Radiation (Ionizing)
Radiation poisoning is a rare but challenging condition. Dependence on nuclear energy and the expanded use of radioactive isotopes in industry and medicine have increased the possibility of accidental exposures. Ionizing radiation is generated from a variety of sources. Particle-emitting sources produce beta and alpha particles and neutrons. Ionizing electromagnetic radiation includes gamma rays and x-rays. In contrast, magnetic fields, microwaves, radio-frequency waves, and ultrasound are examples of nonionizing electromagnetic radiation.
Management of a radiation accident depends on whether the victim is contaminated or only irradiated. Irradiated victims pose no threat to health care providers and can be managed with no special precautions. In contrast, contaminated victims must be decontaminated to prevent the spread of radioactive materials to others and the environment.
A terrorist “dirty bomb” (dispersion bomb) will likely contain commonly acquired radioactive materials such as the following: americium (alpha emitter, found in smoke detectors and oil exploration equipment); cobalt (gamma emitter, used in food and mail irradiation); iridium (gamma emitter, used in cancer therapy); strontium (gamma emitter, used in medical treatment and power generation); and cesium (gamma emitter, used to sterilize medical equipment and for medical and industrial uses). Psychological effects (eg, panic) may overshadow medical concerns because significant acute radiation exposure by contamination is generally confined to the immediate blast area. Long-term exposure may increase the risk for cancer while adequate decontamination can be problematic, potentially making the blast area uninhabitable.
Mechanism of toxicity
Radiation impairs biological function by ionizing atoms and breaking chemical bonds. Consequently, the formation of highly reactive free radicals can damage cell walls, organelles, and DNA. Affected cells are either killed or inhibited in division. Cells with a high turnover rate (eg, bone marrow and epithelial coverings of skin, GI tract, and pulmonary system) are more sensitive to radiation. Lymphocytes are particularly sensitive.
Radiation causes a poorly understood inflammatory response and microvascular effects after moderately high doses (eg, 600 rad).
Radiation effects may be deterministic or stochastic. Deterministic effects are associated with a threshold dose and usually occur within an acute time frame (within a year). Stochastic effects have no known threshold and may occur after a latency period of years (eg, cancer).
Toxic dose. Various terms are used to describe radiation exposure and dose: R (roentgen) is a measure of exposure, whereas rad (radiation absorbed dose) and rem (radiation equivalent, man) are measures of dose. In the United States, rad is the unit of radiation dose commonly referred to in exposures, whereas rem is useful in describing dose-equivalent biological damage. For most exposures, these units can be considered interchangeable. The exception is alpha particle exposure (eg, plutonium), which causes greater double-stranded DNA damage and a higher rem compared with rad. The International System of Units (SI units) has largely replaced the rad and rem nomenclature. For conversion purposes, 1 gray (Gy) = 100 rad and 1 sievert (Sv) = 100 rem.
Toxicity thresholds
Acute effects. Exposure over 75 rad causes nausea and vomiting. Exposure over 400 rad is potentially lethal without medical intervention. Vomiting within 1–5 hours of exposure suggests an exposure of at least 600 rad. Brief exposure to 5000 rad or more usually causes death within minutes to hours.
Carcinogenesis. Radiation protection organizations have not agreed on a threshold dose for stochastic effects, such as cancer.
Recommended exposure limits
Exposure to the general population. The National Council on Radiation Protection (NCRP) recommends a maximum of 0.5 rem per person per year. The background radiation at sea level is about 35 millirem (0.035 rem) per year.
Occupational exposure to x-rays. The current US exposure standards are set at 5 rem/y to the total body, gonads, or blood-forming organs and 75 rem/y to the hands or feet. For comparison, a single chest radiograph results in radiation exposure of about 15 millirem (mrem) to the patient and about 0.006 mrem to nearby health care personnel (at a distance of 160 cm). A CT scan exposes the head to about 1 rad; an abdominal CT scan may expose that region to as much as 2–5 rad.
Radiation during pregnancy. Guidelines vary but generally recommend a maximum exposure of no more than 50 mrem per month (NCRP). Exposure to the ovaries and fetus from a routine abdominal (KUB) film may be as high as 146 mrem, whereas the dose from a chest radiograph is about 15 mrem.
Exposure guidelines for emergency health care personnel. To save a life, the NCRP recommends a maximum whole-body exposure of 50–75 rem for a rescuer.
Clinical presentation
Acute radiation syndrome (ARS) consists of a constellation of symptoms and signs indicative of systemic radiation injury. It is described in four stages (prodrome, latency, manifest illness, and recovery). The onset and severity of each stage of radiation poisoning are determined largely by the dose.
The prodromal stage, from 0–48 hours, may include nausea, vomiting, abdominal cramps, and diarrhea. Severe exposures are associated with diaphoresis, disorientation, fever, ataxia, coma, shock, and death.
During the latent stage, symptoms may improve. The duration of this stage varies from hours to days, but it may be shorter or absent with massive exposures.
The manifest illness stage, from 1 to 60 days, is characterized by multiple–organ system involvement, particularly bone marrow suppression, which may lead to sepsis and death.
The recovery phase may be accompanied by hair loss, disfiguring burns, and scars.
Gastrointestinal system.
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