How is carbon monoxide and carbon dioxide harmful to living things?

Carbon monoxide is more dangerous. because it is quickly absorbed by the hemoglobin in the blood which then can no longer absorb oxygen from the air. Carbon dioxide merely displaces oxygen in the air but is not absorbed. You can survive in a much higher concentration of Carbon Dioxide than Carbon Monoxide. Think of coal miners trapped for days without fresh air. As long as there is still oxygen in the air you can breath Carbon Dinoxide. Even if there was plenty of oxygen available you would die in an atmosphere containing monoxide because your lungs couldn't use it.