Radiation and the Human Body

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Ionizing Radiation

Ionization is the ejection of one or more electrons from an atom or molecule to produce a fragment with a net positive charge (positive ion). The classification of radiation as "ionizing" is essentially a statement that it has enough quantum energy to eject an electron. This is a crucial distinction, since "ionizing radiation" can produce a number of physiological effects, such as those associated with risk of mutation or cancer, which non-ionizing radiation cannot directly produce at any intensity. 电离是指一个原子或分子失去一个或多个电子，从而产生带正电的碎片（正离子）。将辐射分类为“电离辐射”本质上是说明它具有足够的量子能量来击脱电子。这是一个关键的区别，因为“电离辐射”可以引起多种生理效应，如与突变或癌症相关的效应，而非电离辐射在任何强度下都无法直接引起这些效应。

Comparison with non-ionizing radiation 与非电离辐射的比较

The mechanisms of interaction for ionizing radiation in the form of x-rays and gamma-rays include the photoelectric effect, Compton scattering and at high enough energies, electron positron pair production.

Although the precise ionization energy differs with the atom or molecule involved, a general statement is any radiation with quantum energy above a few electron volts is considered to be ionizing radiation. The threshold for ionization lies somewhere in the ultraviolet region of the electromagnetic spectrum, so all x-rays and gamma-rays are ionizing radiation. All forms of nuclear radiation are also ionizing radiation because of their extremely high energies.

Non-Ionizing Radiation

Ionization is the ejection of one or more electrons from an atom or molecule to produce a fragment with a net positive charge (positive ion). In the electromagnetic spectrum, radiation in the visible or longer wavelength range does not have sufficient quantum energy to ionize an atom, so we classify it as non-ionizing radiation. The threshold for ionization occurs somewhere in the ultraviolet range, with the specific threshold depending upon the type of atom or molecule. It typically takes a photon with energy in the range of a few electron volts to ionize an atom.

If an atom absorbs a photon of electromagnetic radiation and remains intact, there is a strong tendency for it to return to its ground state. Just as water runs downhill, all physical systems will tend to move to lower energy levels. If the quantum energy of the radiation absorbed is higher than the average thermal energy of the molecules (that is, infrared or visible radiation), then the downward transitions may emit radiation that leaves the material, or it may be gradually transformed into general thermal energy in the material. Radiation in the microwave or longer wavelengths generally just contributes to the random molecular motion which we have described as thermal energy.

The net result of the absorption of non-ionizing radiation is generally just to heat the sample. Of course if it heats it enough, then chemical changes are likely to occur, but those chemical changes would be expected to be the same changes that would occur as a result of any other source of heating.