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All energy is radiation. There are two types, known as ionizing and non-ionizing radiation, and both are omnipresent on Earth. The characteristics and differences between both ionizing and non-ionizing radiation are important to understand, given both the potential harm and usefulness of radiation upon the human body. While both are potentially harmful, ionizing radiation is more dangerous than non-ionizing radiation, but ionizing radiation has several medical benefits as well.
Ionization is the process by which electrons are removed from their orbit around a particular atom, causing that atom to become charged, or ionized. This process can occur when radiation of sufficient strength interacts with normal atoms. Radiation that is not powerful enough to trigger this process is known as non-ionizing, and is capable instead of simply exciting the movement of atoms and heating them up. The division between ionizing and non-ionizing radiation occurs in the ultraviolet (UV) range, which is why that range is split into UV-A and UV-B rays, and the latter is more powerful and dangerous.
Examples of non-ionizing radiation include infrared, microwaves, and light along the visible spectrum. Just because it does not strip electrons from atoms does not mean non-ionizing radiation is harmless. It is still capable of exciting atoms and in turn heating them up. This is the theory behind microwave ovens, and human biological tissue is not fundamentally exempt from this effect. Exposure to types of non-ionizing radiation whose wavelengths are smaller than the body can lead to dangerous burns. This is why exposure to the sun's rays causes the skin to cook and eventually burn.
Though it does not generate heat, ionizing radiation is even more dangerous than non-ionizing to living tissue. By fundamentally changing the chemical makeup of an atom, this kind of radiation can cause molecular damage and the unchecked cellular growth known as cancer. If exposed to human reproductive organs, ionizing radiation can also lead to future birth defects in unborn children.
The sun produces both ionizing and non-ionizing radiation. Though the sun is responsible for a great deal of the naturally occurring radiation a human may be exposed to, only a small fraction of that which reaches the surface of the Earth is ionizing. In fact, it is radon gas that is estimated to contribute the greatest percentage of ionizing radiation that is absorbed by humans, followed by other radioactive elements like plutonium and radium, which occur in rock formations and other geologic features.
Ionizing radiation does possess valuable properties, however, and has proven vital in the field of healthcare. Medical imaging, including X-rays and magnetic resonance imaging (MRIs), both rely on small doses of man-made ionizing radiation. Radiotherapy is used to treat conditions, including cancer, by obliterating targeted areas of tissue. Unsurprisingly, the same dangers that occur from natural radiation are present with the manufactured kind, and side effects from high doses of radiation treatment can be serious in and of themselves.