Subipedia
NITROGEN – THE INVISIBLE ENEMY
NITROGEN – THE INVISIBLE ENEMY
During my decades-long career as a scuba diving instructor, I have noticed that most divers know nitrogen is part of diving - just like assembling their scuba gear. Terms such as decompression stop, decompression times, no-decompression limits, and decompression dives are more or less familiar. However, far fewer divers truly understand what nitrogen does inside our body while diving, and why it can become dangerous. The better I know my enemy - and the better I understand what it does and why - it becomes much easier to protect myself. Because if I ignore it, things can become very dangerous.
That is reason enough to take a closer look at nitrogen and its role in diving.
Starting Point / Basic Principles
The air we breathe consists of 78% nitrogen (N₂), 21% oxygen (O₂), and 1% other gases such as argon and carbon dioxide. (Note: the % figures are rounded)
What many people don’t realize is that nitrogen and oxygen exist as molecules in the air we inhale. A molecule forms when two atoms bond together.
(Note: on the notation of a molecule: this is indicated by the subscript 2. This means that when nitrogen is written as N₂, it is a molecule of 2 nitrogen atoms)
This is important because nitrogen molecules have different properties than oxygen molecules. The oxygen molecule (O₂) is reactive. It is our essential energy supplier and supports combustion and oxidation processes by reacting chemically. The nitrogen molecule (N₂) is inert, meaning it is basically just there to fill space in the air. In short: oxygen helps things burn, nitrogen just watches.
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Gas Exchange
Both molecules - oxygen (O₂) and nitrogen (N₂) - enter the lungs when we inhale.
From there, the gases move from the lungs into the body. This process is based on two physical laws:
- Gases always move from higher pressure to lower pressure until equilibrium is reached (Fick’s Law of Diffusion).
- Gases dissolve in liquids under pressure (Henry’s Law).
Nitrogen, just like oxygen, is involved in gas exchange. The difference is that nitrogen is not needed or used by the body in any way. It is simply present - without purpose or effect. And now comes the big and crucial difference:
This is only true as long as we are breathing air at the surface and not breathing compressed air.
Gas Exchange While Diving
We know that nitrogen (N₂) makes up 78% of our breathing air and oxygen (O₂) makes up 21%. Together, that equals 100%. At the surface (sea level), the total pressure of the inhaled gas mixture is 1 bar. Therefore, nitrogen has a partial pressure of 0.78 bar. As soon as we descend, the surrounding pressure increases with every meter of depth. At 10 meters, we have twice the amount of nitrogen. At 20 meters, three times as much, and so on. At the maximum recommended recreational diving depth of 40 meters, it is five times as much. Because we breathe compressed air underwater, we inhale more gas molecules with every breath. This process is called nitrogen saturation.
Nitrogen N₂ Saturation
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We now know that nitrogen N₂ enters the bloodstream via the lungs and is distributed throughout the body. The deeper we dive, the more nitrogen our body absorbs. According to Henry's law, three factors are primarily responsible for nitrogen N₂ saturation:
- The pressure, i.e., for divers, the current diving depth.
- The time during which this pressure acts on the body.
- The type of fluid or tissue into which the nitrogen penetrates.
Complete nitrogen saturation is reached when the amount of nitrogen N₂ in the air breathed into the lungs is equal to the amount in the blood and affected tissues. The human body consists largely of water, which serves as a solvent for gases such as nitrogen. Under normal atmospheric conditions, i.e. on land, the amount of nitrogen N₂ dissolved in the body is low. When diving, the ambient pressure increases. This allows more nitrogen N₂ to enter the bloodstream and be transported to the tissues via the blood circulation. Because our bodies have different types of tissue, these are divided into groups - known as compartments in technical terms - for the purpose of calculating nitrogen saturation. This is because the saturation of tissues occurs at different rates. The group with rapid saturation, taking only 5 to 10 minutes, includes the blood and organs (kidneys, liver, brain). The group with a medium saturation time, from 30 to 120 minutes, includes the skin and muscles. The group with slow saturation, from 2 to 5 hours, includes the joints, fatty tissue, and bones. Saturation is therefore a dynamic process that depends on ambient pressure, diving depth, duration of stay at depth, and the individual properties of the tissue.
