The International System of Units (SI) is the most popular system of measurement globally. Radiation units have been brought into this. Becquerels measure quantity and coulombs per kilogram are used instead of roentgens to denote exposure.

Grays (Gy) measure the absorbed dose. A gray is defined as the absorption of one joule of radiation energy by one kilogram of any matter, not just air as with the roentgen unit. One gray is equal to 100 rads. Five grays at once is a lethal dose. Diagnostic medical treatments are usually measured in milligrays (mGy).

An abdominal X-ray gives a dose of 0.7 mGy, while a computerised tomography (CT) scan is higher, at about 6 mGy. Cancer treatments exceed the lethal dose, but in small increments. Up to 80 Gy can be given, in doses of 2 Gy at a time.

Any given amount of radiation may not have the same biological effect. This is influenced by differences in the type of radiation and the conditions of exposure. Where X-rays and gamma rays are concerned, the absorbed dose is the same as the equivalent dose. If alpha particles are involved, the biological damage is more severe and weighting factors are applied. The sievert is the resultant unit.

A sievert (SV) is the standard international measurement of equivalent dose, replacing the rem. Sieverts express the potential for damage to human tissue, and are related to grays. One sievert is equivalent to one hundred rems, which would be a lethal dose. A microsievert (µSv) is one millionth of a sievert. One tenth of a microsievert is the natural radiation found in an average banana.

part four ‘Measuring Devices’ to follow