You Are Viewing

Cave diving ‘bad air’ incident, 14 May 2015, Mexico


by Ivo, ATI Cave Exploration Team, owner of the DivingMexico dive school (


Yesterday, May 14 2015 we had a serious “bad air” incident (not with a student, but with myself) with some very important lessons learned. I would urge all fellow cave divers to read this.

Before we acquired our own filling station we used to fill our tanks at the Zero Gravity dive center, which has impeccable (to this day) record for the purity of their gas. On a certain day in March, however, their compressor had a problem and all tanks were outsourced to another filling station, which is very popular among the cave diving community and also has a very good reputation. A number of our tanks were among the ones taken there and filled with Nitrox 32. The next day we collected the tanks and used some of them for a full cave course we were doing at the time. At cenote Cristalino one of our students complained about bad taste and refused to breathe from one of the tanks. I was called in from home to bring a spare tank, some tanks were exchanged and there was much commotion. That evening we went to ZG where we found two sets of doubles returned with the same complaint and then to the filling station in question where there were a few other tanks returned for the same reason. A certain person from the filling station admitted that the filters should have been changed but weren’t. We were upset, but since nothing serious had happened and it all amounted only to some commotion, and also it turned out that only a few tanks were affected, the incident was soon forgotten and very few people learned about it. Although samples were taken by the filling station owners for analysis no one heard anything about this afterwards. Some tanks were emptied and refilled, but one tank was forgotten at home, full. We have quite a lot of tanks and some are rarely used, this was one of those.

Fast forward to yesterday, May 14-th. We go for a serious dive at our cave, the Ixtlan system, (we own the cenote and control the access) where I was set to to survey an extensive section – something I had been postponing for some time. I took the tank in question as a stage without thinking twice, assuming that it was filled at our place. Me and Deyan went to the cave and started our dive. Now please take note of the sequence of events, as it is very telling:

During the pre-dive check in the water I noticed that my stage had less than 220 bars. I was surprised, since I always fill to 235 for exploration and we have no leaking tanks, and I remarked this to Deyan. We both wondered why, but then dismissed it and continued. We both missed the first sign that something was strange, but anyone would have done the same. Then I got in the water and immediately noticed a strange taste in my mouth. The thought of bad gas crossed my mind but thinking about it I figured that it was not really possible. Since the second stage regulator I was breathing from had not been used for awhile I assumed that the taste was some kind of mold, or other stale smell… Next we scooter at max speed, since we know precisely where we are going and want to conserve gas. At about 20 minutes into the dive I started getting a headache. A thought again crossed my mind that there could be something with the gas, but I saw that Deyan was perfectly fine and I continued to assume that the tank was filled at our place, so there could not be any such problem. Third very serious sign missed. When we reached our goal I gave my scooter to Deyan and started the survey “on foot”. It is a very long section with more than 50 stations, and it goes relatively deep. It took me about 45 minutes to survey, during which time my headache was getting worse. I am still breathing from the stage tank. At one point I remember noticing that my breathing pattern was not normal. I stopped and thought “why is my breathing so accelerated and deep… that’s very strange.” I still dismissed this, however, deciding that it was because of the exertion, as I have gotten so used to scootering these days that such a long swim was very unusual for me. Fourth sign missed! Now I finish my survey and my stage is half way depleted and is a little below 100 bar (our gas management allowed for this particular stage to be emptied to 50 bar). I switch to my regular nitrox tanks, which were filled on a whim the day before with much richer nitrox mix than what I usually make (!). My headache is bad at this point but the nitrox at this depth stopped whatever was happening and I felt that I was good enough to go on with the plan and lay some new line in a prospective passage. We go on, and everything is normal. At one point the passage became very challenging, the visibility was zero and since my headache was not getting better I finally decided to stop the exploration and head back. At this point we are still deep and the nitrox is keeping things under control. I start to realize that there must be something with the gas after all but I still could not make the connection. I can see that Deyan is OK and I am still sure that our tanks should have been filled as part of the same batch, and we start our exit.

