INSIGHT | H&S – COMPRESSED AIR: 30TH ANNIVERSARY WCA REGS
DECOMPRESSION, DON AND SATURATION
Below are briefings of Prof J S Haldane, a key figure in the history of human exposure to pressure and decompression; and, oxygen decompression, stage decompression, threshold for decompression, Dysbaric osteonecrosis (DON) and saturation.
Prof J S Haldane Prof J S Haldane was a renowned Scottish respiratory physiologist who worked extensively on the problems of human exposure to pressure and decompression. He was a strong advocate for stage decompression. Additionally, his work on carbon monoxide resulted in the 1911 Coal Mines Act requirement to use canaries to detect the gas in coal mines. The mining industry recognised his life-saving contribution by electing him as the President of the Institution of Mining Engineers in 1924. During WW1, he developed a gas mask giving protection against chlorine gas for troops in the trenches.
Oxygen decompression Breathing oxygen during the latter part of the decompression increases the effectiveness of the decompression process in two ways: first, the change from air breathing to oxygen breathing eliminates the further intake of nitrogen and creates a steeper pressure gradient to expedite the washout of nitrogen from the body; second, arterial blood transports oxygen, bound to haemoglobin, to the tissues at a given partial pressure; metabolic processes use some of the oxygen to fuel essential bodily functions and the resulting venous blood returning from the tissues contains both residual oxygen and CO2 waste product. Due to its greater solubility, the CO2
as a metabolic is largely
dissolved in the blood, reducing its partial pressure significantly. Taking both into account – the lower partial pressure of the residual oxygen plus the reduced partial pressure of CO2
physiology and thus gives a more effective decompression in a shorter time than a linear decompression profile.
Threshold for decompression As a result of his experimental work on humans and goats, Haldane discovered that it was acceptable to decompress from an absolute pressure A1 to an absolute pressure A2 where pressure A2 was no less than half the pressure A1. This meant, for example, that it was acceptable to decompress from 3 bar(a) i.e. 2 bar(g) to 1.5 bar(a) i.e. (0.5 bar(g) without decompression stops (minor DCS events were considered acceptable). More recent research has shown that a 2:1 ratio is insufficiently conservative and a ~1.7:1 ratio should be used. Decompression time is unproductive time, and it is considered that the 22 psi (1.5 bar(g)) limit in the 1936 ICE Report reflected the need to placate contractors. This was partially rectified by the 18 psi (1.25 bar(g)) limit in the 1958 Regulations. The 14 psi (1 bar(g)) limit in the Blackpool tables made a further correction. The current Blackpool tables with oxygen have fully corrected this with a decompression threshold of 0.7 bar(g). The 21 cases of DCS that occurred on JLE 105 between 0.85 and <1.0 bar(g) support this correction.
– the partial pressure in the venous blood is therefore less than the partial pressure of oxygen in the arterial blood. This partial pressure reduction is referred to as the ‘partial pressure vacancy’ or ‘oxygen window’ and it can accommodate additional partial pressure of an inert gas such as nitrogen without increasing the overall partial pressure in the tissues, thus reducing the DCS risk arising from nitrogen supersaturation. The downside of oxygen breathing is the need to limit partial pressure of oxygen and overall oxygen dose to protect against oxygen toxicity.
Stage decompression Stage decompression involves a profile consisting of pre-determined rapid drops in pressure to generate inert gas bubbles, followed by a period at that stage pressure to allow the bubbles to be off-gassed through the lungs. As the decompression proceeds, pressure drops become progressively smaller while the time period at each stage pressure becomes progressively longer. This technique reduces the amount of inert gas taken on during the initial stages of a decompression, when the inhaled gas partial pressure exceeds that in the tissues and also creates greater pressure gradients in the tissues to flush out the bubbles than linear decompression. It more closely matches human
Dysbaric osteonecrosis DON is a chronic and potentially disabling form of decompression illness in which areas of necrotic (dead) bone occur as lesions on the joint surfaces (articular lesions) or shafts (shaft lesions) of the long bones, particularly around the knees, hips and shoulders, hence the inclusion of long bone X-rays in medical fitness examinations. The precise triggers are unknown but poor decompression and a susceptibility to Type 1 DCS have been identified as possible exacerbating factors. Improvements in effectiveness of decompression procedures have significantly reduced the incidence of DON. BTS recently sponsored an MSc study of the current incidence.
Saturation Saturation is a natural homeostatic process in which the body works to ensure that the partial pressure of inert gas in the body tissues matches the partial pressure of the inert gas being breathed. For practical purposes, saturation is assumed to be achieved around 12–24 hours after compression. Oxygen is a metabolically active gas that is depleted by combining with fuel in the body to give energy and in the process to form CO2
, which
is exhaled. Change the gas mix being breathed or its pressure and the body will respond over time to match the change. Under increased pressure, the tissues take on inert gas until again saturated, at which point no further gas is taken on. Decompression requirements increase as inert gas is taken on but reach a limit when saturation is achieved. Thereafter no further inert gas is taken on, irrespective of the length of the exposure, and hence the decompression requirement remains the same. This is the principle of saturation exposure techniques – a compression – a long period in constant pressure under saturation and a single decompression at the end of the period. Remember, in normal life, we are all in air saturation at atmospheric pressure!
36 | September 2026
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