A Diver's Guide To Nitrogen Species


Larry "Harris" Taylor, Ph.D.


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This article is a brief review of the nitrogen oxides (and a few other nitrogen-containing species). The author will be the first to admit that most of the information contained in this essay is not necessary for comfortable in-water diving. However, since the author has seen a number of chemically incorrect references in the recreational diving literature, this material provides a diver-accessible reference to more information than most will want to know about  nitrogen compounds.




Nitrogen is a chemical element (atomic weight 14) found in nature as the primary component (~78 % by volume) of the atmosphere. The nitrogen molecule is composed of two nitrogen atoms (N2, Molecular weight 28) connected by an extremely strong triple bond. This triple bond makes the nitrogen molecule very stable. Since animals cannot break the N=N triple bond, nitrogen gas cannot, even under pressures associated with recreational diving, be incorporated into normal metabolism. The inability of animals to incorporate or react with molecular nitrogen from the atmosphere into nitrogen-containing biologically important molecules is the reason that nitrogen gas is considered to be physiologically inert.


Since nitrogen gas cannot be utilized in the body, exposure to increased partial pressures of nitrogen (descent in the water column) increases the tissue concentrations of dissolved nitrogen gas (discussed under Henry's Law in the Gas Law Primer). In general, the greater the exposure (increased depth (pressure) and time), the more nitrogen loading occurs in the body tissues. Upon ascent, the decrease in ambient pressure results in release of the excess nitrogen gas (in the same manner as soda pop fizzing). This bubbling process occurs on every ascent. Decompression sickness occurs when the amount of off-gassing exceeds the body's ability to remove the bubbling gas.


There are a variety of dive tables and computer models that are used to predict minimum risk ascents based on time/depth profiles. I have compared some common dive table profiles here. In general, divers should remember that they bubble on every dive and plan their diving to minimize bubble formation.


One major property of nitrogen is that is does not support combustion. This is desirable because the other major component of our atmosphere is oxygen, which dramatically boosts combustion. In an all-oxygen atmosphere, lighting one match could potentially incinerate the planet, so on a global scale, the large percentage presence of flame-suppressant nitrogen is a good thing.


Nitrogen gas is commercially prepared, like oxygen, from the fractional distillation of liquid air.




Simply put, Nitrox is any binary mixture of nitrogen and oxygen. The air you are currently breathing is Nitrox-21. (The number refers to the percentage of oxygen in the mix.) The Nitrox mixes most often encountered are NOAA Nitrox I (32% O2 or nitrox-32) and NOAA Nitrox II (36% O2 or nitrox-36). Collectively, nitrogen-oxygen binary mixes are known as "enriched air nitrox" (EANx, where x is the percentage of oxygen in the breathing mix) or "oxygen enriched air" to emphasize that the O2 concentration in the mixture is higher than air. One Nitrox training agency (ANDI) uses the marketing trademark of "safe air." The scientific community prefers the term "oxygen-enriched air."


Since oxygen enriched air has less inert nitrogen than air, it is a good choice as a breathing mix for repetitive, shallow water dives since using this mix reduces the nitrogen decompression obligation (compared to air). It is best utilized in depths between the 60-100 fsw. Since lowered nitrogen means more oxygen, the use of oxygen-enriched air requires divers to more closely monitor oxygen exposure to reduce risk of oxygen toxicity hits.


I have summarized the history of oxygen-enriched breathing mixes here.


In addition, I have a brief Nitrox FAQ here. Divers using oxygen-enriched air should take specialty classes to learn the procedures developed for this breathing mix.


Oxygen enriched air is manufactured by either mixing the two components or using devices (like the DNA stick) that enhance the oxygen concentration of air.


Nitrogen Oxides


Any binary nitrogen and oxygen compound can be called a nitrogen oxide. However, because of the chemical nature of elemental nitrogen and oxygen, several different combinations or molecular species are possible. Each different nitrogen oxide has a unique name and set of chemical properties. The chemical reaction of nitrogen with oxygen is termed an oxidation or an oxidative process. 


Nitrous Oxide (dinitrogen monoxide, N2O)


British chemist Joseph Priestley first isolated nitrous oxide in 1793.  N2O is a colorless, pleasant smelling, and slightly sweet tasting gas that has been used in anesthesiology since 1844. It is commonly called "laughing gas" because of its tendency to induce a euphoric state. The easily induced euphoric state has led to this gas being used as a recreational drug. It is considered an abused substance. Unfortunately, use of this drug may reduce oxygen levels in the body below that necessary to sustain life. Besides death, less severe side effects from use of this drug may include permanent neurological damage.


