It could have been 98% Air and 2% CO for all we know...
And it could have been 1 percent kryptonite mixed with vaporized rat ****... however, that premise is illogical... as is carbon monoxide poisoning since that was not represented by his reported behavior and nor did CO poisoning present its quite obvious signs to the medical crew who transported him to hospital.
Therefore, the conclusion that he breathed oxygen at great depth and suffered a classic tonic/clonic episode is a logical consequence of the original premise: poor gas operations/management