Oxygen toxicity is one of those hazards that recreational divers rarely think about and rebreather divers manage on every dive. On open circuit, the gas you breathe matches the tank label. On a closed-circuit rebreather, the loop is mixing oxygen and diluent in real time to hit your setpoint, and the partial pressure of oxygen at your lungs depends on depth, setpoint behavior, cell accuracy, and a stack of small decisions that started before you ever rolled off the boat.
When that mix is correct, oxygen toxicity is a background number you log and move on from. When it goes wrong, the consequences range from cutting a dive short to losing consciousness underwater. That is the entire reason the Inspiration and Evolution platforms ship with redundant cells, dual oxygen controllers, and a handset that nags you when the loop drifts.
If you are buying your first AP Diving rebreather, or you are an experienced CCR diver who wants a cleaner mental model for what your handset is doing during a five-hour cave dive, this is the version of the oxygen toxicity story that actually matters in the loop, not on a textbook page.
What Is Oxygen Toxicity in the CCR Context?
Oxygen toxicity is the body’s response to breathing oxygen at a partial pressure higher than it evolved to handle. Divers usually split it into two flavors. Central nervous system toxicity, or CNS, is the acute version that can show up in minutes when the partial pressure of oxygen, written as PO2, climbs above roughly 1.4 to 1.6 bar. The classic warning signs are visual disturbance, ear ringing, nausea, twitching, irritability, dizziness, and finally a tonic-clonic seizure. The seizure itself is not what kills you. Losing the mouthpiece and aspirating water at depth is.
Whole-body toxicity, sometimes called pulmonary toxicity, is the slow version. It is driven by total oxygen dose over hours and days, not a single spike. Multi-day technical projects and chamber treatments measure this as Oxygen Tolerance Units, or OTUs. The first symptoms are usually a dry cough and a burning sensation behind the sternum after a long dive. You do not seize from it. You feel like you have a chest cold for a couple of days and your gas exchange efficiency drops.
CCR divers care about CNS first because the loop holds you at an elevated PO2 for the entire dive. On open-circuit air, your PO2 is whatever your depth times 0.21 yields. On a Silent Diving-supplied Inspiration or Evolution running a 1.3 setpoint, your PO2 is 1.3 from a few minutes after descent until you start the ascent. That changes the math. A 60-minute open-circuit dive at 30 metres sees a PO2 around 0.8. A 60-minute CCR dive at the same depth sees a PO2 around 1.3 the entire time. Same depth, very different oxygen exposure.
How Do PO2 Setpoints Shape Your Toxicity Exposure?
An AP Diving rebreather runs on a setpoint. The setpoint is the PO2 you have asked the loop to maintain. Inspiration and Evolution divers typically configure a low setpoint of 0.7 for the surface and shallow segments and a high setpoint of 1.3 for the working portion of the dive. Some training agencies cap the high setpoint at 1.2 for student dives and at 1.4 for stage decompression. The numbers vary because every agency draws the safety line in a slightly different place, but they all sit below the 1.6 PO2 ceiling that NOAA and most operational standards treat as the edge of tolerable exposure.
The Vision handset will automatically transition between low and high setpoint at a depth you configure, usually around 10 metres. That keeps your PO2 low during the descent so a solenoid that fires too eagerly does not push oxygen into a loop that is already compressing. Once you cross the threshold, the controller starts firing the solenoid until it sees the cells stabilize near your high setpoint. Looking at the dual oxygen controller architecture on the Inspiration and Evolution helps explain why you see two parallel PO2 readouts on the handset and why both have to agree before the unit trusts the value.
A higher setpoint is not free. It buys you better decompression efficiency, since more oxygen in the loop accelerates inert gas off-gassing, but it costs you CNS exposure. Holding 1.3 for 120 minutes is roughly twice the CNS dose of holding 1.0 for the same time. For long, shallow scientific or photography dives, many CCR divers manually drop the setpoint to 1.0 or 1.1 once the productive work is done, then return to 1.3 only for the deco stops where the off-gassing payoff matters.
What Causes Unexpected PO2 Spikes on a Rebreather?
The dangerous version of CCR oxygen exposure is not the planned PO2. It is the unplanned spike that pushes you past 1.6 before the handset can react. Several mechanisms produce that result, and they all reward divers who recognize the pattern early.
