Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel, fuel oil or coal. It is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.
It often disperses downwind in a pattern called an exhaust plume.
Although the largest part of most combustion gases is relatively harmless nitrogen (N2), water vapor (H2O) (except with pure-carbon fuels), and carbon dioxide (CO2) (except with hydrogen as fuel), a relatively small part of it is undesirable noxious or toxic substances, such as carbon monoxide (CO), hydrocarbons, nitrogen oxides (NOx), partly unburnt fuel, and particulate matter.
Main article: Automobile emissions control
Exhaust gas from an internal combustion engine whose fuel includes nitromethane, contains nitric acid vapour, which when inhaled causes a muscular reaction making it impossible to breathe, and people exposed to it should wear a gasmask.http://www.turbofast.com.au/racefuel6.html
Diesel particulate matter is the main article about diesel exhaust.
Here, conditions in the engine are different from in a spark-ignition engine, because power is controlled by controlling the fuel supply directly, not by controlling the air supply. As a result, when the engine is running at low power, there is enough oxygen present to burn the fuel, and diesel engines only make significant amounts of carbon monoxide when running under load. Diesel exhaust is well known for its characteristic smell; but in Britain this smell in recent years has become much less (and diesel fuel more expensive) because the sulphur is now removed from the fuel in the oil refinery.
In steam engine terminology the exhaust is steam that is now so low in pressure that it no longer can do useful work.
Emission standards focus on reducing pollutants contained in the exhaust gases from vehicles as well as from industrial flue gas stacks and other air pollution exhaust sources in various large-scale industrial facilities such as petroleum refineries, natural gas processing plants, petrochemical plants and chemical production plants. 
One of the advantages claimed for advanced steam technology engines is that that they produce smaller quantities of toxic pollutants (e.g. oxides of nitrogen) than petrol and diesel engines of the same power. However, there is a downside – they produce larger quantities of carbon dioxide.