I got CO poisoned a few years ago when a train I was the conductor on got stuck in a tunnel.
The tunnel in which this took place is nearly a mile long, and at it's construction, was allegedly the largest unsupported tunnel in North America. Consequently, since it was built in 1905, it only has one ventilation shaft, and it's not very effective.
If something happens in this tunnel, we have to immediately cut our locomotives away and get out, or the locomotives will fill the tunnel with smoke. Well, on that day, we had a junk SD70MAC in the lead, and of course, we go into emergency right about the midpoint of the tunnel.
I remember cutting the locomotives off the rest of the train, looking towards the head end, and remember the headlights of the lead locomotive obscured by swirling exhaust. Total time from emergency start to cutting the engines away was under two minutes. I have a hazy memory of climbing up on the steps of the rear locomotive, giving the engineer the go ahead with my light, and feeling really tired. I somehow got up to the head end and made the emergency call on the radio, and the ride in the whambulance to a local hospital. I'd rather never have that happen to me again.
I have plans to install a CO meter in my vehicle, especially when using the fuel fired heater, just in case. A little prevention goes a long way.