As radiation passes through living tissue, energy is deposited that ionizes and excites atoms in the cells. Ionization, the removal of an electron from the atom, and excitation, the elevation of an electron to a higher energy state in an atom, can damage molecules in a cell.
If an ionization or excitation occurs in an atom that’s part of the DNA, the DNA may be damaged so that normal cell functions are disrupted or the cell itself is destroyed. The resultant biological effect depends on whether or not the cell can repair the damaged molecule, the molecule’s importance to the cell’s health and functioning, the radio-sensitivity of the tissue, and how much molecular damage the radiation has caused overall in the cell.
The biological effects of ionizing radiation depend on the dose, that is, the amount of energy deposited in the tissue or organ. In general, as the dose increases, the biological damage increases. Stem cells of a uniform type are significantly more sensitive to radiation than differentiated cell types. For example, the stem cells responsible for the production of red blood cells are much more radio-sensitive than are cells in the muscles and nerves.
The health effects of radiation fall into two categories: deterministic and stochastic. Deterministic effects are those effects that follow radiation doses greater than 100 rem to the entire body or to a limited portion of the body. The severity of the damage increases with the dose, but deterministic effects have a threshold dose below which no damage is apparent. Cataract formation, skin reddening, sterility, reduction in blood-cell counts are all examples of deterministic effects that occur at a dose of 200 to 300 rem.
| The average amount of radiation each person receives is about 0.3 rem annually, and it comes from cosmic rays, radon, radioactive materials in the ground and in our bodies, and medical X-ray examinations.
Radiation is measured in different units, such as Roentgen, rad, rem, Gray, and Sievert. The Roentgen is a unit of exposure to ionizing radiation. Equal doses of different types of radiation, like cosmic particles, electrons, or gamma rays, cause different amounts of damage at the same absorbed dose (measured as rads or Grays). The damage levels are accounted for by multiplying the dose by a weighting factor and then expressing the dose in terms of rems or Sieverts.
For whole-body irradiation up to approximately 50 rem, most individuals will not experience any noticeable effect. At a dose in the range of 50 to 250 rem, individuals may experience fatigue, fever, nausea, vomiting, chromosomal aberrations and other symptoms. Most individuals exposed to this dose range survive with appropriate medical care.
At doses higher than 250 rem, most of the blood-forming stem cells are destroyed, and an individual may die within several weeks of anemia or infection due to the severe suppression of the immune system.
At whole-body doses greater than 500 rem, cells of the gastrointestinal tract are destroyed, and infection and GI-tract bleeding occurs. Death follows within a week. Even higher doses affect the central nervous system, and death results within days or hours from impairment of the central nervous system.
Stochastic effects are biological effects that occur randomly in the human population, such as cancer and birth defects. The probabilities of these effects increase with increasing radiation dose, but there’s no threshold below which no effect occurs.
Stochastic effects are believed to occur at doses as low as 1 rem. Since cancer and genetic effects may not be evident until years after irradiation, they are called late effects. Much is known about the carcinogenic and genetic effects of radiation; for additional information, see reports from the National Academy of Science and National Council on Radiation Protection.