ppO2, oxygen toxicity and the 1.4 and 1.6 limits
How oxygen partial pressure scales with depth, why nitrox raises toxicity risk faster than air, and what the 1.4 and 1.6 ata limits mean.
Why ppO2 climbs with depth
At the surface you breathe one atmosphere of pressure, written as 1 ata. Every 10 m of seawater, or every 33 ft, adds another atmosphere, so at 30 m the ambient pressure is 4 ata. Oxygen keeps the same fraction of the mix at every depth, but its partial pressure is that fraction times the ambient pressure. Air at 21 percent therefore delivers 0.21 ata of oxygen at the surface and 0.84 ata at 30 m. The mix did not change, only the pressure squeezing it did.
Why nitrox reaches the limit shallower
Nitrox raises the oxygen fraction to reduce nitrogen loading and extend no-stop time, but the trade is a faster rise in ppO2. EAN32 delivers 0.32 ata of oxygen per atmosphere against air's 0.21, so it hits any given ppO2 ceiling at a shallower depth. That is why enriched air has a maximum operating depth while air, for recreational depths, rarely does. This calculator makes the trade visible: hold the oxygen percentage steady and watch the ppO2 card cross into red as you push the depth down.
What the 1.4 and 1.6 ata numbers mean
The 1.4 ata figure is the working limit taught for the active, task-loaded part of a dive, where carbon dioxide build-up and exertion make the diver more susceptible to a toxicity hit. The 1.6 ata figure is an absolute exposure limit reserved for resting, shallow stops such as decompression, and many agencies treat it as a ceiling never to be exceeded. This tool checks against 1.4, so a mix that reads over the limit here may still be inside 1.6 for a planned stop. Reading a red label means review the plan, not that a specific outcome is guaranteed.
Reading the two result cards together
The Ambient pressure card confirms the depth conversion the tool used, and the Oxygen partial pressure card applies your mix to it. Comparing the two helps you sanity check the math: if ambient pressure looks wrong for your depth, you probably have the units switch set to the wrong system. Because only whole physics is involved, the same depth always yields the same ppO2, and copying the summary line gives you a compact record of the mix, depth and result for a dive log or plan.