Forgive me if I jump on this bandwagon...it's just the mention of swimming that's the trigger.
A classmate once defended his smoking to me by saying how well he could swim. I argued with my usual passion (having seen 30 years of my mother's life ruined by cigarettes.). Eventually I gave up because of his determined opinion that being able to swim the length of a large pool underwater, meant that not a lot could be wrong. He died in Colchester's St Helena Hospice age 58. Yep, lung cancer.
Back to noxious gasses.
I had a job training new crews in Cardiff many years ago, but every so often, I would have to jump into a Navajo and single crew it to Harwarden and back. Lovely flight, but it was always spoilt by a strange feeling. Slightly giddy, a tad vague, it really fazed me at the time. I went for a full medical and the aircraft was thoroughly checked. Nothing was ever found.
To this day I think that I must have been susceptible to smaller traces of Co2 than my colleagues, as I had flown jet transport, un-pressurized twin turbo prop and the Navajo, all in quick succession. No problems on the other types.