Pollutants Vehicles Emit
This section describes common pollutants that vehicles emit. For information on the health impacts of these pollutants, see How Vehicle Emissions Affect Us, and to see how different transportation options compare, see Air Pollution and Greenhouse Gas Emissions from Different Modes of Transportation.
- Nitrogen oxides (NOx)
- These are created during combustion. Vehicle engines burn a small proportion of the nitrogen that is present in the air plus nitrogen compounds found in vehicle fuels. Diesel engines generally produce much larger amounts of NOx than gasoline engines due to the higher combustion temperatures.
- Fine Particulate Matter (PM2.5)
- Some of these tiny particles are formed during combustion (primary PM). Others are formed in the atmosphere through chemical reactions between the various pollutants found in exhaust (secondary PM). PM2.5 may contain many substances including metals, acids, carbon, and polycyclic aromatic hydrocarbons. Diesel engines emit far greater amounts of PM than do gasoline engines.
- Volatile organic compounds (VOCs)
- VOCs are a large class of carbon-containing compounds. In vehicle exhaust, VOCs come from unburned or partially-burned fuel. Additional VOC emissions come from evaporation of fuel (particularly during refuelling). Gasoline engines emit a higher proportion of VOCs than diesel engines, due to the greater volatility of the fuel.
- Carbon Monoxide (CO)
- CO results from the incomplete combustion of vehicle fuels. Gasoline engines emit a higher proportion of CO than diesel engines, due to the lower combustion temperature.
- Sulphur Dioxide (SO2)
- SO2 is emitted from the combustion of sulphur contained in the fuel. Most SO2 is from diesel engines as diesel has much more sulphur than gasoline.
- Air Toxics
- Vehicles emit toxic air pollutants such as benzene, 1,3-butadiene, acrolein, formaldehyde and polycyclic aromatic hydrocarbons (PAH). Some of these components are VOCs, while others are contained in particles.
- Older vehicles may have air conditioning systems using Freon, an ozone depleting substance, as a refrigerant. This Freon could be emitted through leaks, or during repairs. Newer vehicles use non-ozone-depleting coolant. The coolants in newer vehicles are still pollutants as they act as greenhouse gases.
- Other emissions
- The production, distribution, storage and marketing of transport fuels also cause air pollution emissions. An example is the emission of hydrocarbon vapours during refuelling of vehicles.