In technical diving, the equivalent air depth (EAD) is a way of expressing the narcotic effect of breathing gases such as trimix and heliox.

The EAD is the depth of a dive when breathing air that would have the same partial pressure of nitrogen as the breathing gas in question, which has a different proportion of nitrogen and is being breathed at a different depth.

The EAD can be calculated as follows:

EAD = ( ( (Fraction of N2 × ((Depth in metres / 10) + 1)) / 0.79 ) − 1) × 10

For example, for a gas mix containing 40% nitrogen being used at 60 metres, the EAD is:

EAD = ((0.4 × (( 60 / 10 ) + 1) / 0.79 ) − 1) × 10

EAD = ((0.4 × 7 / 0.79 ) − 1) × 10

EAD = ((2.8 / 0.79 ) − 1) × 10

EAD = (3.544 − 1) × 10

EAD = 25.44 metres

So at 60 metres on this mix, the diver would feel the same narcotic effect as a dive on air to 25 metres.

The severity of nitrogen narcosis depends on the proportion of nitrogen in the gas mix and the depth of the dive. Nitorgen narcosis is a major factor limiting the depth of dives where air is breathed. Air consists of 79% nitrogen. Other gas mixes, such as trimix, heliox and nitrox, contain different proportions of nitrogen.