Editing Decompression sickness (section)

== Predisposing factors ==
{{See also|Physiology of decompression#Factors influencing decompression stress and risk}}

Although the occurrence of DCS is not easily predictable, many predisposing factors are known. They may be considered as either environmental or individual. Decompression sickness and arterial gas embolism in [[recreational diving]] are associated with certain demographic, environmental, and dive style factors. A statistical study published in 2005 tested potential risk factors: age, gender, body mass index, smoking, asthma, diabetes, cardiovascular disease, previous decompression illness, years since certification, dives in the last year, number of diving days, number of dives in a repetitive series, last dive depth, nitrox use, and drysuit use. No significant associations with risk of decompression sickness or arterial gas embolism were found for asthma, diabetes, cardiovascular disease, smoking, or body mass index. Increased depth, previous DCI, larger number of consecutive days diving, and being male were associated with higher risk for decompression sickness and arterial gas embolism. Nitrox and drysuit use, greater frequency of diving in the past year, increasing age, and years since certification were associated with lower risk, possibly as indicators of more extensive training and experience.{{r|DeNoble 2005}}

=== Environmental ===
The following environmental factors have been shown to increase the risk of DCS:

* the magnitude of the pressure reduction ratio&nbsp;– a large pressure reduction ratio is more likely to cause DCS than a small one.{{r|Dehart | Vann1 | Fryer}}
* repetitive exposures&nbsp;– repetitive dives within a short period of time (a few hours) increase the risk of developing DCS. Repetitive ascents to altitudes above {{convert|5500|m|ft}} within similar short periods increase the risk of developing altitude DCS.{{r|Dehart | Fryer}}
* the rate of ascent&nbsp;– the faster the ascent the greater the risk of developing DCS. The ''[[U.S. Navy Diving Manual]]'' indicates that ascent rates greater than about {{convert|20|m/min|ft/min||abbr=on}} when diving increase the chance of DCS, while recreational dive tables such as the [[Bühlmann tables]] require an ascent rate of {{convert|10|m/min|ft/min||abbr=on}} with the last {{convert|6|m|ft||abbr=on}} taking at least one minute.{{sfn|Lippmann & Mitchell|p=232}} <!-- There are some doppler bubble studies to support this --> An individual exposed to a rapid decompression (high rate of ascent) above {{convert|5500|m|ft}} has a greater risk of altitude DCS than being exposed to the same altitude but at a lower rate of ascent.{{r|Dehart | Fryer}}
* the duration of exposure&nbsp;– the longer the duration of the dive, the greater is the risk of DCS. Longer flights, especially to altitudes of {{convert|5500|m|ft|abbr=on}} and above, carry a greater risk of altitude DCS.{{r|Dehart}}
* underwater diving before flying&nbsp;– divers who ascend to altitude soon after a dive increase their risk of developing DCS even if the dive itself was within the dive table safe limits. Dive tables make provisions for post-dive time at surface level before flying to allow any residual excess nitrogen to outgas. However, the pressure maintained inside even a pressurized aircraft may be as low as the pressure equivalent to an altitude of {{convert|2400|m|ft|abbr=on}} above sea level. Therefore, the assumption that the dive table surface interval occurs at normal atmospheric pressure is invalidated by flying during that surface interval, and an otherwise-safe dive may then exceed the dive table limits.{{r|Bassett | FAD | pmid17672177}}
* diving before travelling to altitude&nbsp;– DCS can occur without flying if the person moves to a high-altitude location on land immediately after diving, for example, scuba divers in [[Eritrea]] who drive from the coast to the [[Asmara]] plateau at {{convert|2400|m|ft|abbr=on}} increase their risk of DCS.{{sfn|Lippmann & Mitchell|p=79}}
* [[Altitude diving|diving at altitude]]&nbsp;– diving in water whose surface pressure is significantly below sea level pressure – for example, [[Lake Titicaca]] is at {{convert|3800|m|ft|abbr=on}}. Versions of [[decompression table]]s for altitudes exceeding {{convert|300|m|ft|abbr=on}}, or [[dive computer]]s with high-altitude settings or surface pressure sensors may be used to reduce this risk.{{r|Bassett | Egi}}

=== Individual ===
[[File:Atrial septal defect-en.png|thumb|alt=Diagram of the four chambers of the heart. There is a gap in the wall between the upper-left and upper-right chambers|Atrial septal defect (PFO) showing left-to-right shunt. A right-to-left shunt may allow bubbles to pass into the arterial circulation.]]

