I had a photograph of my own to illustrate this post, but it was a bit rubbish. I was inspired to write about helium when I discovered the wreckage of a mylar-foil helium balloon, like the one pictured above, tangled in a gorse bush on the slopes of Newtyle Hill. It’s the second foil balloon I’ve discovered on the hill, and (like the first one) I stuffed it into my rucksack and carried it down for disposal. I took a photograph to illustrate what a non-biodegradable blot on the landscape these things are, but in the photo the balloon looked like just another bit of plastic debris.

The picture above is actually more useful, because it demonstrates the key fact about helium gas, the one thing that pretty much everyone knows about it, and the property from which many of its other interesting qualities derive—it’s lighter than air.

The reason it’s lighter than air is because its atoms are considerably less massive than the molecules that make up air. Helium is a monatomic gas, made up of individual atoms, and the mass of a single helium atom is about four daltons.* (For comparison, the mass of a common carbon atom is 12 daltons, and the commonest kind of hydrogen atom weighs in at around one dalton.) Air, on the other hand, is mainly composed of two diatomic gases, nitrogen and oxygen. Their molecules, N₂ and O₂, come in at a 28 and 32 daltons, respectively, giving air an average molecular mass of 29 daltons.

The fact that individual helium atoms have a low mass feeds into two other important properties of helium.

Firstly, its atoms are small—just a single electron shell containing two electrons. A small atom with tightly bound electrons is reluctant to redistribute its charge in response to nearby polar molecules. This means that its relatively immune to the intermolecular Van der Waals forces which cause atoms and molecules to transiently adhere to each other, which in turn means that helium gas isn’t very soluble.

Secondly, at any given temperature the atoms in helium gas move faster, on average, than the atoms or molecules of heavier gases. This is because temperature is a measure of the kinetic energy of gas particles, and kinetic energy scales with both velocity squared and mass. A low mass means velocity must be higher to produce the same kinetic energy. Since helium is only 4/29 the mass of an average air molecule, the mean velocity of its atoms is correspondingly higher by the square root of 29/4, or about 2.7.

So: helium is light, fast and not very soluble. I’ll come back to each of these as we go along.

Firstly, lightness. It turns out that, at equal temperature and pressure, equal volumes of different gases contain the same number of fundamental particles (to a good first approximation). So a litre of helium is only 4/29 the weight of a litre of air. The only less dense gas is hydrogen, which has diatomic molecules massing about two daltons. So both hydrogen and helium are so buoyant in air that they’re able to lift considerable additional mass as they rise—making them ideal fillers for balloons, large and small. Hydrogen, being half the mass of helium, is by far the better lifting agent, but it has one significant disadvantage:

That’s a photograph of the German dirigible “Hindenburg”, fatally aflame at Lakehurst, New Jersey, in 1937. Hydrogen is flammable; helium is not. In fact, helium is notoriously chemically unreactive, being the lightest of the so-called “noble gases” (the others are neon, argon, krypton, xenon and radon). All of these elements have full outer electron shells, rendering them almost completely chemically inert. Which is why modern balloons and dirigibles are filled with helium, not hydrogen.

Next, speed. The faster gas molecules move, the more readily they diffuse through a barrier—which is why a rubber balloon full of helium will lose its shape within a day, and why helium balloons are often made of less-permeable mylar foil, like the one in the photograph at the head of this post. (Because they’re not biodegradable, foil balloons are supposed to be used only indoors—my experience of finding two on the open hillside shows how well that rule is working in practice.)

The rapid movement of helium gas atoms also affects the speed of sound, because sound waves travel through a gas at a velocity roughly comparable to the average speed of the gas molecules. At 0ºC, the speed of sound in air is about 330m/s; for helium it’s 970m/s, almost three times faster. So if you have a resonant cavity full of helium, it will resonate at a frequency about three times higher than it would if filled with air. And that’s what causes the “duck voice” effect we hear when someone breathes a gas mixture containing helium. Their vocal cords vibrate at exactly the same frequency as usual—but the resonant gas cavities of their larynx and airways pick out and emphasize the higher-pitched harmonics of their voice.

Some people achieve this effect by taking a breath from a helium-filled party balloon, which is very much not a good idea, since it violates The Oikofuge’s First Law:

Never breathe anything that contains no oxygen

Breathing gas that contains no oxygen causes oxygen to leave your circulation and diffuse into the gas in your lungs—your circulating oxygen levels therefore fall very rapidly indeed, and a single deep breath can take you to the edge of unconsciousness.

To illustrate the duck-voice effect of someone breathing helium, here’s a recording of a saturation diver, breathing a helium/oxygen mixture in a pressurized underwater habitat:

Which leads us to wonder why deep divers breathe helium and oxygen (a mixture referred to as Heliox), rather than air.

The ambient pressure rises with depth underwater, by about one atmosphere for every ten metres of descent. To counterbalance this, divers must breathe gas at the ambient pressure. But the higher the pressure of gas we breathe, the more of it dissolves in our tissues—and it turns out nitrogen is an anaesthetic agent at high pressures. Its effects are detectable at depths as shallow as ten metres, where the pressure is twice that at the surface. And by the time divers descends to 30 or 40 metres (four or five atmospheres), their judgement becomes sufficiently impaired by nitrogen narcosis that they’re a potential danger to themselves and others.