The euphoria associated with nitrous oxide use is often cited as an equivalent for the "diver's high" associated with nitrogen narcosis in warm water with good visibility. (Those in cold water with limited visibility often feel a sense of dread, of being stalked. or extreme loss of self-confidence).) Although the exact mechanism, most likely a complex mixture of physiological events, is still not totally understood, the classical explanation for the narcosis phenomena is the Meyer-Overton hypothesis that anesthetic gases work by dissolving in the fatty layers of nerve tissue which disrupts nerve conduction. For divers, this hypothesis is a reasonable way to conceptualize the narcosis process since greater depth would mean a greater pN2 that would drive more nitrogen into tissues. This is consistent with increased time/depth exposure resulting in greater diver impairment.


N2O is also a propellant in whipping cream aerosols. Nitrous oxide does not react with most foods and, unlike freon gasses, does not damage the ozone later.


Some automobile racers will use nitrous oxide to boost compression in their racing engines. Under cylinder pressure ignition, nitrous oxide decomposes


2 N2O ===> 2 N2 + O2


This reaction increases the gas volume inside the engine cylinder, which boosts compression. In addition, the enhanced oxygen concentration improves fuel burning with an overall increase in total engine performance. So, introducing nitrous oxide into an automobile engine is associated with rapid acceleration.


N2O is manufactured commercially by burning ammonia (NH3) in an oxygen-rich environment:


 4 NH3  + 5 O2  ===> 4 NO + 6 H2O


Nitric Oxide (nitrogen monoxide, NO)


Nitric oxide, a colorless, toxic, non-flammable gas, is the most stable nitrogen oxide. This is chemically interesting since the molecule is paramagnetic (has an unpaired electron). This unshared electron makes the NO molecule highly reactive.


In animals, nitric oxide is a neurotransmitter implicated in mediating a number of physiological processes including relaxation of smooth muscles, inhibition of platelet aggregation, increasing kidney filtration and urine production, penile erection (Viagra works by generation of NO), and inhibition of inflammation. Since it is a gas, it rapidly diffuses through the cell to its site of biological activity. But, since it is highly reactive, it is rapidly inactivated.


NO is generated in the body by the reaction of an enzyme Nitric Oxide Synthetase (NOS) in the reaction


Arginine + O2 ===>  Citrullene + NO



There are two importance items to notice about this reaction:

1. Oxygen, not nitrogen is the gaseous component of the reaction

2. An increase in pO2 will drive the reaction to the right (Le Chatelier's Principle) and produce more NO.


This suggests that the sharp threshold of oxygen toxicity at depth may be related to NO (a very reactive oxidizing agent). In other words, increasing pO2 will produce an increasing excess of an excitatory neurotransmitter. The body has a variety of defense mechanisms to remove or inactivate highly reactive oxidizers (like NO), but eventually, too much NO would overwhelm protective mechanisms. So, a simplistic way of conceptualizing an oxygen toxicity hit, is to view CNS oxygen toxicity as overwhelming the bodies defenses with too much oxidative, excitatory neurotransmitters.


Finally, some other NO factoids:


Nitroglycerin used in reducing angina attack works by generating NO which leads to pain-relieving heart muscle vasodilatation.


Nitric oxide is implicated in the initiation of firefly light emission.


Nitric oxide has been used in the treatment of surgical pulmonary embolisms. Its vasodilatation ability reduces blood pressure in the lungs.


Nitric oxide is a common component of automobile pollution. It is a primary component in photochemical smog.


Because of its chemical properties, the molecule often dimerizes to form N2O2.


Nitrogen Dioxide (NO2)


Nitrogen dioxide (NO2) a brown paramagnetic gas having an unshared electron exists primarily in a dynamic equilibrium with the colorless diamagnetc (no unshared electron) dinitrogen tetroxide (N2O4).  Nitrogen dioxide is used in commercial organic synthesis as an oxidizer. a nitrating agent, increasing wet-strength of paper, bleaching flour, a rocket propellant and liquid explosive. 


Nitrogen dioxide readily forms a white solid dimer (N2O4, dinitrogen tetroxide, sometimes called nitrogen tetroxide). The dimer, an excellent oxidizer, is often used in "hypergolic" (self-igniting) mixtures. These self-igniting fuels require no external ignition systems and, as such, form extremely reliable rocket fuel mixtures; the combination of monomethylhydrazine as fuel and dinitrogen tetroxide as oxidizer was used by the American Lunar Lander to supply the absolutely reliable lift-off required for the journey from the moon's surface to the waiting Apollo spacecraft in Lunar orbit.


