Most excess carbon monoxide comes from gasoline vehicles. While exhaust from gasoline engines consists mostly of non-toxic gases (nitrogen, carbon dioxide and water), it contains some carbon monoxide. Older vehicles without catalytic converters are the worst carbon monoxide emitters. Newer vehicles with a catalytic converter in the exhaust system are much better because it changes most of the carbon monoxide to carbon dioxide.
Oxygenated gasoline can reduces carbon monoxide emissions. Engines emit more carbon monoxide when they are fed rich air/fuel mixtures -- mixtures with more fuel than oxygen in the mixture. Rich air/fuel mixtures are used during engine startup and warm-up and at full throttle. Oxygenated gasoline requires less oxygen (from the air) for complete burning than the same volume of conventional gasoline. Adding oxygenate is like adding more air. So, for the same carburetor or fuel injection setting, changing an engine's fuel from conventional gasoline to oxygenated gasoline produces a l