Man dies in an atmosphere of carbon monoxide, because it

Carbon monoxide is poisonous gas
Since it combines with haemoglobin of blood and forming the stable carboxy haemoglobin complex irresveresibly hence the oxgyen transportation by haemoglobin decreases . Hence man dies in an atmoshere of high concentration of carbon monoxide
The anti dote used for the poisonous effect of carbon monoxide is carbogen. carbogen is a mixture of 

carboxy haemoglobin is 300 times more stable than Oxy haemoglobin