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Complete saturation is achieved when the tissue cell contains the same amount of nitrogen N₂ as the air we breathe, i.e., the lungs. This is referred to as the equilibrium of the pN2partial pressure between the tissue cells and the air in the lungs. Remember that saturation with nitrogen N₂ occurs without the diver feeling it. This is because nitrogen N₂ is inert and does not cause any chemical reactions in our body.
Nitrogen (N₂) Desaturation / Decompression
As soon as we begin to end the dive, i.e., leave the depth and slowly ascend, the ambient pressure also changes. As a result, the partial pressure of nitrogen N₂ in the tissue cells is higher than that in the air we breathe. Consequently, nitrogen N₂ migrates from the tissue cells via the bloodstream to the lungs and from there into the exhaled air. This process is called desaturation.
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It is subject to the same laws as saturation, only in reverse. With one crucial exception. We can dive down as quickly as possible. This is not the case when surfacing. This is because our bodies only allow a maximum permissible difference in nitrogen pN2partial pressure between the cells and the air without causing us harm. This is reached when the difference is no greater than twice the initial value.
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This also means that even if we end a dive while following all safety rules (ascent rate, safety stop, decompression stops), we still have more nitrogen in our body tissues than the air at the surface contains. Over the course of hours - without us noticing - it is slowly exhaled. Depending on the level of nitrogen saturation, this can take up to 24 hours. If we dive again too soon, for example after a 2-hour lunch break, we carry the remaining nitrogen with us. This changes the starting conditions compared to the first dive, and it must be considered.
Decompression Sickness
If we ascend too quickly, faster than the recommended 10 meters per minute, it becomes dangerous. This is because as soon as the pressure difference of nitrogen N₂ in the tissue cells is more than double the initial value, nitrogen gas bubbles form in the body. In the truest sense of the word, the nitrogen bubbles out. Just like when you shake and open a bottle of champagne. As shown above, our body consists of fast, medium, and slow-saturating tissue cells. As a result, nitrogen only bubbles out in the area of the body where the partial pressure difference exceeds the permissible value.
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Popping the cork is fun - but not inside the tissue cells of our body. Because nitrogen bubbles are the cause of a decompression accident. Depending on where they form, symptoms may include tingling sensations, skin discoloration or spots, joint pain, or - in severe cases - paralysis and life-threatening complications. The invisible enemy strikes with full force.
Inert Gas Narcosis/Nitrogen Narcosis/Depth Intoxication
The increased partial pressure of nitrogen is also responsible for this. However, this only occurs at a nitrogen pN2partial pressure of 3.12 bar. This corresponds to a diving depth of 30 meters. What exactly does the increased nitrogen pN2partial pressure do?
It acts like an anesthetic in the brain and disrupts signal transmission between nerve cells. Similar to the problem most of us are familiar with. We are talking on the phone with our smartphone and have a very good, interference-free connection. If this is impaired by external influences - an overloaded mobile network, a change in our location, a dead spot - this leads to interference and even the call being dropped.
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With nitrogen narcosis, the signs come on gradually and increase with increasing depth.
From 30 meters down, thinking and reaction times slow down, followed by reduced concentration, misjudgements, and perceptual disturbances. The air we breathe tastes strange, there are inexplicable noises similar to a passing train, and our focus is disturbed, sometimes sharp, sometimes blurred. From 40 meters down, it can become dangerous. Fear and panic or euphoria, similar to a state of intoxication. Hence the term "rapture of the deep". Divers of the older generation are familiar with the Martini rule.
It states: At depths of 30 m and below, the intoxication of depth feels like having drunk 2 to 3 martinis. At depths of 40 m and below, it feels like 4 martinis. Clearly too many martinis. As soon as you feel the first signs of nitrogen narcosis, signal to your diving partner to leave the current depth and ascend a few meters, preferably to around 20 meters. This reduces the pN2partial pressure and the effects of nitrogen narcosis subside abruptly. You are mentally alert and fit again.
This happens without any after-effects such as a "hangover" from alcohol intoxication after a night of heavy drinking.
Summary/Conclusion
Now that we know more about our invisible enemy, nitrogen, and understand the effects of nitrogen in our bodies, we can take this into account when planning and carrying out a dive, allowing us to continue to enjoy the underwater world safely and have unforgettable diving experiences.
Photos & sketches: Johann Vifian
Sources: Extensive technical literature