international unit

1. The quantity of a biologically active substance, such as a hormone or vitamin, required to produce a specific response.
2. A unit of potency for similarly active substances, based on this quantity and accepted as an international standard.

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Used as a measure of comparative potency of natural substances, such as vitamins, before they were obtained in a sufficiently pure form to measure by weight. Still sometimes used (3.33 iu vitamin A = 1 μg; 40 iu vitamin D = 1 μg; 1 iu vitamin E = 1 mg).

electromagnetics (Metric) The units developed under the title electromagnetic units, forming the e.m.u. system, were adjusted by decimal multiplication to give the practical units, adopted at the first International Electrical Conference in 1881 as suitable for everyday electrical work. Accompanying explicit laboratory definitions of the ohm, ampere, and volt were established, making them base practical units instead of derived units as in the e.m.u., where these electrical units were defined in terms of mechanical equivalents. These new definitions were soon found to be discrepant with the intended and, despite some adjustments, are continuingly so (albeit by less than 0.05%). They were not even mutually self-consistent until the volt was made a derived unit, at the IEC of 1908, when the residual discrepancies were accepted via the use of the distinctive label ‘international ohm’, etc.

By Act of Congress in 1984, the USA created its own definitions of the ‘international units’, slightly different from the others, hence creating a set of US international units.

The true international units were in turn discarded at the end of 1947 in favour of new definitions within the metric m.k.s.A. system that became the SI system, these latest units often being called ‘absolute’, and generally agreeing with the original practical units rather than the ‘international’ ones. Typical values in SI terms are as follows:

A unit used to measure the vitamin content of food, based on the vitamin's biological activity. Although IUs are still used, vitamin content is usually expressed as equivalent amounts of a pure standard form. For example, in the UK vitamin A content is given in microgrammes of pure retinal equivalent.

1. immunizing unit.
2. International Unit.

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