I think it's unnecessarily worrying.

When your lungs are full of fresh air, it's 80% N2, 20% O2, and a tiny bit of CO2. When your lungs have absorbed O2 and replaced by CO2, they start to "burn". (Don't know what the percentages are here).

When you've breathed enough of the cockpit air, it will reach the same composition as the "stale lung" air. And your lungs will burn. And you should either roll up or wet exit. I don't think you'll pass out before you feel the burn.

Can the human respiratory system produce CO? I don't think it can; I don't think you'll supersaturate the boat with CO. There shouldn't be any more there
than you started with.

Eating excessive Mexican food will result in the expulsion of COOH!

Always exhale when coming up to prevent an air embolism.

Shawn

: Hi Erez. Personally, I wouldn't want to breathe in C02 while under water.
: But, my understanding is: your lungs will not accept C02/they will accept
: Carbon Monoxide... so Monoxide is far more dangerous. You might try a
: simple test (seated, on dry land) by rebreathing into a balloon to see
: what happens. I haven't tried it but I think your lungs will tell you if
: you are running out of air way before you pass out (could be wrong).

: My first choice by far would be to somehow rig it to let you breath regular
: outside air. Maybe practice in a pool with a longer hose attached to a
: buoy.