Which gas is considered the most dangerous emitted from an internal combustion engine?

Carbon monoxide is considered the most dangerous gas emitted from an internal combustion engine due to its high toxicity and the way it affects human health. This colorless, odorless gas is produced when fuel doesn't burn completely and is particularly hazardous because it can easily be inhaled. Once inside the body, carbon monoxide binds with hemoglobin in red blood cells more effectively than oxygen does, which can lead to oxygen deprivation in vital organs and tissues. This can result in severe health consequences, including headaches, dizziness, confusion, and in extreme cases, loss of consciousness or death.

In contrast, while carbon dioxide is a significant greenhouse gas contributing to climate change, it is not acutely toxic to humans in normal concentrations. Hydrocarbons can also pose environmental risks by contributing to air pollution and smog but do not have the same immediate toxic effects as carbon monoxide. Nitrogen oxides are problematic as well, contributing to respiratory issues and ozone formation, but they are not as immediately life-threatening as carbon monoxide.