“I love to breath. Oxygen is sexy.”  — Kris Carr

How long can you hold your breath? One minute? Two minutes? Pearl divers have been known to hold their breaths for over 10 minutes. When we’re breathing, though, it’s at a rate of between 8 and 20 breaths per minutes. So, many find it difficult to believe that just two breathes of an oxygen-free atmosphere, such as the nitrogen atmosphere that we use to inert our storage tanks and reactors, is enough to cause unconsciousness and lead to death.

When We Breathe

When we breathe in, the normal composition of the atmosphere is 78% nitrogen, 21% oxygen, 0.04% carbon dioxide, and 1% other gases (mostly argon), and our lungs inflate to about six liters. When we breathe out, the typical composition is 78% nitrogen, 17% oxygen, 4% carbon dioxide, and 1% other gases (still mostly argon), and our lungs deflate to about three liters. These compositions are of dry gas; they don’t take water into account. The water content of atmosphere we breath in depends on the relative humidity and temperature of the air. The composition of the mix we breath out, however, is constant: 100% relative humidity (completely saturated) at 98.6 F (37 C), which is 6.3%.

The composition of the three liters of gas that remain in our lungs after we breathe out is the same as the composition of the gas we breathe out. When we breathe in, the air mixes with the gas that remains in our lungs. That means that our lungs never contain pure air. Instead, the composition fluctuates between 17% oxygen/4% carbon dioxide after we breathe out and 19% oxygen/2% carbon dioxide after we breathe in. Given that one molecule of carbon dioxide (CO2) is produced from one molecule of oxygen (O2), it should make sense that the decrease in oxygen concentration is matched by an equal increase in carbon dioxide concentration. Because respiration doesn’t use nitrogen, argon, or any of the other gases in air, it should also make sense that the concentration of those gases remain constant throughout.

When we hold our breath, our body continues to extract the oxygen in our lungs and replace it with carbon dioxide. If it were possible, we could hold our breathe until the composition shifts all the way to 0% oxygen/21% carbon dioxide. But we can’t. While our bodies don’t detect the drop in oxygen, they do respond to the increase in carbon dioxide. That pain you feel in your chest as you hold your breath longer and longer is a response to the increase in carbon dioxide.

Just Two Breaths

Now imagine that instead of breathing air or holding your breath, that you breath something besides air. For instance, nitrogen. On your last regular breath, the three liters of gas in our lungs is 78% nitrogen, 17% oxygen, 4% carbon dioxide, and 1% other gases. When we breathe in pure nitrogen to inflate our lungs to six liters, the composition is then 89% nitrogen, 8½% oxygen, 2% carbon dioxide, and ½% other gases. Notice how the carbon dioxide just drops back to where it is supposed to be after taking a breath, so our body will not notice a problem.

Our body then extracts oxygen from the lungs and replaces it with carbon dioxide. It won’t be quite as effective as when the oxygen concentration is higher, but let’s assume that it will be. So, just before we breath out, the composition in our lungs is 89% nitrogen, 6½% oxygen, 4% carbon dioxide, and ½% other gases.

Now, let’s take our second breath of pure nitrogen. The composition is now 94½% nitrogen, 3¼% oxygen, 2% carbon dioxide, and ¼% other gases. Carbon dioxide is still normal, so there is no warning, but the oxygen is so low that it assures loss of consciousness. This is in just two breaths; continued breathing of pure nitrogen will continue to drive the oxygen concentration down.


Loss of consciousness happens after two breaths. It’s not until after a minute without oxygen that brain cells begin to die. Survival is still possible, but an unconscious person cannot save themselves. After 3 minutes, serious brain damage is likely, and after 10 minutes without oxygen, so many brain cell will have died that the victim is unlikely to recover.

As a young engineer, my plant manager told me a story about another young engineer. Perhaps it was apocryphal, but it made a huge impression on me. The young engineer had a project to install a new carbon steel vessel. When it arrived at the plant, the fabricators had covered the entryways and other nozzles with plywood blanks and the whole thing was wrapped in plastic film. The engineer went out to inspect his new vessel. About fifteen minutes later, they found him slumped in the entryway, dead. At first, they suspected that the fabricator had inerted the tank with nitrogen, but that wasn’t the case. Eventually, they concluded that the natural rusting of the interior had consumed the oxygen, leaving a nitrogen rich atmosphere in the tank. Had someone been there, the engineer would have still passed out when he stuck his head in (how long does it take to take two breaths?), but they could have performed a non-entry rescue, and if necessary, performed artificial respiration.

Helium Balloons

As you might imagine, I’m not a fan of breathing helium from a balloon to make a squeaky cartoon voice. As a simple asphyxiant, helium is just as deadly as nitrogen. If you are going to do it, though, take only one breath before doing the voice. And breathe from a balloon, which is typically at a pressure of around 1 psig, rather than from the helium cylinder used to fill the balloon, which is at 2,000 psig, regulated down to 200 psig.

Inert, but Not Harmless

It is easy to forget just how quickly an otherwise harmless substance like nitrogen (or helium or argon or neon) can cause a death, just by displacing oxygen in the atmosphere we breathe. Just two breaths. If these “harmless” gases displace the air in a confined space, or even in a regular space that is small and poorly ventilated, that space can become a fatal hazard. If someone inadvertently connects their air-supplying respirator to a plant nitrogen system instead of the plant breathing air system, they’ll be down before they know what happened.

So, be aware of this hazard. If you use nitrogen as an inerting gas at your plant, treat every use point that can leak is a potential hazard. But there are also other causes for atmospheres with depleted oxygen level, oxidation being foremost among them. Confined space entry permits can address many of these hazards, but not all. Awareness is the best safeguard.

Remind yourself and your co-workers: Just Two Breaths.


  • Mike Schmidt

    With a career in the CPI that began in 1977 with Union Carbide, Mike was profoundly impacted by the 1984 tragedy in Bhopal and has been working on process safety ever since.