Flying and Diving – A Sojourn Into Physics and Physiology

Departing a little from complimentary therapy applications today, we discuss diving and flying. Since hyperbaric technologies are in fact diving technologies, and diving accidents and illnesses are treated in hyperbaric chambers, it makes for an interesting sojourn.

Remembering of course that in therapeutic treatments (diving or otherwise), we breathe pure oxygen and accordingly incur no decompression penalty. This discussion relates to actual diving under the water when breathing air or some other mix containing an inert gas such as nitrogen. A brief departure into some of the physics and physiology involved in hyperbarics and diving alike.

Most divers know all to well that flying after diving is a bad idea. Thanks to Henry’s Law, one of the gas laws we use in diving and diving technologies (hyperbaric technologies), we can demonstrate why this is.

Henry’s Law States:

At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

To understand this, we must also look at Dalton’s Law of Partial Pressure or simply, Dalton’s Law.

Dalton’s Law States:

In a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.

In simpler terms this means that in air for example, 78 % of the ‘mix’ is nitrogen and approximately 21% is oxygen with the other 1% made up of trace gasses. At sea level, which is 1 atmosphere of pressure or 1 bar depending on your preferred unit of measurement, the pressure exerted by the nitrogen is 0.78 of that one atmosphere, and the oxygen makes up it’s relative proportional 0.21 of that atmosphere. This is expressed as 0,78 ata (atmospheres absolute) of nitrogen, and 0.21 ata of oxygen.

Strictly speaking, and to be precise, 1 atmosphere = 1.01325 bar. In diving we consider them equal though as this very small difference is inconsequential for our purposes.

Coming back to Henry’s Law, we can now appreciate that the nitrogen portion of the air mix will dissolve into the tissues and drawing on previous writings, the oxygen component will be used up in the metabolic process. This presents an inherent risk for divers, however with proper training and procedures, risk is mitigated. Clearly the higher the concentration of oxygen the lower the inert gas absorption. In cases of pure oxygen, as is the case in complimentary and medical therapies, there is no inert gas and therefore no such consideration.

Building on this, it follows that the higher the pressure as is the case when going deeper under the water, the greater the amount of nitrogen will dissolve into blood and tissues. This is the reason that the deeper a diver dives, the less time they will be able to stay at the bottom. ‘Bottom time’ is reduced the deeper you go if you wish to remain in the ‘no-decompression’ zone. This is the time allowed on the bottom without the need to conduct decompression stops on the way back to the surface.

Ascending from a dive reduces the pressure and in the reverse of the above process, nitrogen begins to come out of its dissolved solution in the blood and tissue. In order to avoid this process causing the formation of bubbles, divers must ascend slowly within prescribed ascent rates, and conduct any necessary decompression stops they may have incurred if they have stayed on the bottom longer than the no decompression limits allow. These limits are published in diving tables. Ascent itself is also considered decompression, since as the pressure is relieved, the diver is decompressed.

Both ascent and decompression stops are considered decompression, with ‘stops’ being periods of time that the ascent is halted at pre-determined depths for pre-determined times tabulated in dive tables. Strictly speaking, every dive is therefore a decompression dive, but dives without pre-determined decompression stops are considered ‘no decompression’ dives, referring to the absence of the requirement to make a specific stop.

This brings us back to flying. When we ascend to altitude, the pressure is reduced. (More on altitude in the overview). It follows that a further reduction in pressure following a dive could result in aggravating the formation of bubbles in the tissues resulting in decompression sickness (the bends). It takes time for the body to completely offload the nitrogen which has dissolved into the tissues during a dive (residual nitrogen). Ordinarily the rule of thumb is that it takes up to 24 hours to completely equalise again in terms of dissolved gasses on the blood. The body will ‘off-gas’ the additional nitrogen until reaching equilibrium with the atmosphere once again.

Accordingly, there are flying after diving guidelines as published by every agency involved in any kind of diving. It matters little to chamber staff whether a diver is commercial, recreational, technical, military or anything else. There is no diving bravado when the chips are down. (For the divers among us… we all bend the same). The advice below is in accordance with most commercial and recreational training standards.

These guidelines apply to air dives followed by flights at cabin altitudes of 2,000 to 8,000 feet (610 to 2,438 meters) for divers who do not have symptoms of decompression sickness (DCS). As we know cabins are pressurised to lower altitudes than the plane actually flies. These altitude or cabin altitudes represent the altitudes likely to be experienced in commercial aviation. It then goes without saying that flying in unpressurised cabins is an even worse idea.

