Deterministic Effects 3.2 . 3.2.1 Factors Influencing the Clinical Expression of Acute Radiation Injury Early clinical expression of radiation-induced biological damageis influenced by several physical and biological factors. These include radiation type, radiation dose and dose rate, the radiosensitivity of the irradiated tissues, the area of the body irradiated, and variations in individuals’ biological response to radiation. The type of radiation determines its penetrating power, a key factor in considering the clinical consequences of exposure. For present purposes, distinction is made between penetrating radiations (i.e., X and y rays, and neutrons), and those having less penetrating. power(i.e., a and B particles). X and y rays are sparsely ionizing electromagnetic waves emitted, respectively when a metal target is bombarded by electrons in a vacuum, from nuclear fission process or during radioactive decay of fission products. Neutrons are uncharged particles that typically are released in the fission process but that also can be produced in cyclotrons and linear accelerators. They also occur naturally in cosmic radiation. Being uncharged, neutrons do notinteract directly with biological targets. In traveling through tissue they are absorbed by interaction with the nuclei of atoms in the tissue thereby releasing high energy particles that cause ionization of molecular materials. Penetrating radiations can deliver dose to anytissue irrespective of whether the activity is internally or externally distributed. Alpha particles are densely ionizing; they have a penetrating power of only a few microns, equivalent to one or two layers of cells and thus are not a health hazard when external to the body. Beta particles are sparsely ionizing; they may penetrate up to a few centimeters oftissue depending on their energy. When nearor in contact with skin, B radiation can induce acute radiation burns locally but has a limited whole-body effect. Internally deposited B particles can induce local and wholebody effects depending on their energy and distribution. The type, incidence and severity of radiation-induced deterministic effects are directly related to the magnitude of the radiation dose and the rate at which it is delivered. A | single dose “<n delivered in a short period of time(i.e., acutely) will have greater 5 OU 2G?