The following individual factors have been identified as possibly contributing to increased risk of DCS:

* [[dehydration]]&nbsp;– Studies by Walder concluded that decompression sickness could be reduced in aviators when the serum surface tension was raised by drinking isotonic saline,{{r|Walder}} and the high surface tension of water is generally regarded as helpful in controlling bubble size.{{r|Fryer}} Maintaining proper hydration is recommended.{{sfn|Lippmann & Mitchell|p=71}} There is no convincing evidence that overhydration has any benefits, and it is implicated in [[immersion pulmonary oedema]].<ref name="UKDMC" />
* [[patent foramen ovale]]&nbsp;– a hole between the atrial chambers of the [[heart]] in the [[fetus]] is normally closed by a flap with the first breaths at birth. In about 20% of adults the flap does not completely seal, however, allowing blood through the hole when coughing or during activities that raise chest pressure. In diving, this can allow venous blood with microbubbles of inert gas to bypass the lungs, where the bubbles would otherwise be filtered out by the lung capillary system, and return directly to the arterial system (including arteries to the brain, spinal cord and heart).{{r|Moon}} In the arterial system, bubbles ([[arterial gas embolism]]) are far more dangerous because they block circulation and cause [[infarction]] (tissue death, due to local loss of blood flow). In the brain, infarction results in [[stroke]], and in the spinal cord it may result in [[paralysis]].{{sfn|Lippmann & Mitchell|p=70}}
* a person's age&nbsp;– there are some reports indicating a higher risk of altitude DCS with increasing age.{{r|38uhms | Fryer}}
* previous injury&nbsp;– there is some indication that recent joint or limb injuries may predispose individuals to developing decompression-related bubbles.{{r|38uhms | Karlsson}}
* [[wikt:ambient|ambient]] temperature&nbsp;– there is some evidence suggesting that individual exposure to very cold ambient temperatures may increase the risk of altitude DCS.{{r|38uhms | Fryer}} Decompression sickness risk can be reduced by increased ambient temperature during decompression following dives in cold water,{{r|Gerth2}} though risk is also increased by ingassing while the diver is warm and peripherally well-perfused, and decompressing when the diver is cold.<ref name="Pollock 2023" />
* body type&nbsp;– typically, a person who has a high body fat content is at greater risk of DCS.{{r|38uhms | Fryer}} This is because nitrogen is five times more soluble in fat than in water, leading to greater amounts of total body dissolved nitrogen during time at pressure. Fat represents about 15–25 percent of a healthy adult's body, but stores about half of the total amount of nitrogen (about 1 litre) at normal pressures.{{r|fatness}}
* alcohol consumption&nbsp;– although [[alcohol (drug)|alcohol]] consumption increases dehydration and therefore may increase susceptibility to DCS,{{r|Fryer}} a 2005 study found no evidence that alcohol consumption increases the incidence of DCS.{{r|leigh2005}}

==="Undeserved" decompression sickness===
A common misconception or misnomer is "undeserved" decompression sickness, which is more accurately decompression sickness that was not predicted by the algorithm or tables used. The algorithms and tables calculated from them use a simplistic mathematical model to predict decompression sickness which uses the dive profile as input and a mathematical approximation of tissue gas dynamics which omits a number of factors which are known or suspected to exist, but for which there is no known tested and reliable model to take the effects of these factors into account. In several cases there is also no currently available way to measure the factors in a meaningful way, so they are either simply omitted, or an arbitrary manual or programmed input used to produce a more conservative, therefore "safer" profile, but the reduction in risk is generally unknown. When these necessarily oversimplified models occasionally fail, some divers claim that the decompression symptoms were "undeserved" as they followed the directions of the computed schedule.<ref name="Pollock 2023b" />