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In common usage, emission is most often the giving off of gases from industrial processes and the engine exhausts of transport vehicles (automobiles, trucks, airplanes, trains and ships). As they occur on an industrial scale, even relatively harmless gases can have an undesired effect (such as carbon dioxide contributing to the greenhouse effect).

In chemistry, emissions are the products of a reaction, either chemical or nuclear.

In physics and physical chemistry, emissions are outputs of electromagnetic radiation or particles.

Also in physics, emission theories assumes that light leaves the object that emits it at a particular speed. This idea is most commonly identified with Isaac Newton and Walter Ritz.

In the history of optics, light was supposed by adherents of emission theory (vision) to be emitted by the eyes. Visual perception was accomplished by such rays of light acting like feeling hands.

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Air pollution

AP 42 Compilation of Air Pollutant Emission Factors

Atmospheric pollution dispersion modeling

Automobile emissions control

Emissions trading

Greenhouse gas

Pollution