When I use a word . . .: Now concentrate
Authors: J. Aronson
Journal: BMJ (1999)

Dictionaries were originally selective glossaries, often of so called “hard words.” Nowadays, however, we expect them to be comprehensive, from a to zyzzogeton. Nevertheless, occasionally even an established word is omitted. For instance, “liar” was mistakenly omitted from the 1972 edition of Chambers Twentieth Century Dictionary and “bondmaid” from the fascicle in the Oxford English Dictionary for which it had been prepared (Battentlie-Bozzom, 1887). I have also read that James Murray omitted “appendicitis” from the first edition of the OED on the advice of Sir William Osler, then regius professor of medicine at Oxford, who said that it was medical jargon that wouldn’t last; the word gained currency when, just before he was due to be crowned in 1902, King Edward VII, already on the throne, fell ill and had his inflamed appendix removed by Frederick Treves.

“Concentration” is defined in the Dent Dictionary of Measurement (J M Dent, 1994) as “The amount of a particular substance in a mixture or solution.” Now, excellent though it is, I would not normally quote a definition from Dent’s dictionary. I have done so here because it is missing from the OED.

Of course, “concentration” is a headword in the OED, but none of its seven main definitions is the meaning defined above. When I drew this to the attention of John Simpson, one of the current editors of the OED, he agreed that the dictionary had lapsed and he opened a file. However, such is the richness of the OED that entries under other headwords contain quotations that illustrate the use of the word.

Making a quick search of the whole text on CD Rom, I unearthed “concentration” in the sense given above in many quotations under other headwords. The earliest examples are, as it were, concentrated in the years 1897-9, surprisingly late, given the great advances that occurred in the science of chemistry during the nineteenth century. For example, “It is the difference in the concentration of the chemotropic substance in two layers ... which controls movement” (Am Naturalist 1897;31:719). In earlier examples than this it is often hard to distinguish between this meaning of concentration and its parent meaning, “the strengthening of a solution by contraction of its volume.”

“Level” in the sense of “concentration” is also not defined as such in the OED, but again examples of its use in this way can be found under other headwords. For example, under “para-1”: “The extraction of a parathyroid hormone which will prevent or control parathyroid tetany and which regulates the level of blood calcium” (J Biol Chem 1925; 63:395).

“Level” in this sense is now used more often than “concentration.” In 118 056 papers published in the bioscience literature in 1965-97 “level” was used in 54.4% of cases. However, this average figure conceals an unusual phenomenon: “blood level” was used markedly more often than “blood concentration” (68.5% vs 31.5%) while “plasma level” was used less often than “plasma concentration” (48.5% vs 51.5%). More research might show why, but for the moment I cannot offer a good explanation for this difference.

Using “level” to mean “concentration” may be colloquial, but it is hardly scientific. This is a matter that is best contemplated at leisure in the bath, where you have time to consider the difference between the level of water in the tub and the concentration of bathsalts in the water.