Diving impacts many processes in the body. The common denominator for the affected systems is the gas the diver is breathing. One critical element of gas mixtures is partial pressure.
Partial Pressures and Dalton’s Law
In a gas mixture, the portion of the total pressure contributed by a single gas is called the “partial pressure” of that gas.
Dalton’s Law deals with partial pressures. It is relevant to a number of issues in diving, including decompression, gas toxicity, use of breathing mixtures other than air, and maximum operating depths for diving. This article focuses on gas toxicity.
Dalton’s Law states that:
The total pressure exerted by a mixture of gases is equal to the sum of the pressures that would be exerted by each of the gases if it alone were present and occupied by the total volume.
In other words, as pressure increases, partial pressure increases and vice versa.
Dalton's Law Formula
The mathematical formula describing Dalton's Law is:
Ptotal = Pp1 + Pp2 + Ppn, where .
Ptotal = total pressure of the gas
Pp1 = partial pressure of gas component 1
Pp2 = partial pressure of gas component 2
Ppn = partial pressure of other gas components
Air Mixtures and Partial Pressures
The air jammed into a diving cylinder—and which humans naturally breathe—is for all practical purposes a gas mixture comprised of 79 % nitrogen molecules (N2) and 21 % oxygen molecules (O2).
While these gas percentages define the ratios between the number of gas molecules in the mixture, it is the partial pressures of each that is responsible for the physiological effects of the individual gas.
Examples of Air Partial Pressures at Depth
Adding Dalton to Boyle’s Law and doing the appropriate calculations (not covered here), the following are examples of conditions and partial pressures encountered by divers at depth, and illustrate one of the reasons it is important to pay close attention to the Dive Tables:
- At the surface, or 0 feet seawater (fsw), the air in a diver’s lungs is under one atmosphere of pressure (1 ata). The partial pressures of N2 and O2 in atmospheres and breathed by the diver are, respectively, 0.79 ata and 0.21 ata.
- At a depth of 33 fsw, the weight of the water column is equal to another atmosphere, so now the pressure on the diver’s body is two atmospheres absolute (2 ata). The diver’s lungs are now compressed to one-half their surface volume. The partial pressures of N2 and O2 in atmospheres are, respectively, 1.58 ata and 0.42 ata.
- At a depth of 66 fsw, the pressure is 3 atmospheres absolute (3 ata), and the diver’s lungs are one-third their surface volume. The partial pressures of N2 and O2 in atmospheres and breathed by the diver are, respectively, 2.37 ata and 0.63 ata.
- At a depth of 99 fsw, the pressure is 4 atmospheres absolute (4 ata), and the lung volume of the diver is one-quarter their surface volume. The partial pressures of N2 and O2 in atmospheres and breathed by the diver are, respectively, 3.16 ata and 0.84 ata.
- At a depth of 132 fsw, or two feet past the depth the Navy Dive Tables allow any time for non-decompression diving, the pressure is 5 atmospheres absolute (5 ata), and the diver’s lung volume is one-fifth their surface volume. The partial pressures of N2 and O2 in atmospheres and breathed by the diver are, respectively, 3.95 ata and 1.05 ata.
Oxygen Toxicity
Remember--it is the partial pressures of each that is responsible for the physiological effects of the individual gas. As the examples above illustrate, the deeper a diver goes, the higher the partial pressures exhibited by the component gases in their breathing gas.
Breathing oxygen is vital to sustain life, but under the right conditions oxygen becomes toxic. Diving too deeply while breathing air offers those conditions. The max O2 partial pressure a human nervous system can stand is 1.6 ata. Should a diver breathing air lose track of their depth and pass below 200 feet seawater, (decompression concerns aside!) they would be nearing a death zone—for at 218 fsw, their PO2 would exceed 1.6 ata. This oxygen concentration would essentially short circuit the nervous system. They would convulse, most likely lose their regulator, and drown.
Dive Safely
Diving is a fantastic sport and past time, but it is not without risks. It is important that divers learn the gas laws governing diving physiology. Doing so helps strengthen understanding as to why the dive tables impose the limits they do. As long as a diver minds their tables or dive computer, from an oxygen toxicity standpoint they should be fine. They are one step closer to diving safely.
References
Joiner, James T (editor). 2001. Physics of Diving. In NOAA Diving Manual: Diving for Science and Technology, 4th Edition. National Oceanic and Atmospheric Administration. Best Publishing Company, Flagstaff Arizona, pp. 2.1-2.18.
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