Single no-decompression dive in a day: A minimum pre-flight surface interval of 12 hours is recommended.
More than one dive per day or multiple days of diving: A minimum pre-flight surface interval of 18 hours is recommended.
For dives requiring decompression stops, it is usually a requirement in commercial, technical and recreational diving to exercise a 24-hour minimum pre-flight surface interval.
In cases where saturation techniques or extreme exposure techniques have been used, this can be extended to multiple days or even as much as a week following final decompression.

To err on the side of safety, many divers plan a 24-hour surface interval and spend their time engaging in other activities and visiting other attractions.

Altitudes below 2000 feet have proven safe for the author who has engaged in flying out of necessity following diving. The pilot was asked to stay below a ceiling of 2000 feet crossing the Zanzibar Channel. While we don’t advocate this, it was one of the best fights ever. Wales and fisherman were in full and clear view and it felt like we would touch the wave crests.

In addition to flying after diving, there are also further activities to avoid following diving of any kind.

Activities to Avoid Following Diving

Driving to altitude. There a multitude of reasons to do this, either sightseeing or further activity such as hiking and such. Some divers live at altitude and dive at sea level. This can be just as problematic as flying. It is noted though that when flying, one reaches altitude far quicker than driving and this creates a very steep difference in nitrogen pressures in tissue and the surrounding atmosphere. This is called a gradient. If the outward gradient for nitrogen becomes too steep, decompression sickness usually follows, and one quickly becomes a patient or client of a hyperbaric facility. In this instance, the chamber takes on the identity of “re-compression chamber” since we need to re-compress the individual to reduce the bubble size and have it re absorbed into tissue reducing the damage it can cause. There is little evidence on driving to altitude, however if it can be avoided, it should be.

The same rules apply to diving at altitude. Some of the world’s best dive sites are already at altitude. This essentially means that an individual immediately arrives at altitude upon surfacing. There are tables in every agency, for use in safe altitude diving. The tables are more conservative than standard sea level tables and account for the increased outward gradient following a dive. They usually use a concept called equivalent depth. This means the diver, while diving at a given depth, will use a table for a greater depth which is decided on depending on the altitude. Specialised altitude tables will also include more conservative limits and decompression requirements.

Excessive exercise can lead to increased blood flow to muscles and tissues and can result in bubble formation. Stay clear of the gym or the track for the same intervals associated with flying. This can also be said of any activity that would increase blood flow. Certain types of massage can be associated with this however there is little actual evidence that it raises the risk. No harm in playing it safe though.

Hot water should be avoided. This includes a soaking hot bath or a visit to the Jacuzzi or shower. It is very common for divers to develop decompression sickness following even dives in relatively warm water when jumping straight in the shower or bath. The heat increases blood flow and reduces load carrying capabilities in the tissues. It also follows that the lungs don’t get the chance to offload the nitrogen coming out of solution quickly enough. Usually the DCS will present in the skin as a rash. This is not say don’t shower or bath, just don’t make it too hot. Rather warm up with a cup of hearty soup if it’s a cold one. The solubility of gas is inversely related to temperature, tissues will also hold less in solution as they warm. This coupled with the inability to handle the load the circulation can develop bubbles.

Excessive alcohol consumption can also be problematic. This is a tough one for divers sometimes but should be observed as alcohol will not only dilate blood vessels, it will thin the blood and dehydrate an individual. All risk factors for decompression sickness. The load carrying capability of tissue is also affected and this can lead to bubble formation.

Free-diving should also be avoided along the same lines as flying following SCUBA diving. It’s not a grand idea to free dive following a SCUBA dive. The yo-yo effect can stimulate bubble formation in tissues in the same way it does in diving on compressed air. There is little data on flying following free diving, however DCS is not unheard of. During a free dive the same physics apply, and it is conceivable that after multiple breathe hold dives, the nitrogen in the lungs will dissolve into the blood just as it does in SCUBA diving. The air in the lungs becomes compressed as a free diver descends, and, although in much lesser amounts, will dissolve into tissues. Most of us however, don’t stay down long enough for this to become problematic however there have been as many as 90 recorded cases of DCS following free dives. (US National Institute of Health). World record holder Herbert Nitsch suffered DCS and was nearly paralysed for life. So, if you’re a top-level free diver it would certainly be a good idea to wait until flying as well. While shallow snorkelling is OK, don’t engage in open water free-diving on the same day following a SCUBA dive