RADIATION, IONIZING

Electromagnetic waves of extremely short wavelength (X-rays and gamma rays) and accelerated atomic particles (such as electrons, protons, neutrons, and alpha particles) deposit enough localized energy in an absorbing medium to dislodge electrons from atoms with which they interact and to disrupt chemical bonds. The loss of electrons creates particles known as "ions," and these types of radiation are termed "ionizing radiation." Natural sources of such radiation, which are ubiquitous and to which all people are exposed, include cosmic rays, radioactive elements in the earth's crust, internally deposited radionuclides, and inhaled radon. Artificial sources include the use of X-rays in medical and dental diagnosis; radioactive materials in building materials, phosphate fertilizers, and crushed rock; radiation-emitting components of TV sets, smoke detectors, and other consumer products; radioactive fallout from atomic weapons; and nuclear power. Additional sources are encountered by workers in certain workplace environments.

As ionizing radiation penetrates a living cell, it collides randomly with atoms and molecules in its path, giving rise to ions, free radicals, and other molecular alterations that may injure the cell. Any molecule in the cell can be altered by radiation, but DNA is the most critical biological target because of the limited redundancy of the genetic information it contains. A dose of radiation that is large enough to kill the average dividing cell causes hundreds of lesions in the cell's DNA molecules. Most such lesions are reparable, but those produced by a densely ionizing radiation (such as a proton or an alpha particle) are generally more complex and less reparable than those produced by a sparsely ionizing radiation (such as an X-ray or a gamma ray). Any damage to DNA that remains unrepaired or is improperly repaired may result in a mutation or chromosome aberration, and both of these types of effects appear to rise in frequency in proportion to any increase in the dose in the low-dose domain.

Damage to the genetic apparatus may be lethal to cells, especially dividing cells—the depletion of which in a given organ may cause severe damage. In radiation accident victims, for example, the depletion of blood-forming cells in the bone marrow is typically a cause of early death. Although the production of an overt clinical reaction generally requires a dose that is large enough to kill many cells, smaller doses can suffice to cause malformations and other disturbances of development in an embryo. Although adverse health effects have not been demonstrated at the low exposure levels characteristically associated with natural background irradiation, it is noteworthy that at higher dose levels many of the cellular alterations that are precursors to cancer, as well as the risks of some forms of cancer themselves, appear to increase in frequency as linear-nonthreshold functions of the dose.

The risks to human health and to the environment from exposure to ionizing radiation have been reviewed repeatedly by the National Research Council, the National Council on Radiation Protection and Measurements, the International Commission on Radiological Protection, the United Nations Scientific Committee on the Effects of Atomic Radiation, and various other national and international organizations. Such organizations have generally concurred in the conclusion that the existence of a threshold for risks in the low-dose domain cannot be excluded, but that the weight of existing evidence supports the hypothesis that the genetic and carcinogenic effects of radiation increase in frequency as linear-nonthreshold functions of the dose. Assessments of the risks of low-level radiation for public health purposes are, therefore, generally based on the use of linear-nonthreshold dose-response models, their inherent uncertainties notwithstanding. In other words, there is an assumption that there is no threshold for the cancer-causing effects of ionizing radiation and that any increase in radiation exposure causes a corresponding increase in cancer risk.

RADIATION, IONIZING

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