DCS, DCI, and AGE: What the Decompression Terms Actually Mean
The short answer
Decompression illness (DCI) is the umbrella term in dive literature; it covers both decompression sickness (DCS), which involves inert gas coming out of solution during or after ascent, and arterial gas embolism (AGE), which involves gas entering the circulation from the lungs. DCS itself is conventionally divided into Type I and Type II categories. This page explains what those words mean so you can read a dive manual, a training module, or a medical form without getting lost in acronyms.
What this page is not: it is not a guide to evaluating anyone, and it contains no checklist of signs, no timelines, and no instructions. Anything that involves a person's condition is a matter for emergency services and physicians. The reference libraries to know about are Divers Alert Network's health resources and the federal CDC Yellow Book chapter on scuba diving.
Why the terminology exists at all
Dive planning counts nitrogen because nitrogen is the gas that accumulates in tissue under pressure and has to leave again on the way up. That single fact generates nearly every number on a dive table — bottom time limits, surface intervals, residual nitrogen. The physiology behind it, including inert gas uptake, off-gassing, and the factors researchers associate with risk, is laid out in the peer-reviewed StatPearls chapter on decompression sickness.
If the underlying concept is new, the plain-English starting points on this site are what is residual nitrogen in diving and why dive tables track nitrogen and not oxygen.
The term hierarchy, in order
- Decompression illness (DCI) — the broad category. Used when writers want to refer to pressure-related injuries without committing to a mechanism.
- Decompression sickness (DCS) — the subset specifically tied to inert gas leaving solution as ambient pressure falls. Historically called the bends and caisson disease, names that come from tunnel and bridge caisson work, not from diving.
- Arterial gas embolism (AGE) — a different mechanism, involving gas from the lungs entering the arterial circulation. Grouped under DCI because the two are not always separable in practice, which is part of why the umbrella term is used.
- Type I and Type II DCS — a long-standing descriptive split, with Type I covering the milder presentations traditionally described in the literature and Type II covering those involving the nervous system or cardiorespiratory system. The categories are descriptive conventions used in the literature, not a severity scale a layperson applies.
Terms you will see nearby
Hyperbaric chamber. A pressure vessel used by medical personnel in the treatment of DCI. For a diver reading equipment or logistics documents, the relevant point is that chambers are medical facilities with staff, not devices available to divers independently, and chamber availability is a travel-planning consideration on remote trips.
Flying after diving. A planning constraint that appears in both training materials and travel guidance, because cabin pressure is lower than sea-level pressure. The federal travel chapter above and dive-specific references both address it; the surface-interval guidance for any particular itinerary comes from your training agency and your physician, not from a general-interest page.
Fitness to dive. The medical screening side of the same subject. In recreational diving this runs through a standardized questionnaire; what it asks and who has to sign it is covered in the diver medical screening form, with the official current form hosted by the Undersea and Hyperbaric Medical Society.
How the vocabulary connects back to planning
The reason tables and computers exist is to keep a dive inside a profile that, by the model being used, does not call for a decompression schedule. That boundary is the no-decompression limit — see what is a no-decompression limit — and crossing it changes a dive into one that generates required stops, as described in safety stop vs. decompression stop.
Knowing the words is dive literacy. Acting on anything involving a person is not a literacy problem; it is a call to emergency services and to physicians trained in diving medicine.