glass-like at liquid nitrogen temperature; a spill on a mild-steel floor plate or a rubber hose can crack it.
- Expansion. One litre of liquid yields about 694 litres of gas at 21 °C. A dewar that cannot vent will burst; a dewar that can vent will displace air. Never transport a dewar in the passenger compartment of a vehicle, and never ride in a lift with one.
Liquid nitrogen also condenses oxygen out of the air onto uninsulated cold surfaces, producing a liquid that is oxygen-enriched; the puddle under a frosted pipe is not pure nitrogen.
Indian specification and colour
Industrial nitrogen in India is specified by IS 1747:2023, which abolished the old technical and pure grades and now carries four: N 2.5 (≥ 99.5 %, oxygen ≤ 5,000 ppm), N 4.5 (≥ 99.995 %), N 5.0 (≥ 99.999 %) and N 5.5 (≥ 99.9995 %). KIGL's industrial nitrogen is supplied to the N 2.5 grade. The cylinder is identified under IS 4379:2021 Table 7 by a French grey (630) body, a Graphite Black RAL 9011 shoulder and a Bottle Green RAL 6007 band. The standards and their editions are set out on the nitrogen standards page.
For licensing, nitrogen is a non-flammable non-toxic gas; Rule 44(b)(ii) of the Gas Cylinders Rules, 2016 permits up to 200 cylinders for own use without a licence. Oxygen depletion, not the law, is the constraint on where you may keep them.
Rules that matter
Rules to apply
- Measure, do not sense. Wear a personal oxygen monitor alarmed at 19.5 % wherever nitrogen is piped, vented or stored, and bump-test it each shift.
- Treat every purged vessel as unbreathable until an air purge and a documented gas test prove otherwise, and follow the permit procedure in inert gas confined-space entry.
- Vent nitrogen outdoors, never into a room, a corridor or a pit.
- Never enter to rescue a collapsed colleague without breathing apparatus; raise the alarm and ventilate.
- Handle liquid nitrogen with cryogenic PPE, in ventilated areas, and never in a lift or a closed vehicle.
- Keep the 19.5 % and 23.5 % alarms together: the companion guide on oxygen enrichment and deficiency explains why both matter in the same shop.
This page is an explainer. The full SDS at /sds/nitrogen carries the first-aid, fire-fighting and transport sections that an emergency responder needs.
Nitrogen safety — frequently asked questions
Is nitrogen gas flammable?
No. Nitrogen is an inert gas: it does not burn and does not support combustion, which is why it is used to purge and blanket flammable equipment and why it travels as a Class 2.2 non-flammable, non-toxic gas. Its hazard is asphyxiation, not fire: it displaces the oxygen in a space without any smell or warning.
Is nitrogen gas toxic?
No. Nitrogen has no toxic action on the body, which is exactly why its SDS carries no health pictogram. It kills by displacement: every cubic metre of nitrogen that enters a space pushes out a cubic metre of air, and the oxygen fraction falls. The medical term is simple asphyxiant. The hazard is physical, not chemical, and no filter respirator offers any protection against it.
Why does the body give no warning in a nitrogen atmosphere?
Because the urge to breathe is driven by carbon dioxide building up in the blood, not by a lack of oxygen. Breathing nitrogen still removes carbon dioxide from the lungs, so the brain senses nothing wrong while oxygen in the blood falls. Victims do not gasp or struggle; they feel slightly light-headed, then lose consciousness. This is why nitrogen asphyxiation is so often fatal to rescuers as well.
How low does oxygen have to fall before it is dangerous?
OSHA 29 CFR 1910.146 treats 19.5 % as the floor for entry without breathing apparatus. Between 15 and 19 % work capacity falls and coordination suffers; between 10 and 12 % judgement fails and lips turn blue; below 10 % a person can faint within a few breaths; below 6 % death follows in minutes. These bands are tabulated in CGA SB-2.
How much nitrogen does it take to make a room unsafe?
Less than most people expect. In a sealed room, oxygen falls to 19.5 % once nitrogen equal to about 7 % of the room's volume has been released and mixed. For a 50 m³ store that is 3.5 m³ of gas, which is 5 litres of liquid nitrogen or well under one full industrial cylinder. Real rooms leak, but pits, tanks and cold rooms do not.
Can I enter a vessel that has been purged with nitrogen once the gas is turned off?
Not on the strength of the gas being off. The nitrogen is still inside. The vessel must be ventilated with air, the atmosphere tested at top, middle and bottom to show at least 19.5 % oxygen, and the test repeated continuously during entry. EIGA Doc 237 makes the oxygen-in-nitrogen test the final check before anyone enters an acetylene generator, and the same logic applies to any purged vessel.
What are the hazards of liquid nitrogen specifically?
Three. Cold burns from contact at −196 °C, which are instantaneous and deep; brittle fracture of carbon steel, rubber and plastics that touch it; and the 694-fold expansion, which means a spilled dewar in a lift, a cold room or a laboratory can displace the air in it within minutes. Liquid nitrogen vapour is also heavier than air while cold, so it pools before it warms and disperses.
What oxygen monitor should a nitrogen user carry?
A personal electrochemical oxygen monitor with a 19.5 % low alarm and a 23.5 % high alarm, bump-tested before each shift and calibrated to the maker's schedule. For fixed installations where nitrogen is piped, a wall-mounted oxygen depletion alarm at breathing height with an external beacon at the door is common good practice.
References
- Bureau of Indian Standards. (2021). Identification of the contents of industrial gas cylinders (IS 4379:2021, Second Revision).
- Bureau of Indian Standards. (2023). Nitrogen, compressed gas and liquid (IS 1747:2023, Second Revision).
- Compressed Gas Association. (n.d.). — Oxygen-deficient atmospheres (CGA SB-2).
- European Industrial Gases Association. (n.d.). — Hazards of inert gases and oxygen depletion (EIGA Doc 44).
- Linstrom, P. J., & Mallard, W. G. (Eds.). (n.d.). NIST Chemistry WebBook (NIST Standard Reference Database 69). National Institute of Standards and Technology. [Nitrogen thermophysical properties] https://webbook.nist.gov/cgi/cbook.cgi?ID=C7727379
- NIOSH — Worker deaths in confined spaces and oxygen-deficient atmospheres
- Occupational Safety and Health Administration. (n.d.). Permit-required confined spaces (29 CFR 1910.146).
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