What deadly substance is found in exhaust gases?

Carbon monoxide is a colorless, odorless gas that is produced during the incomplete combustion of carbon-containing fuels, such as gasoline and diesel, which are commonly used in small motors. When these fuels are burned, especially in poorly ventilated areas, carbon monoxide can accumulate in exhaust gases to dangerous levels. Inhalation of carbon monoxide is particularly harmful because it binds to hemoglobin in the blood more effectively than oxygen, which can lead to serious health issues, including hypoxia, brain damage, and even death.

Other gases listed, such as hydrogen sulfide, nitrous oxide, and methane, do have their own hazards, but they are not typically regarded as the primary deadly substance in exhaust gases produced by small motors. Hydrogen sulfide is more associated with particular industrial processes and is notable for its rotten egg smell. Nitrous oxide, while a greenhouse gas and an anesthetic, is not directly associated with motor vehicle exhaust in a life-threatening context. Methane is a flammable gas but does not have the same acute toxicity and physiological impact on the body that carbon monoxide does. Thus, among the options provided, carbon monoxide is the most relevant and dangerous substance related to exhaust gases.