Silent Hunter said..Any gas that displaces air will do. Two lovers died at Lane Cover river in 1963, supposedly from hydrogen sulphide, aka rotten egg gas.
The ABC made a show about it in 2006 called
Who Killed Dr Bogle and Mrs Chandler?
As dad would say, dying in the mud from fart gas while cheating on your wife is no way to go through life son.
Carbon monoxide is more subtle than that. It has orders of magnitude higher affinity to haemoglobin than oxygen does - and then prevents oxygen binding to the haemoglobin, and thus transport to the tissues. Only very small concentrations of carbon monoxide are required to develop severe toxicity.
And then even when you get the victim out of the toxic gas environment, the problem hasn't gone away, as the haemoglobin is effectively taken out of circulation.
Gas displacement can be rapidly fatal through a different mechanism. If you get someone breathing 100% nitrogen, then oxygen in the body will move down its concentration gradient into the lungs and be breathed out - rapid onset of severe hypoxia and death. But give them oxygen and they improve very rapidly.
Hydrogen sulphide is different again - it has direct effects on metabolism, blocking oxidative metabolism of energy sources. At low concentrations, the human nose is exquisitely sensitive. But at high concentrations we cannot detect it
