Micro-

Micro (Greek letter μ, mu, non-italic) is a unit prefix in the metric system denoting a factor of 10−6 (one millionth).[1] Confirmed in 1960, the prefix comes from the Greek μικρός (mikrós), meaning "small".

This article is about the SI prefix. For other uses, see Micro (disambiguation).
Look up micro- in Wiktionary, the free dictionary.

It is the only SI prefix which uses a character not from the Latin alphabet. In Unicode, the symbol is represented by U+03BC μ or the legacy symbol U+00B5 µ . The prefix "mc" is commonly used in healthcare or when the character "μ" is not available; for example, "mcg" commonly denotes a microgram.[2] The letter "u" is sometimes used instead of "μ" when non-Latin characters are not available.

Examples
SI prefixes
PrefixBase 10 Decimal Adoption
[nb 1]
NameSymbol
quettaQ1030 10000000000000000000000000000002022[4]
ronnaR1027 1000000000000000000000000000
yottaY1024 10000000000000000000000001991
zettaZ1021 1000000000000000000000
exaE1018 10000000000000000001975[5]
petaP1015 1000000000000000
teraT1012 10000000000001960
gigaG109 1000000000
megaM106 10000001873
kilok103 10001795
hectoh102100
decada10110
——1001—
decid10−1 0.11795
centic10−2 0.01
millim10−3 0.001
microμ10−6 0.0000011873
nanon10−9 0.0000000011960
picop10−12 0.000000000001
femtof10−15 0.0000000000000011964
attoa10−18 0.000000000000000001
zeptoz10−21 0.0000000000000000000011991
yoctoy10−24 0.000000000000000000000001
rontor10−27 0.0000000000000000000000000012022[4]
quectoq10−30 0.000000000000000000000000000001
Notes
  1. Prefixes adopted before 1960 already existed before SI. The introduction of the CGS system was in 1873.

Symbol encoding in character sets

The official symbol for the SI prefix micro is a Greek lowercase mu (μ).[6] For reasons stemming from its design, Unicode has two different character codes for the letter, with slightly different appearance in some fonts, although most fonts use the same glyph. The micro sign (µ) is encoded in the "Latin-1 Supplement" range identical to ISO/IEC 8859-1 (since 1987), at U+00B5 (Alt+0181),[7] residing at this code point also in DEC MCS (since 1983) and ECMA-94 (since 1985). The Greek letter (μ) is encoded in the Greek range at U+03BC (Alt+956). According to The Unicode Consortium, the Greek letter character is preferred,[8] but implementations must recognize the micro sign as well. This distinction also occurs in some legacy code pages, notably Windows-1253.

In circumstances in which only the Latin alphabet is available, ISO 2955 (since 1974,[9] withdrawn 2001[10]), DIN 66030 (since 1980[11][12]) and BS 6430 (since 1983) allow the prefix μ to be substituted by the letter u (or even U, if lowercase letters are not available), as, for example, in um for μm, or uF for μF, or in the common abbreviation UC for microcontroller (µC). Similarly, capacitor values according to the RKM code defined in IEC 60062 (since 1952) can be written as 4u7 (or 4U7) instead of 4μ7 if the Greek letter μ is not available.

Other abbreviating conventions

In some health care institutions, house rules deprecate the standard symbol for microgram, "μg", in prescribing or chart recording, because of the risk of giving an incorrect dose because of the misreading of poor handwriting.[13] The two alternatives are to abbreviate as "mcg"[13][2] or to write out "microgram" in full (see also List of abbreviations used in medical prescriptions). The alternative abbreviation may be ambiguous in rare circumstances in that mcg could also be read as a micrigram, i.e. 10−14 g; however the prefix micri is not standard, nor widely known, and is considered obsolete. This deprecation, focused on avoiding incorrect dosing in contexts where handwriting is often present, does not extend to all health-care contexts and institutions (for example, some clinical laboratories' reports adhere to it, whereas others do not[13]), and in physical sciences research, "μg" remains the sole official abbreviation.

In medical data exchange according to the Health Level 7 (HL7) standard, the μ can be replaced by u as well.[14]

See also

References
  1. International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), ISBN 92-822-2213-6, archived (PDF) from the original on 2021-06-04, retrieved 2021-12-16
  2. "ISMP List of Error-Prone Abbreviations, Symbols, and Dose Designations: Abbreviations for Doses/Measurement Units". Recommendations. Institute for Safe Medication Practices (ISMP). 2021-02-05. Archived from the original on 2022-12-24. Retrieved 2022-12-24. Error-Prone Abbreviations, Symbols, and Dose Designations: µg […] Intended Meaning: Microgram […] Misinterpretation: Mistaken as mg […] Best Practice: Use mcg […]
  3. Biology by Campbell & Reece, tenth edition. Ch. 6 "A Tour of the Cell". p. 98.
  4. "On the extension of the range of SI prefixes". 2022-11-18. Retrieved 2023-02-05.
  5. "Metric (SI) Prefixes". NIST.
  6. Prefixes of the International System of Units, International Bureau of Weights and Measures (page visited on 9 May 2016).
  7. (Unicode 1.0, 1991)
  8. Unicode Technical Report #25
  9. ISO 2955-1974: Information processing - Representations of SI and other units for use in systems with limited character sets (1st ed.). 1974.
  10. "Table 2". ISO 2955-1983: lnformation processing - Representations of SI and other units for use in systems with limited character sets (PDF) (2nd ed.). 1983-05-15. Retrieved 2016-12-14.
  11. DIN 66030: Darstellungen von Einheitennamen in Systemen mit beschränktem Schriftzeichenvorrat (in German) (1st ed.). 1980.
  12. "Neue Normen für die Informationsverarbeitung". Computerwoche (in German). 1981-01-09. Archived from the original on 2016-12-14. Retrieved 2016-12-14.
  13. Burtis, Carl A.; Ashwood, Edward R.; Bruns, David E. (2012), Tietz Textbook of Clinical Chemistry and Molecular Diagnostics (5th ed.), Elsevier Health Sciences, ISBN 978-1455759422.
  14. "Commonly Used UCUM Codes for Healthcare Units". HL7 Deutschland e.V. 2015-11-21. Archived from the original on 2022-10-06. Retrieved 2022-12-24.
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