minutes, hours or days. In somecell systems, acute radiation injury may not become clinically evident for several weeks, months or years post-exposure. The various types of radiation injury can occur alone, in combination with each other, or with physical trauma or with other medical conditions or complications of the injury. The clinical effect of such combined injuries has been shown to be synergistic so that the acute response to a given whole- or partial-body dose ofradiation is apparently greater in the presence of other radiogenic or non-radiogenic injuries or in complications such as infection, than it would have been if received alone (28). For the purposes ofthis report, the following topics are addressed in the context of acute (high dose, high dose-rate) exposures to radiation: 3.2.2.1 Whole-Body Exposures Exposures in excess of threshold levels to the whole-body or substantial portions ofit cause irreversible biological damage that is expressed in a group of dose-related signs and symptoms that comprise the Acute Radiation Syndrome (ARS), sometimes referred to as “acute radiation sickness." The ARSis characterized by an acuteillness that follows a four-phase clinical course. The severity and duration of each phase are inversely related to radiation dose. The prodromal phase (Phase I) is characterized by symptoms that result from acute cell death and from effects on the gastrointestinal and central nervous systems mediated by direct injury to the parasympathetic nervous system. Symptoms include fatigue, lassitude, anorexia, nausea and apathy. At higher doses vomiting, diarrhea, hyperexcitability, ataxia, erythema, perspiration and fever can occur. Radiation-induced conjunctivitis has a threshold of approximately 200 mGy to the eye. During this phase the earliest detectable clinical signs are of bone marrow depression beginning at doses of0.51.0 Gy with the absolute lymphocyte and granulocyte counts being the parameters of interest. The developmentof signs and symptoms during the prodromal phase serve as a basis for radiological triage and assessmentof medically stable patients. Thelatent period (Phase IT) is characterized by the disappearance or decreased severity of the prodromal symptoms and an apparent improvementin the patient’s well-being. At lower doses the patient can proceed to recovery. At higher doses cell systems becomeincreasingly