Although we breathe
nitrogen every day, many people are not aware that it can be potentially very dangerous.
Each year, approximately eight to ten people are killed in North America due to nitrogen asphyxiation. Almost all of these are as a result of accidents in industrial settings, where nitrogen is commonly used for purging, blanketing, and cooling.
How it happens
Contrary to our intuition, we are not motivated to breathe by a lack of oxygen, but rather by a build-up of carbon dioxide in our bloodstream.
When we eat, carbon containing compounds in our food are broken down by the body to recover the energy in the chemical bonds. A by-product of this metabolization is carbon dioxide, and the body has to get rid of this by expelling it through our lungs.
When carbon dioxide enters the bloodstream, it forms traces of
carbonic acid. This causes a drop in pH which is sensed by chemical receptors in the arteries, and they send a signal to the brain to breathe.
When we hold our breath, the carbonic acid level builds up, and the signals to exhale become stronger and stronger.
The system works great, as long as we continue to breathe in oxygen.
Asphyxiation
In situations where there is too much nitrogen (or other asphyxiant gases such as helium or argon) our bodies are not getting any oxygen so they don’t produce any carbon dioxide, and there is no signal to breathe. (Low oxygen levels are also a trigger to breathe – but they are a much weaker signal than high carbon dioxide levels in the blood.)
It only takes a few breaths of an inert gas before all the carbon dioxide is expelled, and the body decides it doesn’t have to breathe anymore.
This is why nitrogen asphyxiation is so deadly. It happens quickly and people don’t realize they are in distress until it’s too late.
Cases
“I tried to breathe, but I couldn’t – it felt really awkward, like trying to take another breathe when you have just filled your lungs with air.”
This is how the feeling was described by the host of an Australian TV show who almost lost consciousness after breathing in pure helium for a science demonstration. Luckily, he began breathing again on his own.
Less lucky were two NASA workers in 1981, shortly before the launch of the first space shuttle mission. They lost consciousness and one of them died after they entered the Orbiter compartment which was pressurized with pure nitrogen as a precaution against fire.
In 1999, a Scottish lab assistant died, apparently from asphyxiation, after liquid nitrogen spilled in a basement storage room.
This is also one reason for specialized fittings on nitrogen
hoses, to prevent accidentally hooking up nitrogen hoses to pneumatic tools, which would then vent pure nitrogen into the workspace.
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