Today’s fuels are liquid or gaseous hydrocarbons, possibly with a small proportion of bound oxygen (alcohols), additives, and impurities. An ideal combustion of hydrocarbons oxidizes carbon to carbon dioxide and hydrogen to water. In reality, the exhaust gas from combustion engines contains other pollutants with different health impacts, predominantly as intermediates in incomplete combustion or through the oxidation of natural components of the air (nitrogen) and the fuel (sulfur). These pollutants are primarily particles, carbon monoxide (CO), nitrogen monoxide (NO), nitrogen dioxide (NO2), sulfur oxides (SOx), hydrocarbons, and aldehydes.