Choosing the Right PPO₂ for Scuba Diving: Why 1.4 Is a Working Limit, Not a Bargaining Position

Benjamin Hadfield   Sep 28, 2026

Choosing the Right PPO₂ for Scuba Diving: Why 1.4 Is a Working Limit, Not a Bargaining Position

In conversations with students and customers, I come back to the same practical PPO₂ limits. Guidance from DAN, NOAA, NEDU, Scripps, and other diving sources supports a clear distinction: keep working or bottom gas at or below 1.4 PPO₂; reserve a PPO₂ as high as 1.6 for trained divers following a planned deco dive.

The other day, a diver was sitting at my counter and telling me about several dive-rule violations. Then they said, “Okay, I will compromise with you and only use 1.5 PPO₂.”

I asked, “Who are you compromising with?”

They were not negotiating with me. They were negotiating with pressure, oxygen exposure, and their own physiology. The ocean was not part of the conversation, and it never signed that agreement.

My practical recommendation is to plan for a working PPO₂ of 1.4 ATA or less. A higher PPO₂ may be used during planned deco by divers trained and experienced in that kind of diving. The dive phase, workload, gas, and plan matter. A decompression PPO₂ is not a new bottom-gas limit.

What PPO₂ means

PPO₂ is the partial pressure of oxygen in the gas you are breathing. It is not just the oxygen percentage on the cylinder label. As a diver descends, ambient pressure increases, so the pressure exerted by each gas in the breathing mix increases too.

Dalton’s law gives us the basic calculation:

PPO₂ = oxygen fraction × absolute ambient pressure

At 115 feet of seawater, ambient pressure is about 4.48 ATA. A mix containing 31 percent oxygen gives a PPO₂ of about 1.39 ATA there: 0.31 × 4.48. Go deeper on that same gas and PPO₂ rises. This is why each gas has a maximum operating depth, or MOD.

How to calculate MOD

First, choose the PPO₂ limit you are planning to use. For working or bottom gas, that is commonly 1.4 ATA. Then use the oxygen fraction in the gas as a decimal. For example, 32 percent oxygen is 0.32.

For salt water, a simple calculation is:

MOD in feet of seawater = [(planned PPO₂ ÷ oxygen fraction) − 1] × 33

For EAN32, using a planned maximum PPO₂ of 1.4:

[(1.4 ÷ 0.32) − 1] × 33 = about 111 feet

That is the calculated ceiling, not a target depth. Plan below it. DAN lists the MOD for EAN32 at a 1.4 limit as about 112 feet, reflecting the rounding used in common tables and tools. I would rather plan conservatively below the ceiling than spend a dive arguing over one foot.

The MOD changes with the oxygen percentage. Using the same 1.4 working limit, EAN36 calculates to about 95 feet. If the analyzer reads a different oxygen percentage than expected, the MOD changes too. Analyze the cylinder, use that actual mix when setting your computer, check the displayed MOD, and plan the dive around the deepest point you intend to reach. DAN advises divers to verify the cylinder’s oxygen content themselves rather than relying only on the label or fill station.

Do not take the same formula, substitute 1.6, and treat the answer as permission to use that gas on the bottom. A higher PPO₂ belongs only in the phase of a dive where your training, gas, and decompression plan allow it.

Why nitrox can extend your NDL

Nitrox contains more oxygen and less nitrogen than air. Henry’s law helps explain why the lower nitrogen fraction matters: as the partial pressure of a gas rises, more of that gas can dissolve into body tissues. At the same depth, a nitrox mix with less nitrogen generally means less nitrogen uptake than air.

That can give a diver a longer NDL when using the appropriate nitrox tables or computer setting. It can also be used as added conservatism by planning the dive using air limits. Those are different ways to use the reduced nitrogen exposure. You do not get to spend the whole margin on extra bottom time and still claim the same safety margin. Nitrox is useful. It is not a bottom-time coupon.

Choosing a mix that brings PPO₂ close to—but does not exceed—1.4 at your planned maximum depth may also reduce the nitrogen fraction compared with air. The lower nitrogen content is what can improve the NDL. PPO₂ at 1.4 is not a magic setting that creates bottom time by itself.

Why I use 1.4 for the working part of the dive

DAN describes a PPO₂ at or below 1.4 ATA as a “green light” range for open-circuit scuba. It describes 1.4 to 1.6 as a warning range, with very little room for an unexpected depth change, individual differences, or strenuous work. NOAA’s decompression-planning form makes a similar distinction: bottom mixes at or below 1.4 ATA and decompression gases at or below 1.6 ATA.

That distinction matters. At the bottom, you may be working against current, hunting for lobsters or fish, managing equipment, handling a problem, or helping your buddy. Workload can increase quickly. During a planned decompression stop, a trained diver will be holding a stable position and keeping effort very low and controlled.

A PPO₂ of 1.6 does not mean a seizure is certain. It does not mean the exposure is harmless either. DAN says the chance of CNS toxicity at 1.6 during a resting decompression stop is low, but not absent. Individual responses vary, and the margin is narrower than at 1.4. If an emergency forces you to work hard at that moment, the conditions have changed.

