[reconeil]

Boats,
Sorry for all your health issues and problems, Youngster (got you by about 10)
Neil

Regardless, I dug up more on the computer about how good long time breathing in ones own exhaling (carbon dioxide) is good for you.

Here's something from: Lambertsen, C. J. (1971). "Carbon Dioxide Tolerance and Toxicity". Environmental Biomedical Stress Data Center, Institute for Environmental Medicine, University of Pennsylvania Medical Center. IFEM. Philadelphia, PA. Report No. 2-71.

Too Much Carbon Dioxide Is Toxic

However, if you breathe high concentrations of carbon dioxide or re-breathe air (such as from a plastic bag or tent), you may be at risk for carbon dioxide intoxication or even carbon dioxide poisoning. Carbon dioxide intoxication and carbon dioxide poisoning are independent of oxygen concentration, so you may have enough oxygen present to support life, yet still suffer from the effects of rising carbon dioxide concentration in your blood and tissues.

The condition of excess carbon dioxide concentration in the blood is called hypercapnia or hypercarbia. Symptoms of carbon dioxide toxicity include high blood pressure, flushed skin, headache and twitching muscles. At higher levels, you could experience panic, irregular heartbeat, hallucinations, vomited and potentially unconsciousness or even death.