On Wed, 02 Sep 2009 09:51:05 +0000, SpamTrapSeeSig wrote:

Methane may not be actually poisonous (unlike CO), but it will
still asphyxiate.

Correct.

Oxygen is transported around the body by haemoglobin in the blood stream.

Carbon monoxide binds more readily and more tightly to haemoglobin than
oxygen so that at sufficient levels, though very much lower than needed
for asphyxiation alone with other gases, the body is deprived of oxygen
and dies.

From http://www.ebi.ac.UK/interpro/potm/2005_10/Page2.htm

QUOTE

Carbon monoxide (CO) binds to haemoglobin with a higher affinity (200x
greater) than oxygen, and at the same binding site.

Consequently, carbon monoxide will bind haemoglobin preferentially over
oxygen when both are present in the lungs - even small amounts of carbon
monoxide can dramatically reduce the ability of haemoglobin to transport
oxygen.

Levels as low as 0.02% carbon monoxide can cause headaches and nausea,
while a concentration of 0.1% can lead to unconsciousness.

....

People who smoke heavily can block up to 20% of the oxygen binding sites
in haemoglobin with carbon monoxide

....

By contrast, carbon dioxide (CO2), which is produced as a waste product
after aerobic respiration, binds to haemoglobin at a different site,
therefore does not compete with oxygen for binding to haemoglobin.

UNQUOTE