Carbon monoxide
Sorry, this answer is incorrect. Carbon monoxide provides no protecting barrier for the Earth in the stratosphere. It is a dangerous pollutant that may contribute to the greenhouse effect and global climate change. Carbon monoxide is dangerous and life-threatening to humans and other forms of air-breathing life, as inhaling even small amounts can lead to neurological damage and possibly death. Carbon monoxide outcompetes oxygen when binding to hemoglobin and therefore causes oxygen starvation throughout the body.