So for deep diving, nitrogen has to go. But it can’t be replaced by pure oxygen, because oxygen is toxic at higher-than-normal pressures, damaging the lungs and causing convulsions. Indeed, the need to keep the partial pressure of oxygen in the breathing mixture close to what we’re used to at the surface means that, with increasing depth and pressure, oxygen must make up a lower and lower percentage of the breathing mixture by volume.

Helium is a good replacement for nitrogen, for several reasons. Firstly, its low solubility and chemical inertness mean that it doesn’t produce any anaesthetic effect. Secondly, because helium is less soluble than nitrogen, less of it dissolves in the diver’s tissues during a long dive at high ambient pressure, so there’s less of it to get rid of during decompression at the end of the dive, and therefore less risk of gas-bubble formation in the blood and tissues as ambient pressure decreases. Such bubbles are the cause of decompression sickness (“the bends”), and in order to avoid their formation, divers are forced to make their return to the surface slowly. But because helium dissolves in smaller volumes than an equivalent pressure of nitrogen, there’s less risk of bubble formation, and so a faster safe decompression.^† And finally, the low density of helium comes into play again—because it’s less dense, it’s easier to breathe at high pressures.

Indeed, that last advantage is present even at one atmosphere of pressure. When a person’s airways are narrowed by disease or inflammation, air flow through the narrowed regions can shift from smooth, laminar flow to turbulent flow, which produces a higher resistance to flow through the airways and makes breathing more difficult. The transition from laminar to turbulent flow is determined, in part, by the density of the breathing gas. And, once turbulent flow occurs, the resistance to flow is higher for a denser gas. Substituting helium for nitrogen in the patient’s breathing gas drops its density by 60%, which delays the onset of turbulent flow, and causes less resistance to flow if turbulence occurs. That serves to reduce the work of breathing, decrease distress, and get a bit more oxygen into the patient—which is all good stuff.

So are there any disadvantages for divers breathing helium (apart from the funny voices)? There are. One is caused by that high average velocity of helium atoms—as well as conducting sound faster, helium is also more conductive of heat, with a thermal conductivity almost six times faster than nitrogen. Divers in a helium atmosphere find it harder to stay warm, and when submerged they lose heat to the water very quickly if they have helium filling their dry-suits. (So they often fill the insulating space in their dry-suits with argon, which has an even lower thermal conductivity than air.)

And finally, it turns out that the absence of anaesthetic effects with helium is actually a disadvantage for the deepest of dives. Below depths of about 150-300 metres (fifteen to thirty atmospheres of pressure), divers breathing Heliox develop a condition called High Pressure Nervous Syndrome (HPNS), associated with an apparent overactivity of the nervous system—tremors, muscle jerks, nausea, dizziness and cognitive impairment. No-one’s quite sure why this happens—it was at first blamed on a stimulant effect of helium that appeared only at high pressure, but it now seems more likely that it’s a direct pressure effect on nerve cell membranes, which are reduced in volume by such high ambient pressures. Ironically, the symptoms of HPNS can be damped down by introducing the sedative effects of nitrogen back into the mix, using a breathing mixture of nitrogen, helium and oxygen generically referred to as Trimix. Things get very technical at that point—not only must the ratio of helium and nitrogen be adjusted to minimize the effects of HPNS, but the proportion of oxygen in the mixture must be reduced with increasing depth, in order to limit the pressure of oxygen to a non-toxic level.

But there’s a problem with Trimix, which is that nitrogen at high pressures is difficult to breathe because of its density. What low-density gas could we substitute for nitrogen? Hydrogen, half the density of helium and a fourteenth as dense as nitrogen, turns out to be mildly anaesthetic at high pressures, and therefore it also limits the symptoms of HPNS.

But wait a minute, I hear you cry, glancing back at that photograph of the Hindenburg. Hydrogen is flammable. Can a breathing mixture containing hydrogen and oxygen be safe?

Well, yes it can. Remember that we have to wind down the proportion of oxygen in the breathing mixture as we go to greater depths, to keep the partial pressure of oxygen within safe limits. At thirty atmospheres pressure, a gas mixture containing just 1% oxygen provides an oxygen pressure equivalent to 30% oxygen at sea level—a little more than the 21% we’re used to, but within safe limits. Hydrogen/oxygen mixtures are flammable over a wide range of proportions—from 4% hydrogen in 96% oxygen to 95% hydrogen in 5% oxygen. But not at lower proportions of oxygen. So the low proportion of oxygen required for safety at great depth means that the hydrogen/oxygen ratio sits outside the flammable range. These Hydreliox mixtures are very experimental, but they’ve been used successfully, with 1% oxygen and roughly equal proportions of helium and hydrogen, at depths in excess of 500 metres.

And that’s about it for helium gas. Liquid helium, of course, has all sorts of interesting properties, but that’s perhaps a topic for another day.

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* The dalton, also called the Atomic Mass Unit, is named after John Dalton, who first codified the idea that chemistry was due to atoms interacting with each other in a very systematic way.

^† There’s an important distinction here, though. The advantages of helium’s lower solubility only appear in what’s called “saturation diving”—when divers stay at depth in pressurized habitats for long periods, so that their tissues become saturated with dissolved breathing gas. But divers who descend and then reascend relatively quickly (called “bounce diving”) are never at depth long enough for their tissues to become saturated with nitrogen. For them, helium paradoxically produces a worse risk of decompression sickness than nitrogen, because helium diffuses so much faster than nitrogen. The volume dissolved in the tissues rises very quickly initially, and in the short term may exceed what would be reached by nitrogen in the same time period. Like this: