Exhaust gas

Exhaust gas, also called flue gas, is the gas discharged from an engine or combustion process through an exhaust pipe, stack or propelling nozzle, and once released it often disperses downwind in a visible pattern called an exhaust plume. It results from burning fuels such as natural gas, gasoline, diesel, fuel oil, biodiesel blends or coal, and motor vehicles and stationary internal combustion engines are its most common sources, alongside crankcase blow-by and fuel evaporation. Most combustion gas is relatively harmless, consisting mainly of nitrogen, water vapor and carbon dioxide, but a smaller share carries pollutants produced by incomplete combustion or high temperatures, including carbon monoxide, unburned hydrocarbons, nitrogen oxides and soot particulates. Air pollution from burning fossil fuels is estimated to kill more than five million people worldwide each year, and motor vehicle exhaust is a major contributor to urban smog.