tracted or fractionated doses are less than acute doses because the numbers of cells being killed by the radiation over time will be less than the number of new cells being produced in the body's tissue systems during the same period and because repair of radiation injury occurs within most cells. Distribution of doses over long time periods can occur with external exposures and when long-lived radionuclides are deposited inside the body. The likelihood of alpha and beta emitters deposited inside the body causing generalized symptoms of radiation exposure early in the postexposure period is minimized by their limited penetrating power, which restricts their biological effectiveness to nearby cells.

Another key factor in the body's response to ionizing radiation is the relative sensitivity to radiation of the various cell types that comprise body tissues. Bergonié and Tribondeau's Law (1906) implies that rapidly dividing cells (e.g., cells of the blood forming tissues and certain groups of immature sperm cells) are among the most sensitive to acute effects of radiation. The more highly differentiated cells (e.g., muscle and nerve cells) are less vulnerable to acute injury by radiation. Other factors that influence the expression of the deterministic effects of radiation include the region of the body irradiated and variation between individuals in their physiologic response to radiation.

A small group of deterministic effects tends to appear beyond the characteristic early (2 month) postexposure period. This group reflects irreparable DNA damage incurred at the time of exposure and subsequent cell death. It includes cataracts, infertility in males and females, suppression of thyroid gland function, and fibroatrophy as a consequence of radiation-induced damage to connective tissue and blood vessels. These effects are associated with practical threshold doses that are typically higher than those of concern in this report.

Of special concern in the modern military would be the radiation-induced damage that could occur in the embryo or fetus as the result of the inadvertent exposure to radiation of a pregnant soldier. A dose of greater than 50 mSv (5 rem) to the embryo or fetus is associated with an increase in risk (relative to the nonexposed) of nonspecific deterministic effects in the forms of embryonic