The first is a solenoid that sticks open. Oxygen continues to flow into the loop after the controller has commanded it to close, and the PO2 climbs until you notice the alarm or smell the oxygen-rich gas character in the loop. The fix is a manual diluent flush, which dilutes the loop with low-PO2 diluent and brings the partial pressure down while you sort the cause.
The second is operator error during manual oxygen addition. A short intentional bump of oxygen at depth is normal during a setpoint transition or after a flush. A long-pressed manual add is not. Working the manual valve while distracted is one of the most common ways a CCR diver creates their own spike. The same reflex that keeps you alive when the solenoid stops responding can hurt you when the solenoid is fine and the diver is the one adding gas without thinking.
The third is a cell reading that no longer reflects reality. A current-limited cell that has run out of headroom will read low when the real PO2 is much higher. The controller, trying to bring a falsely low reading up to setpoint, will keep firing oxygen and create the spike it cannot see. The Vision handset’s cell-discrepancy alerts on the handset exist for exactly this case, and they are the reason divers should not silence them. Treat repeat warnings as a reason to abort, not a reason to keep diving the unit.
The fourth is descent compression. As you descend, depth multiplies the PO2 of whatever oxygen is already in the loop. If the unit started the dive with a slightly hot loop, descending faster than the controller can compensate produces a brief but real spike at depth. Slower descents and a clean pre-dive flush solve this one before it ever happens.
How Do You Track CNS and OTU Exposure Across Dives?
The Vision handset and most external dive computers used with CCR units track CNS as a running percentage of the NOAA exposure limit. At a constant 1.3 PO2, the single-exposure limit is 180 minutes before you hit 100 percent. At 1.4 PO2, it is 150 minutes. At 1.6 PO2, it is 45 minutes. Most CCR planners keep CNS below 80 percent on any single dive and below 300 percent across a 24-hour day, with the residual decaying at a half-life of about 90 minutes.
OTU tracking is less commonly displayed in real time. Divers running multi-day expeditions, especially deep technical projects, use logbook math: a 1.3 PO2 for 60 minutes deposits roughly 41 OTUs, and the standard daily ceiling for operational diving is 300 OTUs with about 670 OTUs across a five-day exposure window. Saturation divers operate under very different rules, but no recreational or technical CCR diver should be approaching those numbers.
The right time to think about cumulative exposure is during planning, not during the dive. Rebreather dive planning that pairs setpoint shifts with decompression strategy lets you decide before you splash where the oxygen budget will be spent. If your plan calls for two 90-minute dives at 1.3 with a one-hour surface interval, you are at roughly 60 percent CNS before lunch. If a third dive is on the schedule, you will need to lower the setpoint or trim the bottom time to stay in margin.
The newer external displays and the AP Vision both surface this number on a glance. Use it. CNS tracking is one of the few real-time indicators on a CCR that maps directly to a hard physiological limit, and the only one where ignoring the number can produce an injury inside a single dive.
How Should You Respond to a PO2 Spike Underwater?
If the handset alarms on PO2 high, or if you notice tunnel vision, lip twitching, or an odd metallic taste, treat the situation as a hyperoxic event whether or not the numbers fully agree. A CCR diver’s response to a suspected spike follows a short sequence that is worth memorizing on the surface so the muscle memory is there when it is needed.
Drop the setpoint. Most Vision handsets let you switch to the low setpoint in two button presses. Doing that stops the solenoid from adding more oxygen and lets the loop drift back toward a safer range as you breathe down the oxygen already present.
Flush with diluent. A two- or three-second manual diluent add at depth dilutes the loop. Many divers run a second flush 30 seconds later to confirm the cells move with the change. If the cells do not respond, you may be looking at a current-limited cell, and the priority shifts to bailout.
Ascend at a controlled rate. Lower depth means lower PO2 for the same gas mix in the loop. A slow ascent of three to five metres per minute drops your PO2 mechanically without changing anything in the unit, and it gives the controller time to settle. Skipping decompression because you are worried about toxicity is not the answer. The risk profile of acute hyperoxia at depth is worse than a controlled ascent that respects your deco obligation.
Log everything. After surfacing, write down what the handset showed, what setpoint you were running, how the cells were behaving, and how long you have been on those cells. That log is what lets you and the service team decide whether the unit needs new cells or a full controller check. The way oxygen cells age across twelve to eighteen months of normal use is well understood, but only your dive log tells you which cell on which dive started reporting a soft millivolt curve.
