COMMENT: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, then sought treatment, recovered fully and resumed diving.
DAN is currently helping divers who think they may have experienced breathing gas contamination by supplying them with a breathing gas analysis test kit. If you have experienced an incident possibly related to breathing contaminated gas and have lawful control of the tank, please preserve your tank and contact DAN Research by phone (919-684-2948, ask for the Research department) during regular office hours (8:30am-5:00pm EST) or through the online incident reporting system.
~ Peter Buzzacott, MPH, PhD