Guide
Helium safety data explained: why the balloon gas that changes your voice is an asphyxiant and a cryogen
Helium is not flammable and not toxic. Helium (UN 1046) is an inert simple asphyxiant 7 times lighter than air that collects under roofs and in the top of enclosures. Inhaling it from a cylinder or balloon displaces oxygen in the lungs and has caused deaths. Liquid helium at −269 °C expands about 750-fold.
- Transport identity
- UN 1046
- Density and buoyancy
- 0.1786 kg/m³ at 0 °C
- Boiling point and expansion
- Liquid helium boils at −268.9 °C
- Voice change
- The speed of sound in helium is about 1
- Oxygen-deficient limit
- Below 19.5 % oxygen by volume; helium displaces oxygen with no smell
Helium is the gas people laugh at, and it has a record of deaths among people who were not at work. Its safety data sheet is as short as argon's, because helium reacts with nothing, and like argon's it hides its one serious hazard in a single phrase: simple asphyxiant. This page explains the figures behind the sheet at /sds/helium, why the voice change is a warning and not a joke, and what the liquid at −269 °C adds.
What the SDS classification means
| Item | Value | Source |
|---|---|---|
| UN number | 1046 (compressed gas); 1963 (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 | 7440-59-7 | NIST Chemistry WebBook |
| Molar mass | 4.0026 g/mol | NIST Chemistry WebBook |
| Boiling point | −268.9 °C (4.2 K) at 1 atm; no solid phase at atmospheric pressure | NIST Chemistry WebBook |
| Critical temperature and pressure | −268.0 °C (5.2 K); 2.3 bar | NIST Chemistry WebBook |
| Gas density | 0.1786 kg/m³ at 0 °C and 1 atm; relative density 0.14 (air = 1) | NIST Chemistry WebBook |
| Liquid density at boiling point | about 125 kg/m³; roughly 750 volumes of gas per volume of liquid at 20 °C | NIST Chemistry WebBook |
| Speed of sound at 20 °C | about 1,000 m/s in helium; 343 m/s in air | Standard acoustic data |
| Helium in normal air | about 5 ppm by volume | Standard atmosphere |
Two numbers carry the page. The relative density of 0.14 makes helium the second-lightest gas after hydrogen, so it rises and gathers at the highest point of any enclosure. And the boiling point of 4.2 K makes liquid helium the coldest liquid in use anywhere, cold enough to freeze every other gas including nitrogen and oxygen.
Rules that matter
Helium does nothing chemically in the body; it is breathed in and out unchanged. But a lungful of helium is a lungful without oxygen, and the body's breathing drive, which responds to carbon dioxide rather than to oxygen, gives no alarm. The mechanism is identical to nitrogen asphyxiation and is described in full under nitrogen safety and asphyxiation; the thresholds are the same too, with 19.5 % oxygen as the entry floor under OSHA 29 CFR 1910.146 and EIGA Doc 44, and collapse possible below about 10 %.
What makes helium different is the way it is misused. A single breath from a balloon, exhaled at once, rarely harms a healthy adult, and that is where the idea that it is harmless comes from. The deaths occur in three ways:
Why inhaling helium is not a joke
- Repeated breaths, each one replacing the previous lungful of helium with another, so that blood oxygen falls with no sensation of suffocation and the person collapses while still laughing.
- Inhalation from a bag or with the head covered, which removes the air between breaths.
- Inhalation directly from a pressurised cylinder or a balloon-filling nozzle, which adds barotrauma: the pressure can rupture lung tissue and force gas into blood vessels, a fatal injury independent of oxygen.
The voice change is itself the warning. Sound travels about three times faster in helium than in air, so the resonant frequencies of the throat and mouth rise and the voice sounds thin and high; the pitch of the vocal cords does not change. A high voice is a direct demonstration that the lungs are full of helium, and the sensible response is to stop, not to repeat it. Helium has real work to do in leak detection, welding, laboratories and medicine, and the supply and conservation guide explains why a scarce imported gas should not be spent on balloons at all.
