A Diver Feels Unwell at Depth

A diver suddenly starts feeling very unwell, ascends and breaths oxygen for symptom relief.

Reported Story

Five minutes into the dive and at 50 fsw (15 msw) I started not feeling well. I was getting confused, nauseated and tunnel vision started. No matter how hard I inhaled I felt I was not getting enough gas (I was breathing Nitrox). I managed to get my breathing under control, ended the dive and made a controlled ascent to the surface with my instructor right beside me. During this dive I was taking a dive rescue course. It took a lot of will power not to take my regulator out and bolt for the surface. There was nothing wrong with my gear or the quantity of gas, tank was fully on (we checked later). After surfacing, we returned to our villa and I was put on oxygen for an hour. I felt much better and cautiously returned to diving the next day.


There are a number of possibilities to explain this diver suddenly feeling very unwell at depth. On further questioning, some of the possibilities were deemed less likely candidates but it appears that excess carbon dioxide (CO2) could not be ruled out, nor carbon monoxide (CO) or hydrocarbon poisoning.

CO2 can be generated within a diver, such as by “skip breathing”, working excessively, using a regulator with a high breathing resistance, wearing too tight a wetsuit and/or a number of other mechanisms, or it can be delivered to a diver, for example in the tank, by standing next to an engine exhaust, etc. Oxygen is not necessarily the recommended treatment for excess CO2, breathing air may just as effectively wash out the CO2, but it would not normally make the diver’s condition worse.

Oxygen is the recommended treatment for CO poisoning though, which may present with similar symptoms. CO is not produced by divers and can only be introduced from an external source. The same applies to hydrocarbon poisoning, which may be introduced by the compressor used to fill a tank.

We may never know what caused this diver to suddenly feel unwell underwater but, regardless, he kept a cool head, ended the dive, made a controlled ascent, and then sought treatment, recovered fully and resumed diving.

Peter Buzzacott, MPH, Ph.D.