Thank you for your inquiry to Environment Canada. We receive numerous public enquiries concerning the composition and appearance of contrails and whether or not contrails pose a public health risk. Recent years have seen the emergence of popular opinions often carried by the media and seen on many websites suggesting that these visible contrails from planes are chemically laden. Hence, the term chemtrail. However, there is currently no scientific evidence to support the existence of chemtrails and Environment Canada cannot provide any scientific evidence.
The phenomena that you are referring to are commonly called aircraft condensation trails or contrails. Airplane contrails are long white condensation trails made up of ice crystals left behind by aircraft flying primarily at high altitudes. They form when hot water vapour produced by the burning of aviation fuel cools rapidly as it comes into contact with the much colder air. The airplane's exhaust increases the amount of moisture in the air. As the moist air cools it reaches saturation and the water vapour sublimates to form ice crystals. The cooling of the air and subsequent formation of ice crystals is very rapid so that the contrails appear almost immediately behind the aircraft. This process is analogous to seeing ones breath on a winter day. Because visible contrails are composed primarily of water/ice, they do not pose a health risk to humans. The appearance of contrails depends on many factors including the prevailing atmospheric conditions, type of aircraft and engine, wind direction, altitude and amount of sunlight. If the air at high altitude is very dry, the condensation trails will dissipate rapidly. If the air at high altitude is relatively moist, the contrails typically last from minutes to hours. Under certain atmospheric conditions, contrails can linger in the air for more than a day and usually grow wider and more dissipated over time. Some of these contrails change in appearance by the presence and the strength of the upper level winds. If the wind is very strong, contrails will move away from the area where they were formed, often changing in appearance and opacity as they are moving. Contrails can sometimes appear to stop and start as the aircraft encounters areas in the upper atmosphere that contain different amounts of moisture. The paths of aircrafts criss-crossing each other may explain the unusual appearance of contrails. Increase in air traffic in the last two decades has inevitably caused an increase in the number of visible contrails in the sky, especially in the vicinity of larger airports.
With respect to aircraft emissions, aircraft engines and fuel have very stringent emissions standards set by national and international organizations. To minimize the impact of aircraft emissions on the environment, these organizations constantly update their standards to ensure that aircraft engine manufacturers use the best available technology. In Canada, aviation fuel specifications are controlled by the Canadian General Standards Board (CGSB) which includes fuel suppliers, fuel users and other stakeholders. CGSB sets limits on all important fuel parameters and prescribes test methods for ensuring consistent fuel quality. Aviation fuel such as petrol/gasoline (piston engines) or paraffin/kerosene (jet engines) consists primarily of hydrocarbons. When the fuel is burned, the carbon combines with oxygen to form carbon dioxide; the hydrogen also combines with oxygen to form water, which emerges as steam (i.e. water vapour) in the exhaust. There have been no new fuel additives in recent years that could suggest that new foreign substances are being injected into the atmosphere by aircraft engines.
Finally, the web sites that discuss chemtrails contain numerous suggestions that chemtrails are a form of weather or climate modification. Under the Weather Modification Information Act, any weather modification experiments legally require that Environment Canada be formally notified. Environment Canada has no such notification from any group, and has no knowledge of any weather modification experiments related to aircraft contrails.
In conclusion, there is no scientific evidence, data or information related to aircraft emissions that supports the idea that contrails from aircraft are chemically laden. Similarly, there is no evidence that aircraft contrails pose a health risk. I hope that this information adequately addresses your concerns. (less info)