Lesson 1, Topic 1
In Progress

Air Composition and Nitrogen Effect

Any gas in contact with a liquid tends to dissolve in it, up to a certain limit. When this limit is reached it is said that the liquid is saturated, that is, it cannot dissolve more gas. This dissolution limit is related to the pressure to which the liquid and gas are subjected.

Thus, in the respiratory act, the components of the air we breathe will dissolve in the blood and other organic tissues, until they reach a maximum value, according to the pressure to which they are subjected.

The air we breathe is a mixture of gases composed of:

  • 79% nitrogen (N2);
  • 20.9% Oxygen (O2);
  • 0.03% Carbon Dioxide (CO2);
  • 0.07% of water vapor and rare gases.

Taking into account the characteristics of the dives allowed to the diver CMAS One Star Diver, which can only evolve to the depth of 20m, and assuming that the respiratory technique of the practitioner is satisfactory, we are only interested in analyzing the behavior of the nitrogen.


Nitrogen is an inert gas that dissolves in the tissues of the body, although it has no respiratory utility. The volume of dissolved nitrogen depends on the depth reached and the duration of the dive: the greater the depth (pressure) and the dive time, the greater the amount of dissolved gas.

All this nitrogen that dissolves in the body during diving is released when the return to the surface begins (decreased pressure). This release is made through respiratory exchanges.

This is not to say that when reaching the surface the diver has already eliminated all the nitrogen dissolved during the dive. In fact, upon reaching the surface the organism continues with a certain amount of nitrogen, which will break free after the dive is finished.

Therefore, our body can support a certain additional amount of dissolved nitrogen, in addition to what it normally contains when it is at sea level (about 1 litre).

In order for the diver to reach the surface with the nitrogen dissolved within the limits that the organism can withstand, limits of depth and dive time are imposed, which are related in the form of tables (Module 13 – Tables and Computers) .

Failure to comply with these limits can cause serious problems for the diver. In fact, if the amount of dissolved nitrogen exceeds the capacity supported by the body, nitrogen bubbles are released into the tissues during the ascent. It’s a phenomenon similar to what happens when you take the cork off a bottle of sparkling wine, which has gas dissolved under pressure.

These blisters cause a physiological change of greater or lesser severity, called “decompression sickness” or “decompression accident”.


  • Tiredness;
  • Dehydration;
  • Cold;
  • Violent physical efforts during and after diving;
  • Poor physical form;
  • Emotional factors;
  • Age;
  • Obesity;
  • Alcohol consumption;
  • Drug use;
  • Successive dives;
  • Dive profile.

They are all factors that may also predispose to the decompression accident, even when the limits imposed on the depth and time of the dive are respected.

The sudden decrease in pressure following a dive, for example on a plane trip or on a road trip, can also cause a decompression accident, even if the imposed limits of depth and time have been respected.