---
title: "Gas density and narcosis"
description: "What gas density does to work of breathing, how END measures narcosis, and the two density thresholds the planner warns at."
---

> Documentation Index
> Fetch the complete documentation index at: https://docs.divelogic.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Gas density and narcosis

import { GasCard } from "@/components/react/PlannerScreens";

Gas density is the mass of one litre of the gas you are actually breathing, at
the depth you are breathing it, in grams per litre, rising in direct proportion
to depth. It decides how hard a dive is to breathe, and it has nothing to do
with decompression.

Denser gas flows turbulently rather than smoothly, work of breathing rises,
ventilation falls behind and carbon dioxide accumulates. CO₂ retention
contributes directly to oxygen toxicity and to loss of control at depth, which
is why density is a planning constraint rather than a comfort issue. Helium is
roughly a seventh as dense as nitrogen, so replacing nitrogen with helium is the
only effective way to make a deep mix breathable. Reducing oxygen does almost
nothing, because oxygen is slightly denser than nitrogen.

## The two thresholds

| Density | What happens |
|---|---|
| Above **5.2 g/L** | Warning, advisory only. `Elevated gas density — exceeds 5.2 g/L` |
| Above **6.2 g/L** | Error. `High gas density — exceeds 6.2 g/L` |

Both thresholds are applied twice: once by the gas analyser when you inspect a
mix, and once by the planner, which checks the gas you are breathing at depth
and attaches a warning naming the gas and the time. The two can differ slightly
on the same mix, because only the planner accounts for the water type and
altitude you set.

The figures are conventional in technical diving practice, but the citations
attached to them in this build are not consistent with each other, so treat
their source as unconfirmed.

Air at 45 m passes the error threshold, because air is the densest thing you can
put in a cylinder at that depth. On a helium mix taken deeper, density often
crosses 5.2 g/L before the ppO₂ limit is reached.

## Narcosis and END

Equivalent narcotic depth is the depth at which air would produce the same
narcotic load as the mix you are breathing. DiveLogic works it out from the
nitrogen alone: helium is treated as entirely non-narcotic, and oxygen is not
counted at all. It is rounded up to the next 0.1 m, because a deeper equivalent
is the conservative one. Above **30 m** the analyser raises a narcosis warning.

The same number appears under two labels: **EAD** on the planner's gas card, and
**END** in the gas mix wizard and the analyser. There is a second convention in
which oxygen is counted alongside nitrogen, producing a deeper figure for the
same mix; DiveLogic does not use it, so an END quoted from another tool may not
match this one.

Both numbers arrive on the block every gas in a plan carries. This is 21/35
judged at 45 m: **Density** in grams per litre and **EAD** in metres.

## What the numbers do not tell you

Density says nothing about decompression. A dense mix and a light mix at the
same depth for the same time load your tissues according to their inert
fractions, not their mass.

The thresholds are not a personal limit. Work of breathing depends on the
regulator, the scrubber, the exertion, the posture and the diver. 5.2 g/L is a
figure below which most divers on well-maintained equipment ventilate
adequately, not a line at which something changes.

END measures nothing directly either. It rests on the assumption that helium is
not narcotic, and cold, workload, CO₂ and fatigue all change how narcosis
presents at a given END.

## Next

- [Oxygen exposure](/theory/oxygen-exposure) is the other constraint on a mix.
- [Depth limits](/gases/analysing) walks through the numbers in the app.

Source: https://docs.divelogic.ai/theory/gas-density/index.mdx