Heavy Gas – Sulfur Hexafluoride

SF6 is the voice lowering gas that sounds so freaky!

Why does helium make your voice sound like a cartoon character? Chemistry and physics teachers often use this classic science demonstration to illustrate how sound travels through a gas that is six times lighter than air. Steve Spangler looks at the science behind the demonstration and wonders what might happen to sound that travels through a gas that is six times heavier than air.


Even though helium is an inert gas which can be inhaled briefly without risk of death, we do not recommend that you breathe helium… or any gases besides “regular air.” The science teachers and demonstrators in the video were trained by professionals. Do not try this experiment at home (which is a crazy thing to say because our special gas – sulfur hexafluoride (SF6) – is extremely expensive).

Simply typing “helium voice” or “breathe helium” into a Google search will produce a number of interesting articles explaining why your voice changes when you breathe helium. In our search, we found a number of articles stating that your vocal cords vibrate faster in a helium environment than in a heavier “air” environment. The truth, however, is that sound waves travel faster through helium than they do through regular air because helium is less dense than regular air (six times less dense).

Now your wheels are spinning… If helium is six times lighter than the air we breathe, what would your voice sound like if you breathed an inert gas that was six times heavier than air? Hmmm? This sounds like a topic for another experiment, but first a word from our safety officer… Do not try this experiment at home – just watch the trained professionals do it on the video!

So, you’re asking, “What inert gas is about six times heavier than the air we breathe and will not result in death if you breathe it?”  We’re glad you asked. Drum roll… the gas of choice is sulfur hexafluoride. Sulfur hexafluoride (abbreviated SF6) is not poisonous and can affect the sound of a person’s voice if it is inhaled in small quantities (again, don’t try this at home). When SF6 is inhaled, the pitch of a person’s voice decreases dramatically because the speed of sound in SF6 is considerably less than it is in air. Sound travels through air at about 340 m/s but through SF6 at only about 120 m/s. This is the opposite of what is heard when a person inhales helium (about 900 m/s).

Like helium, a person trained by an expert can breathe a small quantity of sulfur hexafluoride without any harm to demonstrate how sound travels through a dense gas. Of course, it is not advisable to inhale or release SF6 in any quantity without proper training and precautions. If you are a science teacher doing this demo, be sure to read the Additional Information section below.

Additional Info

We turned to Dr. Steve Harris who has a fair amount of experience using the gas in his practice. Dr. Harris has written about SF6 extensively and really understands the properties of this inert gas. Dr. Harris writes, “Should be safe to inhale SF6 in the way that science teachers do in their demo so long as you do not fill your lungs repeatedly over a short period of time. You must beware of hypoxia and fainting, though, as you will not get much warning because the human hypoxia sensors are not very good.”

Dr. Harris suggests that anyone doing this science demonstration get a tank of pure oxygen gas. “Breathe pure oxygen for ten breaths before you do the SF6 demo. That will give you lungs full of nearly 100% oxygen, which means a LOT of reserve against blackout if you go too high on the SF6 content. You probably would need at least five tidal breaths (500 mL) of SF6 to go into the danger zone on air, but much more if you start with oxygen at 100%. Figure your total lung capacity is 6000 mL, so each tidal breath is 1/12th of TLC, so 5 tidal breaths of inert gas gives you (11/12)^5 = 65% of former oxygen concentration. Starting with air, that’s down to 13%, which is near danger zone. But starting with 100% oxygen, you’re still at 65%. It takes 20 tidal breaths even to get to 17.5%, which is about what you breathe out already. Of course, you’re probably breathing more than tidal breaths when deliberately breathing a foreign gas (more like 1 L so the figure is (5/6)^n per n breaths. But it’s interesting to do these calcs and then test them with an oxygen sensor for your students.”


How is sulfur hexafluoride used in industry? SF6 is a gas that is used in electrical power equipment. It is colorless, odorless, non-flammable and chemically stable. This means that at room temperature it does not react with any other substance. Stability comes from the symmetrical arrangement of the six fluorine atoms around the central sulphur atom. This stability is just what makes the gas useful in electrical equipment. SF6 is a very good electrical insulator and can effectively extinguish arcs, which makes high and medium voltage apparatus filled with SF6 highly popular. SF6 is used as an insulating gas in substations, as an insulating and cooling medium in transformers, and as an insulating and arc quenching medium in switchgear for high and medium voltage applications. These are all closed systems which are extremely safe and unlikely to leak. In electrical power systems, high and medium voltage switchgear is required to cut off the power in case of a fault in order to protect people and equipment. When power is switched, an electric arc strikes between the circuit-breaker contacts. Breakers filled with SF6 are electrically insulating and effectively control arcing. Gas isolated substations are mainly found in urban areas where you want them to take up as little room as possible and often integrate them into buildings. These substations reduce the magnetic field and remove the electrical field completely. This is a real advantage for installers, maintenance personnel, and people who live in the vicinity of substations.

SF6 is also used in other ways. Mixed with argon, it can be used in insulated windows. SF6 is used in the metal industry, for example, when casting magnesium. Eye surgeons use SF6 as a cooling agent in operations. SF6 can also be used as a fire extinguishing agent because it is non-flammable and cooling. In electrical applications, SF6 is only used in sealed and safe systems which, under normal circumstances, do not leak gas.