Diesel exhaust is produced by the combustion (burning) of diesel fuel. The exhaust is a complex mixture of gases, vapours, aerosols, and particulate substances. The exact nature of the exhaust depends on a number of factors including the type of engine, how well serviced/maintained the engine is, type of fuel, type of oil, speed and load on the engine, and emission control systems. Diesel exhaust may contain:

• Carbon (soot)
• Carbon monoxide
• Carbon dioxide
• Oxygen
• Water vapour
• Ammonia
• Nitrogen
• Oxides of nitrogen
• Oxides of sulphur
• Alcohols
• Aldehydes
• Ketones
• Hydrocarbons
• Aromatic compounds such as benzene, toluene, and polycyclic aromatic hydrocarbons (PAHs)