Jet engines are rather commonplace; most people in the U.S. have flown on a commuter plane at some point in time. However, the exhaust released by jet engines has been tested by scientists such as Karleen Boyle at UCLA, and it's been found to be amazingly hazardous not only to the environment, but to human health as well. This is mostly due to the particulates, or extremely small particles, combined with harmful gases that jet engines pump out of their exhaust valves.
Greenhouse Gases
First and foremost, jet engines release a large number of greenhouse gases as a result of burning jet fuel. Carbon dioxide (which has been labeled as a greenhouse gas) is only one of them. However, unlike automobiles or power plants that release their gases on the ground, jet engines are usually releasing their chemicals in the sky. This means that there's no opportunity for nature to absorb and clean out these gases, and they go directly into the atmosphere, and higher areas of our air, where they can do more damage.
Particulates
Particulates are extremely small particles of material that can do damage not only to the environment, but to people who inhale them. Jet exhaust contains a number of heavy metals, such as mercury, which are poisonous to living organisms, including both animals and humans. The particulates released by jet engines in their exhaust are so small that they can be inhaled by people, and the particulates will penetrate the walls of the lungs and enter the body. This can cause a number of health issues and may eventually result in a person's death if enough particulates build up.
Heat
One of the more immediate dangers of jet exhaust is that it's extremely hot. This heat often dissipates into the atmosphere, and if there's no one around to be hurt by it, then it doesn't make any difference. However, if people, animals or even plants are exposed to the intense heat that jet engine exhaust can generate, it can lead to injury or death. This is one of the reasons that airports need to have so much space around the jet liners that land and take off. To see this in action, watch a plane with a jet engine: the trail it leaves behind is partially generated by the huge amount of heat that the exhaust creates, making the steam trails.
Tags: extremely small, extremely small particles, heat that, into atmosphere, only environment, released engines