Gas Density – A Divers Dilemma

By John Hauxwell, Red Sea Tribe

As divers we manage – through learned behaviour to adapt to the effects of depth without fully understanding the mechanisms behind them. Gas density is a commonly misunderstood dynamic lurking somewhere deep in the background of dive theory. We minimize our work at depth, improve our fitness, add helium to reduce narcosis and work of breathing (WOB), but these are all reactionary responses to what is, typically,  an unknown set of conditions. While our bodies usually can’t discern the difference during a dive between gas density and the accompanying increase in WOB that causes decreased respiratory capacity, or increased CO2 production, and our reduced capability to eliminate CO2. 

CO2 is eliminated from the body by respiring, and the more we breathe the more CO2 we remove. This process of elimination is usually precisely controlled by the  brain – actually an area known as the medulla oblongata –  to keep CO2 in the body at a stable level. If CO2 levels rise, the brain will increase breathing rate and depth to bring CO2 levels back to within normal tolerance. This is a completely automatic function which takes place whether we are aware of it or not.

However, this normal process of CO2 control can be disrupted during a dive. The work of breathing (WOB) increases because we are respiring a denser gas. In some people, when the work of breathing increases the brain seems less sensitive to the rising level of CO2 and it may fail to keep those levels under control. Thus, divers are prone to having body CO2 levels rise, particularly when they overexert themselves, and when the work of breathing (WPB) is high. We refer to this as ‘CO2 retention’.

High levels of retained CO2 can produce symptoms such as headache, shortness of breath and anxiety. At very high levels these symptoms can include panic and may lead to drowning. In addition, high CO2 levels increase narcosis and are a significant risk factor for oxygen toxicity, which is a concern for technical divers. 

One obvious question that arises is this: If increased gas density is one of, if not the main reason(s) why work of breathing increases as we go deeper, and, if this in turn predisposes us to CO2 retention, then what is an acceptable gas density and how do we factor that into our diving planning?

Some agencies (e.g. GUE) have been proactive in modifying standard gases and tailoring training to adapt to these concerns, some however do not, yet hyperbaric research is constantly pushing us towards increasingly conservative gas choices.

So, to quickly summarise WOB and Gas Density (GD).  Work Of Breathing  (WOB) is an integral of pressure as a function of volume. A high WOB means it takes more effort (measured typically in joules) to draw breath (a measure of volume, typically in litres). High WOB results in increased CO2 production, and that CO2 increase can result in hypercapnia, narcosis, and loss of consciousness as well as other severe symptoms. Gas Density (GD) is a measure of mass per unit volume, measured in grams per litre (g/l). A high gas density means a given volume of gas weighs more and takes more effort to move, resulting in increased WOB. Increased gas density also skews the pressure gradient between inspired and arterial CO2, resulting in further decreased CO2 off gassing efficiency (CO2 retention) that results in further complications.

There have been a number of articles recently (as well as questions on the usual message boards and forums) about how gas density and work of breathing should be factored into our gas choices. Much of this discussion has been around the recent research and empirical data by Gavin Anthony and Simon J. Mitchell. Anthony and Mitchell’s work shows that gas density near the 6 g/l mark it significantly increases the risk of dangerous CO2 retention during dives. Within their tests, subjects failed more than half their attempted dives and experienced issues at more than three times the rate of divers using marginally less dense gases (<1g/l). 

Their headline takeaway was that an ideal maximum gas density of 5.2 g/l (equivalent to air at 31m)  should be used when planning dives with a “hard limit” of 6.2 g/l (equivalent to air at 39m). A sensible conclusion would be that we should plan all our gases so that the density lives between these boundaries.

The implications of these results are far-reaching. Recreational and technical divers alike face issues with gas density under these suggested parameters. The use of EANX 32 (32% oxygen, 68% nitrogen) at 34m (MOD, assuming ppO2 of 1.4) exceeds the ideal  recommendation with a gas density of 5.66 g/l but is easily within tolerance of the upper limit. 

Technical divers using trimix 18/35 (18% oxygen, 35% helium, balance nitrogen) will overshoot recommendations, reaching 6.93 g/l at 61m and a ppO2 of just 1.26, and trimix 10/70 (10% oxygen, 70% helium, balance nitrogen) reaches 6.73 g/l at around 120m and 10.29 g/l at 150m, far outside the recommended hard maximum for gas density but still within tolerances for traditional dive planning.

The reality is that gas density is another in a series of dynamic risk factors that divers of all levels must contend with and we should look to treat gas density and WOB similarly to how we plan against DCS.  We should acknowledge and mitigate these hazards through correct training, personal fitness, decreased work at depth, and appropriate dive planning. These factors have been shown to work well historically and they will continue to do so. We should take these factors into consideration when we plan gas choices for our dives and look to managing our “acceptable risk” against knowledge, experience, fitness and training. 

For more information on dive theory please contact us and we will be glad to help you

Sources https://www.omao.noaa.gov/sites/default/files/documents/Rebreathers%20and%20Scientific%20Diving%20Proceedings%202016.pdf