Sievert Health Dictionary

An outdated unit of absorbed radiation dose, now superseded by the sievert. (See also radiation units.)... rem

Hazards from radiation may arise from external sources of radiation or from radioactive materials taken into the body. The effects depend on the dose, the duration of exposure, and the organs exposed.

With some forms of radiation, damage occurs when the radiation dose exceeds a certain limit, usually 1 sievert (Sv) (see radiation unit). This damage may include radiation dermatitis, cataracts, organ failure (which may occur many years later), or radiation sickness.

For other types of radiation damage, the risk that damage will occur increases with increasing doses of radiation. Cancer caused by radiation-induced mutation is the major example of this type of damage. Radioactive leaks from nuclear reactors can cause a rise in mutation rates, which may lead to an increase in cancers, such as leukaemias; to birth defects; and to hereditary diseases. Cancer usually develops years after exposure. Radiation damage can be controlled by limiting exposure. People exposed to radiation at work have their exposure closely monitored to ensure that it does not exceed safe limits. People of reproductive age or younger should have their reproductive organs shielded when having X-rays or radiotherapy.There is no evidence of radiation hazards with visual display units (VDUs).... radiation hazards

The term applied to the acute effects of ionizing radiation on the whole, or a major part, of the body when the dose is greater than 1 gray (1 Gy) of X-rays or gamma rays, or 1 sievert (1 Sv) of other types of radiation. The effect of radiation depends on the dose and the exposure time. Total-body doses of less than 2 Gy are unlikely to be fatal to a healthy adult. At doses of 1–10 Gy, transient nausea and occasional vomiting may occur, but usually disappear rapidly and are often followed by a 2–3 week period of relative well-being. By the end of this period, the effects of radiation damage to the bone marrow and immune system begin to appear, with repeated infections and petechiae (pinpoint spots of bleeding under the skin). Some people are successfully treated with a bone marrow transplant or by isolation in a sterile environment until the bone marrow recovers.

With a dose of 10–30 Gy there is also an early onset of nausea and vomiting, which tends to disappear a few hours later. However, damage to the gastrointestinal tract, which causes severe and frequently bloody diarrhoea (called the gastrointestinal syndrome), and overwhelming infection due to damage to the immune system is likely to result in death 4–14 days after exposure.

Acute exposures of more than 30–100 Gy cause the rapid onset of nausea, vomiting, anxiety, and disorientation.

Within hours, the victim usually dies due to nervous system damage and oedema of the brain; these effects are called the central nervous system syndrome.... radiation sickness

Several different internationally agreed units (called units) are used to measure ionizing radiation. For example, the roentgen (R) measures the amount of radiation in the air, and the becquerel is the unit of spontaneous activity of a radioactive source such as uranium. For medical purposes, the most commonly used units are the gray (Gy) and the sievert (Sv).

The gray is the unit of radiation that is actually absorbed by any tissue or substance as a result of exposure to radiation. 1 Gy is the absorption of 1 joule of energy (from gamma radiation or X-rays) per kilogram of irradiated matter. The gray supersedes an older unit called the rad (1 Gy = 100 rads).

Because some types of radiation affect biological organisms more than others, the sievert is used as a measure of the impact of an absorbed dose. It uses additional factors, such as the kind of radiation and its energy, to quantify the effects on the body of equivalent amounts of different types of absorbed energy. The sievert replaces an older unit, the rem (1 Sv=100 rems).... radiation unit