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This is a highly complex and important safety issue. See also Pregnancy and non-obstetric surgery. Ionising radiation consists of either subatomic particles or electromagnetic waves that have sufficient energy to remove electrons from molecules, thus ‘ionising’ them. Ionising electromagnetic waves are X-rays, gamma rays and the higher-energy ultraviolet part of the electromagnetic spectrum. Ionising subatomic particles are alpha and beta particles and neutrons, which are typically created by radioactive decay. There are also cosmic particles produced by cosmic rays interacting with earth's atmosphere.

Radiation activity

This is how much radiation is being emitted. It is measured in the SI unit becquerel (Bq). One Bq is defined as the activity of a quantity of radioactive material in which one nucleus decays per s.

Units of exposure

The amount of radiation a person or object is exposed to.

  1. The SI unit of exposure for ionising radiation is the coulomb per kg (C/kg).

  2. The older unit is the roentgen (R). 1 R = 2.58 × 10−4 C/kg.

Units of absorbed dose

This is the amount of radiation absorbed by a person or object.

  1. The SI unit is the gray (Gy) and has units of joules/kg.

  2. The rad is an older term: 100 rad = 1 Gy.

Units of equivalent dose

This is a measure of the biological effect on tissue of the radiation dose weighted for harmful effects of different types of radiation. The units are:

  1. Roentgen equivalent man (rem): 1 rem = 1 rad.

  2. The sievert (Sv) is the SI unit of equivalent dose and effective dose: 1 Sv = 1 Gy.

Although highly oversimplified, from the above 1 Sv = 1 Gy = 100 rad = 100 rem.

Radiation risk

6 000 000 mrems exposure has a 100% mortality rate and 450 000 mrems has a 50% mortality rate. Table R1 shows the radiation risks of various exposure sources.

Table R1Radiation risks compared



Anti-emetic second-generation 5-HT3 blocker that is more effective than ondansetron in preventing PONV. It is also used to treat diarrhoea-predominant irritable bowel syndrome.

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