How Do You Make Oxygen Tracking a Daily Habit?
The CCR divers who never have to talk about a hyperoxic event are the ones who treat oxygen tracking as a small daily routine, not a separate skill. Pre-dive, they confirm cell calibration against a known-PO2 reference gas, they verify both controllers agree at the surface, and they record cell millivolt values in the logbook. In the water, they glance at the PO2 readouts every minute or two during the bottom phase and every 30 seconds on ascent.
Post-dive, they note the cumulative CNS percentage, they pay attention to whether they coughed or felt chesty in the days after a long trip, and they retire cells before the cells retire themselves. The cost of a fresh set of cells is trivial compared to the cost of any other gear failure in the loop.
When the unit feels off, even slightly, it is worth pulling it out of rotation until Silent Diving’s authorized service team has eyes on it. Service catches the things that look minor in the kit room and turn into a real problem at 40 metres: a solenoid that is hesitating on close, an O-ring that is starting to weep, a cell connector that is intermittent in cold water. None of those are diver-fixable on a boat. All of them are routine service items for a unit that ships with the parts inventory, warranty workflow, and trained technicians already in place.
For owners in North, Central, or South America, that whole chain lives with Silent Diving. The result is a shorter loop between noticing a problem on Sunday and having a unit you trust again before the next trip.
Frequently Asked Questions About CCR Oxygen Toxicity
At What PO2 Does Oxygen Toxicity Become a Real Concern on a CCR?
CNS oxygen toxicity risk climbs sharply above a PO2 of 1.4 bar, and most operational standards treat 1.6 as the working ceiling. Most CCR planners select a working setpoint of 1.2 or 1.3 to stay below that ceiling while still getting decompression benefit from the elevated oxygen. Time at PO2 matters as much as the peak number itself, which is why the NOAA single-exposure table caps exposure minutes against partial pressure rather than against depth.
What Is the Difference Between CNS Toxicity and Whole-Body Toxicity?
CNS toxicity is the acute version that can produce visual disturbance, twitching, or a seizure within minutes when PO2 is too high. Whole-body or pulmonary toxicity is the slow version, measured in OTUs over hours and days, and it shows up as a dry cough and chest tightness after long projects. CCR divers manage CNS minute by minute and OTUs trip by trip. Both metrics matter, but they fail in very different ways.
Can a Recreational CCR Diver Hit Oxygen Toxicity at Shallow Depth?
Yes, if the loop is hot. A stuck solenoid or a manual oxygen overrun can push PO2 to 1.6 or above even at 20 metres, where the depth alone would never produce that exposure on open-circuit air. This is the reason CCR divers verify cells and watch the handset throughout the dive rather than relying on depth alone for safety. Shallow does not mean low-PO2 on a rebreather.
How Long Can You Safely Dive at a 1.3 PO2 Setpoint?
Using the NOAA single-exposure table, the published limit at 1.3 is 180 minutes before reaching 100 percent CNS. Most operational planning caps a single CCR dive at 80 percent of that and tracks cumulative exposure across the day. If the dive is longer than that allows, the setpoint comes down for the working segment or the dive is split into two sessions with a real surface interval in between.
Does the AP Vision Handset Warn Before You Reach a Toxic PO2?
Yes. The Vision handset provides high-PO2 alarms, cell-discrepancy warnings, and visual cues across both displays well before the loop reaches a clearly toxic level. The warnings should be treated as commands to act, not as nuisance notifications. Silencing or ignoring repeat alarms is one of the patterns the service team sees in post-incident reviews when a unit comes back for inspection after a near miss.
Should I Drop My Setpoint for Long Shallow Dives?
For long photography or science dives at shallow depths, dropping from 1.3 to 1.0 or 1.1 once the productive work is done is a common technique to keep CNS load in margin. Some divers raise back to 1.3 only for the final deco stops, where the off-gassing benefit is highest and the time at the higher PO2 is short. The decision belongs to the planner, not the unit, and it should be documented in the dive plan before the team splashes.
How Do I Recover After an Oxygen Exposure Overrun?
Surface safely, end the dive day, and rest. Do not rejoin the dive schedule until OTUs and CNS have decayed back to baseline, which usually takes 24 to 36 hours for moderate overruns. If you experienced any symptoms in the water, document them and contact a dive medical professional before the next dive. The unit itself should be pulled from rotation and inspected before it is breathed again, even if the rest of the trip is on the line.
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