Oxygen exposure also depends on time. CNS exposure is tracked across a dive and repetitive dives using the method taught in your training. Pulmonary exposure is tracked separately, often with OTUs. Neither clock predicts exactly when an individual diver will have a problem. The practical point is simple: higher PPO₂ means no room for exposure time, depth changes, exertion, or a mistake.

For me, 1.4 is a reasonable upper working limit. A lower PPO₂ may make sense when the conditions or workload call for it. A diver who has used 1.5 before and felt fine has learned only that nothing happened that time. “I got away with it last time” is not a gas-planning method.

CNS and pulmonary oxygen toxicity are different

The immediate oxygen-toxicity concern during a typical scuba dive is usually central nervous system, or CNS, oxygen toxicity. Symptoms can include visual changes, ringing in the ears, confusion, nausea, tingling, muscle twitching, and convulsions. Warning signs do not always appear before a seizure.

A convulsion underwater can dislodge a regulator or cause an injury. The buddy may suddenly need to control the diver, get them to the surface, keep their airway open, and signal for emergency help. DAN notes that the main danger is not the convulsion itself; it is the risk of trauma or drowning.

Pulmonary oxygen toxicity, or POT, is a different problem. It affects the lungs and is more closely tied to prolonged oxygen exposure. DAN describes lung oxygen toxicity as resembling a bad case of the flu and says it most often results from very long recompression treatments. A short recreational dive at 1.5 does not mean your lungs are about to be damaged. Still, PPO₂ and exposure time matter, especially on long or repetitive technical dives. CNS exposure and pulmonary exposure are different calculations; track both as required by your training and plan.

Hunters: the fish is not in charge of the plan

Many hunters are careful, disciplined divers. I am talking about the ones who treat 1.4 as a normal working PPO₂.

A hunt can make a diver work harder than expected. There may be current, surge, repeated descents, equipment to manage, or a fish that does not cooperate with the schedule. A diver focused on the target can miss changes in effort, depth, or how they feel. Exercise and elevated carbon dioxide can increase oxygen-toxicity risk. That is a poor time to have already spent your safety margin.

Keep effort down. Move at a pace you can maintain. If current, task load, or conditions turn the dive into hard work, stop chasing the target and follow your training and dive plan. At deeper hunting depths, PPO₂ is only one concern. Gas density, narcosis, decompression requirements, gas reserves, and the ascent plan matter too. The fish can wait. It has no say in your MOD.

Stay within your training

A certification is not permission to use any gas at any depth. It qualifies you for the activities and conditions covered by that training. Technical dives can involve staged gases, gas switches, mandatory decompression, team procedures, and emergency plans. Do not attempt those parts of a dive unless you have been trained to plan and perform them, and have the current skills to do so.

A computer, a buddy’s confidence, or a story about someone else’s dive does not replace training. If the planned dive is beyond your certification, experience, or current ability, choose a different dive or get the training first. TDI’s technical progression is built around specific skills such as buoyancy, trim, propulsion, gas management, decompression procedures, and team awareness; those skills are practiced before divers move into more demanding courses.

What an oxygen accident can cost

If a diver has an oxygen-related convulsion underwater, their buddy may suddenly be doing a rescue instead of enjoying a dive. The crew may need to stop normal operations, manage an emergency pickup, and get the diver to medical care. The diver’s family is dealing with the phone call and the uncertainty. A bad outcome can mean lasting injury or death.

If the diver survives, treatment may involve emergency care, physician services, transportation, and hyperbaric chamber treatment. DAN’s current FAQ lists chamber treatment at roughly $330 to $1,000 per hour, plus medication and physician fees. The total depends on how many treatments are needed and where the accident happens. DAN also warns that regular medical insurance may exclude diving or cover only a small portion of chamber costs; deductibles and copays alone can reach thousands of dollars. Coverage depends on the exact policy and circumstances, so check the terms with your insurer before the dive.

Then there is the lost work, canceled travel, rehabilitation, and the buddy who has to live with having managed that emergency. A PPO₂ argument can sound theoretical at the shop counter. The consequences are not theoretical to the people who have to respond.

Learn technical diving in Stuart, Florida

At Stuart Scuba, we teach dive training in Stuart, Florida, and TDI technical diving courses including Intro to Tech, Advanced Nitrox, Decompression Procedures, Helitrox, and Trimix. The right next course depends on your current certification, logged dives, skills, and goals. We will talk through that progression with you and help identify the training that fits. You can learn more at StuartScuba.com.

Analyze the gas. Calculate and check the MOD. Set the computer to the analyzed mix. Plan working PPO₂ at or below 1.4 ATA, keep effort controlled, and stay within your training. Use a PPO₂ up to 1.6 only where it belongs in your decompression training and plan.

That is my own humble opinion and observation as an instructor: the diver and plan should come first; the fish second. There are many ways to dive. Not all of them lead back to the surface.