Where helium goes: up, and then it stays
In the open, helium disperses within seconds and is the least dangerous of all gases to leak. Indoors, its buoyancy is the problem. It collects under roofs, in ceiling voids, in the top of cabinets and enclosures, in the roof space of a van and at the crown of any closed vessel. The oxygen-deficient layer forms from the ceiling downward, so a monitor at belt height reads normal until the layer has grown to head height, by which time a person standing on a ladder or a platform has been in it for some time.
| Space | Where helium collects | Where to mount the oxygen sensor |
|---|---|---|
| Workshop or laboratory | Under the roof, above beams and ducts | High wall or ceiling, 19.5 % alarm |
| Closed van or delivery vehicle | Roof of the load space, then the cab | Never carry dewars or leaking cylinders in a closed cab |
| MRI or NMR room | Ceiling, then the whole room on a quench | High wall, with external beacon, and a functioning quench vent |
| Vessel or tank being leak-tested with helium | Crown of the vessel; any manway at the top | At the manway before any entry |
| Glove box or enclosure with helium atmosphere | The entire enclosure | Interlock the door to the purge |
The rules are the mirror image of those for argon and carbon dioxide. Vent helium rooms from the highest point, mount detectors high, and treat the top of any enclosure as the hazard. A helium leak test, which is the commonest industrial use because the small atom finds leaks nothing else can, leaves the test vessel full of helium afterwards; it must be purged with air and tested before anyone opens a manway.
Rules that matter
Where helium is delivered or stored as a cryogenic liquid, the SDS adds H281 and three hazards that go beyond those of liquid nitrogen:
Liquid helium: the coldest liquid there is
- Cold burns at −269 °C. Contact is instantaneous and deep; cryogenic gloves and a face shield are mandatory, and a splash into an eye is an emergency.
- Expansion. One litre of liquid produces about 750 litres of gas, so a sealed dewar will burst. Helium dewars carry relief valves and bursting discs, and the relief must never be blocked, iced or taped. A dewar that is venting steadily is behaving normally; one that has stopped venting may be building pressure because its neck has iced.
- Air condensation. Any surface cooled by liquid helium is far below the −196 °C at which air liquefies and the −183 °C at which oxygen does. Uninsulated transfer lines and bayonets drip liquid air, and the liquid that forms is enriched in oxygen because oxygen condenses first. That puddle is an oxidiser on a floor that may hold oil or combustible packing.
Below 2.17 K helium becomes a superfluid that flows without viscosity and creeps over surfaces, which is a laboratory curiosity rather than an industrial hazard. Separately, helium cannot be frozen at atmospheric pressure at all, so any solid plug found in a helium dewar neck is frozen air or water ice, never helium, and it can block the vent.
The MRI quench
The largest single use of liquid helium is cooling superconducting magnets, and the hazard specific to them is the quench. A superconducting MRI magnet sits in a bath of liquid helium at 4 K and stores a great deal of magnetic energy. If any part of the winding warms enough to lose superconductivity, that energy is dumped into the helium as heat and the bath boils off very rapidly. A magnet holding on the order of 1,000 litres of liquid releases roughly 750,000 litres of gas, enough to fill a scan room many times over. The quench pipe is designed to carry this outdoors; where it is blocked, undersized or leaks into the building, the room becomes unbreathable within moments and the pressure rise can prevent the door from opening. Hospitals treat the quench vent as a life-safety system, inspect it as one, and mount oxygen monitors high on the scan-room wall with a beacon outside the door.
Indian specification, colour and licensing
The Indian product standard is IS 15583, Compressed helium gas — Specification, published in 2005 and revised in 2026; the standards position is set out on the KIGL helium product page. Industrial helium cylinders are identified under IS 4379:2021 Table 7 by a Middle brown (411) body with the shoulder in the ground colour and no band, and IS 3933:2021 gives medical helium the same brown, with medical oxygen-helium mixtures carried in a Graphite Black cylinder whose shoulder is painted in white and brown quarters. For licensing, helium is a non-flammable non-toxic gas and Rule 44(b)(ii) of the Gas Cylinders Rules, 2016 allows up to 200 cylinders for own use without a licence.
Helium is a permanent gas with the lowest critical temperature of any substance, so the cylinder is filled to a pressure referred to 15 °C and the gauge is a fair measure of contents. Because the atom is so small, helium cylinders and fittings are also the ones most likely to show a slow leak at a valve gland or a regulator seat; a cylinder that reads lower after a week on the shelf than it did on delivery has probably leaked rather than been short-filled, and the fitting should be soap-tested.
Rules that matter
Rules to apply
- Never inhale helium, from a balloon, a bag or a cylinder, and refuse to sell it for that purpose.
- Vent high, detect high; the hazard is at the ceiling, not the floor.
- Treat every vessel that has been helium leak-tested as a permit space until it has been air-purged and gas-tested; follow inert gas confined-space entry.
- Handle liquid helium with cryogenic PPE, keep dewar reliefs clear, and never ride in a lift or a closed vehicle with a dewar.
