Can you tell me how is carbon monoxide poisonous ?

Stops oxygen use: Carbon monoxide binds the the part of red blood cells that carry oxygen. As more of those binding sites get clogged with the CO less oxygen is carried to the body's organs. The binding is reversible so once the exposure stops the CO will fall off the hemoglobin after several hours and the red cells will work properly. That's why the treatment for exposure is to give 100% oxygen.