Radiation is present, usually in low levels, nearly everywhere in the environment; this widespread radiation is known as background radiation and is usually not detrimental to life. Radiation in the environment is, for the most part, completely natural and comes from natural environmental sources rather than from the actions of humans. There are radioactive substances in the Earth's crust, some of which are even mixed in the soil in small quantities. Radioactive gases also exist in the air in small quantities; these gases tend to come from the Earth as well. Trace amounts of radioactive substances can even be found in the human body.
Radiation is a process through which energy is released by one object and absorbed by another. In many cases, this is a harmless process, and it occurs on a huge scale all the time. Ionizing radiation occurs when subatomic particles released from an object are energetic enough to remove electrons from the atoms contained in another object; this can be harmful. Ionizing radiation comes from such things as nuclear weapons and nuclear power plants.
There are many sources of background radiation in and on the Earth, much of which is consumed by the Earth's organisms, including humans, with little detrimental effect. Background radiation can also come from space in the form of cosmic rays. Many of the cosmic rays that penetrate the atmosphere come from the sun, though there are many more distant sources that also send cosmic rays. Such rays are typically not harmful, but extended exposure could lead to skin problems.
Much of the background radiation found in the atmosphere exists because radon gas, which is radioactive, enters the atmosphere from its source in the Earth's crust. It then attaches itself to various airborne particles which can be harmful in large quantities. Uranium miners, for example, have been known to get lung cancer because of radon exposure. Radon also has been said to have some medical and therapeutic uses, though these are controversial because of the risks inherent in exposure to significant amounts of the gas.
While most background radiation does come from natural environmental sources, a small percentage of the radiation in the environment can be attributed to human activities in science and industry. Small sources, such as radioluminescent paints, contribute to background radiation only to a small degree. The use and testing of nuclear weapons, however, has resulted in an increase in global levels of background radiation. Nuclear power plants also contribute to environmental radiation, as does the recycling of nuclear fuels. Even radioactive medical devices, such as x-rays, contribute to background radiation levels to a small degree.