- Keep the quench vent of any superconducting magnet inspected and unobstructed.
- Read Sections 2, 7, 9 and 14 of the SDS before the first delivery; the guide on how to read an industrial gas SDS explains each.
The full sheet at /sds/helium carries the first-aid, fire-fighting and transport sections that this explainer leaves to the SDS.
Helium safety data explained — frequently asked questions
Is helium flammable or reactive?
No. Helium is a noble gas and takes part in no chemical reaction under any industrial condition, which is why it is used to shield arcs, purge rocket tanks and test for leaks. Section 10 of the SDS lists no incompatibilities. Its only hazards are physical: pressure, cold and the displacement of oxygen.
Is it safe to inhale helium from a balloon?
No. One breath from a balloon rarely harms a healthy adult, but every breath of helium is a breath without oxygen, and people have died inhaling it from balloons, party cylinders and bags, usually by taking several breaths in succession or by covering the head. Inhaling directly from a pressurised cylinder is worse: the pressure can rupture lung tissue and force gas into the bloodstream. Industrial helium is not supplied for inhalation.
Why does helium change your voice?
Sound travels about three times faster in helium than in air, which raises the resonant frequencies of the vocal tract so the voice sounds thin and high. The vocal cords vibrate at the same rate. The effect is a direct demonstration that the lungs are full of helium rather than air, which is why safety trainers use it as a warning sign rather than a party trick.
Where does leaked helium go?
Straight up. At one-seventh the density of air it rises and gathers under roofs, in ceiling voids and at the top of enclosures, cabinets and vehicles. A helium leak at floor level in a well-ventilated shop is harmless; the same leak in a closed van, a cold-room roof space or an MRI room with the vent blocked builds an oxygen-deficient layer from the ceiling down, which is why oxygen monitors for helium are mounted high.
What is an MRI quench and why is it a helium hazard?
A superconducting MRI magnet is bathed in liquid helium at 4 K. If the magnet loses superconductivity, the stored energy boils the helium off very rapidly; a magnet holding on the order of 1,000 litres of liquid releases roughly 750,000 litres of gas. The quench pipe must carry it outdoors; if it fails or is blocked, the room becomes unbreathable within moments, and the pressure can prevent the door opening. Hospitals treat the quench vent as a life-safety system.
What are the hazards of liquid helium?
Cold burns at −269 °C, the coldest liquid in industry; roughly 750-fold expansion, so a sealed dewar will burst; and air condensation, because any surface cooled by liquid helium is far below the −196 °C at which air liquefies, so uninsulated transfer lines drip liquid air that is oxygen-enriched. Helium dewars must always have a working pressure relief and must never be transported in a passenger compartment or a lift with people in it.
Do helium cylinders need a PESO licence?
Helium is a non-flammable non-toxic gas, so Rule 44(b)(ii) of the Gas Cylinders Rules, 2016 allows up to 200 cylinders to be held for own use without a licence. Industrial helium cylinders are Middle brown (411) under IS 4379:2021, and the Indian product specification is IS 15583, revised in 2026. Helium is imported into India, and the supply-and-conservation guide explains why it is priced and rationed as it is.
What should I check on the helium SDS?
Section 2 for the gas-under-pressure classification and the simple-asphyxiant note, Section 7 for the high-level ventilation and no-inhalation rules, Section 9 for the relative density of 0.14 that governs where it collects, and Section 14 for UN 1046 and Class 2.2. KIGL's full sheet is at /sds/helium; this page explains it.
References
- Bureau of Indian Standards. (2021). Colour identification of gas cylinders and related equipment for medical use (IS 3933:2021).
- Bureau of Indian Standards. (2021). Identification of the contents of industrial gas cylinders (IS 4379:2021, Second Revision).
- Compressed Gas Association. (n.d.). — Oxygen-deficient atmospheres, and CGA P-14 Accident prevention in oxygen-rich and oxygen-deficient atmospheres (CGA SB-2).
- European Industrial Gases Association. (n.d.). — Hazards of inert gases and oxygen depletion (EIGA Doc 44).
- IS 15583 Compressed helium gas — Specification, Bureau of Indian Standards
- Linstrom, P. J., & Mallard, W. G. (Eds.). (n.d.). NIST Chemistry WebBook (NIST Standard Reference Database 69). National Institute of Standards and Technology. [Helium thermophysical properties] https://webbook.nist.gov/cgi/cbook.cgi?ID=C7440597
- Occupational Safety and Health Administration. (n.d.). Permit-required confined spaces (29 CFR 1910.146).
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