Hazards beyond decompression

What it is

A mature planner warns about what can hurt you besides decompression. Each of the hazards below is computed across your whole profile and surfaced as a warning in the planner, so a plan that is fine on its stop schedule but dangerous in some other way does not slip through silently. Acronyms used here: END (equivalent narcotic depth), MOD (maximum operating depth), HPNS (high-pressure nervous syndrome), MinOD (minimum operating depth), and ICD (isobaric counter-diffusion).

Gas density

Gas too dense to move easily through your airways drives up CO₂, which is a direct contributor to many diving incidents. The engine computes density across the whole profile, pairing each depth with the gas breathed there. It cautions above the ideal limit of about 5.2 g/L and flags critically above about 6.2 g/L, the limit from Anthony and Mitchell’s rebreather respiratory-physiology work (2016). Both thresholds are adjustable in the planner’s deco defaults. Air at 55 m runs over 8 g/L, well into the danger zone, and the planner says so.

Narcosis (END)

The engine computes the equivalent narcotic depth (END), the depth at which breathing air would give the same narcotic load as your actual mix, and flags it once it passes the threshold (30 m by default, adjustable in the planner’s deco defaults). That way a deep-air or thin-helium plan cannot impair your judgement without warning.

Surface hypoxia

A hypoxic bottom mix such as 15/55 trimix (15% oxygen, 55% helium) is fine at depth but dangerous at the surface, where its low oxygen fraction cannot sustain you. 15/55 gives a surface PPO₂ of just 0.15 bar, under the 0.16 bar minimum needed to stay conscious. The engine checks PPO₂ at the surface, not only at depth, a check some tools skip. It applies this surface check to the descent gas only (the gas you actually breathe from the surface), since bottom and travel gases are switched to at depth, not breathed up top.

For any hypoxic mix, the engine also adds a minimum-operating-depth (MinOD) note. This tells you the shallowest depth where the mix still delivers enough oxygen, so you know not to breathe it any shallower.

Isobaric counter-diffusion (ICD)

Switching from a helium-rich to a nitrogen-rich gas at the same depth can drive helium out of slow tissues while fast tissues are still taking nitrogen on. Two gases move in opposite directions at once, which is how bubbles can form even though the surrounding pressure never changed. The engine shows one advisory when a switch crosses the commonly cited threshold (roughly, the helium drop is more than 5× the nitrogen increase), rather than a flag on every gas change.

HPNS (high-pressure nervous syndrome)

At extreme depth, breathing helium can bring on tremors, dizziness, and other neurological symptoms, a condition called high-pressure nervous syndrome. The engine watches any mix with at least 10% helium and flags it once you pass about 80 m (a caution), then critically past about 120 m. Recreational divers will never reach these depths; this is for technical heliox and trimix planning.

Related

• How the deco engine works covers the tissue model and the oxygen clocks.
• Gas Specs lets you check MOD, END, and density for a single mix at any depth.
• Deco Planner surfaces these warnings as you build a plan.
• Further reading collects the papers behind these thresholds, including the gas-density and CO₂-retention sources (Anthony and Mitchell, Warkander, Doolette and Mitchell) and the ICD rule of fifths.