The moment we got to the shallower sections of the cave at 5 meters however and the PPo2 dropped somewhat, I started feeling very weird and was already certain that something out of the ordinary was happening. On my way back I jumped a line without realizing it and had to stop and reverse somewhat just to be sure where I was – and this was in a section of the cave I know by heart and can navigate without any line whatsoever. At this point I am already switching between focus and a totally disoriented state, but having very extensive experience with non-ordinary states of mind from my meditation background I know that I can manage to act coherently despite all this. I knew that I could still function and there was no need to signal distress. I focused, turned the speed of the scooter to the maximum tolerable setting and exited without further incident.

When I got out of the water and thus started breathing normal outside air instead of high pressure nitrox, I started feeling very, very bad, the headache was severe and all I could do was lay down and try to sleep in the sun. I slept on and off for about an hour, slowly regaining my equilibrium and then we went home. At home I breathed some 100% O2, the headache still severe, and we started figuring out what had happened. Our tanks are numbered and we keep meticulous records of every tank filled, the date and everything. We took the notebook out and realized that I had just breathed from the only tank which was not filled at our place and which was filled that day in March at that filling station, and was left full and forgotten! Understandably we got very, very upset.

Now when I am writing this I have been out of the water for 20 hours. My headache is still bad, but it seems that I may live after all. That certainly was a near miss, and it is clear that I have every symptom of CO poisoning. Had I not made a rich nitrox mix and had I not breathed it at depth for a good hour and a half after I switched from the stage, I may have passed out and possibly died there. I missed many repeated signs that something was weird, I dismissed the thought of bad gas on several instances, confident in my own fills, which actually may have saved my life. I had every reason to assume what I assumed, and I can not really blame myself or anyone else, it was a very complex situation.


  1. NEVER, EVER, dismiss the obvious signs of bad gas poisoning, no matter how illogical this may seem. Get out and think it over on dry land!
  2. NEVER trust a filling station which is not fully open on the practices of filter replacement, etc. Insist that you know when the filters were changed. Know what these filters are. Insist that they use Hopcalite in their filters (this is a CO catalyst)! The filling station in question DOES NOT use Hopcalite, for cost reasons. The fact that there has not been a problem for many years means nothing. It takes just one miss in changing the filters for someone to die. If we were at 30 meters with this gas, it would have been RIP.
  3. When filling at commercial filling stations always analyze your gas for CO. Either invest in a CO meter, or insist that the fill station provides one for you. If they refuse, find another filling station.
  4. Remember that a filling station is a commercial operation. They try to minimize costs and to increase profit. Unless the owners have very high integrity, this can often be at the expense of the customer, in every possible way. If you are a dive shop, invest in your own compressor, so that you can have full control over your gas.
  5. Educate yourself about the symptoms and causes of gas poisoning but don’t assume too much and just act conservatively. Although I have excellent knowledge of this and have been teaching this to students for 20 years (and have actually had an instance of gas poisoning before – not diving related), and although I immediately recognized the signs, I assumed (wrongly) that this could not be happening in this particular case. See again point 1.

Ironically, if the bad air had not caused so much fuzz and chaos that day in March, this tank would not have been forgotten at home, but would have been emptied like all the rest. Now we know precisely what happened, but the situation is very unusual for us and many complex factors contributed, as is often the case with such problems.

There is also something else I thought about today, and it is probably relevant to the very few active underwater cave explorers out there and maybe to some technical divers. I realized that from doing daily exploration dives in quite extreme conditions my tolerance for difficult situations under water has gotten quite high. I dived yesterday with strong physical discomfort in a very difficult sidemout cave, exploring a new section in total zero visibility, with complex gas management and I knew that I was still well within the limits of my current ability and skill level. At no point during the dive did I risk my own or my buddy’s safety – psychologically I was still operating very much within my current limits, I could take more than that. I realize now that although this may sound like a good thing, it can actually mask possible problems with bad gas, which can take the situation out of control. We should all know this and think it over very carefully. It can sometimes be difficult to decide. Especially when we are working in teams on complex projects with difficult planning, we probably should not turn a dive out of paranoia just because our nose itches. But we should be mindful of what is happening and we should be informed. Accident analysis is our friend.

It is said that a wise person learns from the mistakes others make, and a fool learns from his own mistakes. In this particular instance I was the fool, but you better be a wise one.

I wish you all safe diving!