The Anhydrides


Compounds that react with water to form acids are called acid anhydrides. There are two commercially important nitrogen anhydrides:


Dinitrogen Trioxide (N2O3)


Dinitrogen trioxide exists only in the cooled state (less than -21 oC (-6 oF)). When heated, it decomposes to a mixture of NO and NO2. Nitrogen dioxide is prepared commercially by oxidizing NO with air. When reacted with water, it forms nitrous acid (HNO2).  


Dinitrogen Pentoxide (N2O5)


Dinitrogen pentoxide is a white solid. However, it is unstable at room temperature and above; it decomposes to N2O4 and O2.  Dinitrogen pentoxide is formed by the dehydration of nitric acid by phosphorus(V) oxide. When reacted with water, it forms nitric acid (HNO3).




The triple bond of nitrogen requires a great deal of energy to break in order for nitrogen to be incorporated into biologically important molecules like amino acids, proteins, vitamins, RNA, and DNA. While higher animals typically cannot incorporate nitrogen into biologically important molecules, microorganisms (particularly those found in the root nodules of legumes) are capable of converting nitrogen found in the atmosphere into ammonia (NH3). Once the nitrogen is reduced to ammonia, the nitrogen is available for biological synthesis processes to manufacture building blocks like amino acids for use in assembling large biologically important macromolecules.


For simplicity, the process of fixation can be thought of as


N2 + 6 H ===> 2 NH3


(Nitrogen plus hydrogen from a biological hydrogen source, not gaseous hydrogen, in the presence of microbes gives ammonia.)


Fertilizers often contain ammonia or ammonium compounds (containing the NH4+ ion)  because, like the microbes found in legume root nodules, they provide the nitrogen necessary for growing plants.


The bacterial process of converting nitrogen into ammonia is termed "nitrogen fixation." This is chemically a reduction process (adding electrons or hydrogen atoms) and not an oxidation (losing electrons or adding oxygen). Thus, it is technically inappropriate to refer to any binary reaction of nitrogen with oxygen as a fixation.



 Nitrate (NO3-)


Nitrate (NO3-) is an anion (negatively charged ion). Nitrates are used as oxidizers (provide non-atmospheric sources of oxygen) in explosives. Since microbes can convert nitrate to ammonia, nitrates are also extensively used in fertilizers. Most nitrates are water soluble. Soil nitrates from fertilizers can accumulate in bodies of fresh water where they promote algae blooms.





Nitrogen species of importance to divers are summarized below:


Nitrogen Species

Significance To Divers



Nitrogen (N2)


Physiologically inert; excess leads to decompression maladies


Nitrox (Oxygen Enriched Air)


Breathing Mix for shallow diving and/or decompression


Nitrous Oxide (N2O)


Mimics warm water narcosis euphoria


Nitric Oxide (NO)

Neurotransmitter; implicated in oxygen CNS toxicity

Nitrate (NO3-


Fertilizer, water pollutant that promotes algae blooms





While the chemical nature of all the nitrogen compounds are of little importance to divers, those divers, especially instructors, who chose to discuss nitrogen oxides should use the proper chemical terminology.


General Chemical References


Braker, W. & Mossman, A. The Matheson Gas Data Book, Matheson, Milwaukee, WI. 1971.


Cotton, F.A. & G. Wilkinson, Advanced Inorganic Chemistry (3 rd Edition), John Wiley, NY. 1972; (Chapter 12: Nitrogen, p. 339-366.)


Eastwood, D.W. Clinical Anesthesia: Nitrous Oxide, F. A. Davis, Philadelphia, PA. 1964.


Research Article


T. Thippeswamy,  J.S. McKay, J.P. Quinn and R. Morris, Nitric oxide, a biological double-faced janus- Is this good or bad? Histol Histopathol (2006) 21: 445-458.





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About The Author:


Larry "Harris" Taylor, Ph.D. is a biochemist and Diving Safety Coordinator at the University of Michigan. He has authored more than 100 scuba related articles. His personal dive library (See Alert Diver, Mar/Apr, 1997, p. 54) is considered one of the best recreational sources of information In North America.

  Copyright 2001-2020 by Larry "Harris" Taylor

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