&gt; Or carbon monoxide<p>You don&#x27;t even need CO, any inert gas other than CO2 will do as long as you remove the oxygen (nitrogen is convenient as the atmosphere is already mostly nitrogen)<p>&gt; the body thinks it&#x27;s oxygen so there&#x27;s no sense of being asphyxiated.<p>CO does compete with O2 which kills you, but the reason the body doesn&#x27;t react is it basically doesn&#x27;t react to anything other than CO2 blood levels. You can asphyxiate with argon, butane, nitrogen, helium, krypton, etc… and not realise it as long as CO2 partial pressure doesn&#x27;t rise.