Radiation sickness results when humans (or other animals) are exposed to very large doses of ionizing radiation.
Radiation exposure can occur as a single large exposure (acute), or a series of small exposures spread over time (chronic). Exposure may be accidental or intentional (as in radiation therapy).
Radiation sickness is generally associated with acute exposure and has a characteristic set of symptoms that appear in an orderly fashion. Chronic exposure is usually associated with delayed medical problems such as cancer and premature aging, which may happen over a long period of time.
The risk of cancer depends on the dose and begins to build up, even with very low doses. There is no "minimum threshold".
Exposure from x-rays or gamma rays is measured in units of roentgens. For example:
- Total body exposure of 100 roentgens/rad or 1 Gray unit (Gy) causes radiation sickness.
- Total body exposure of 400 roentgens/rad (or 4 Gy) causes radiation sickness and death in half of the individuals who are exposed. Without medical treatment, nearly everyone who receives more than this amount of radiation will die within 30 days.
- 100,000 roentgens/rad (1,000 Gy) causes almost immediate unconsciousness and death within an hour.
The severity of symptoms and illness (acute radiation sickness) depends on the type and amount of radiation, how long you were exposed, and which part of the body was exposed. Symptoms of radiation sickness may occur immediately after exposure, or over the next few days, weeks, or months. Bone marrow and the gastrointestinal tract are especially sensitive to radiation injury. Children and babies still in the womb are more likely to be severely injured by radiation.
Because it is difficult to determine the amount of radiation exposure from nuclear accidents, the best signs of the severity of the exposure are: the length of time between the exposure and the onset of symptoms, the severity of symptoms, and severity of changes in white blood cells. If a person vomits less than an hour after being exposed, that usually means the radiation dose received is very high and death may be expected.
Children who receive radiation treatments or who are accidentally exposed to radiation will be treated based on their symptoms and their blood cell counts. Frequent blood studies are necessary and require a small puncture through the skin into a vein to obtain blood samples.