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Which units/terms are usually used to discuss blood alcohol levels in English? Wikipedia suggests that people use percent in the US and basis points in Great Britain. On websites devoted to the topic, I found the expression BAC (blood alcohol content) followed either by a percentage or a number. In my native language German, the unit permille is typically used, but I don't recall seeing permille used in English in this context.

How do native speakers express the fact that a person had a certain BAC, that is, how would they complete sentences such as

According to the police report, his blood alcohol content was ...

or

The legal limit for driving a car is ...

(Strictly speaking, there are several ways of expressing a certain BAC because there are several different units. However, I am most interested in the expressions used in everyday English, rather than those used in legal or scientific documents.)

  • "per mille" is quite unusual in English, but not wholly unheard of; there's even a symbol for it (‰). It's not really used for blood alcohol levels, though. – Nathan Tuggy Dec 30 '14 at 0:43
  • @NathanTuggy Interestingly, in my language (and perhaps German too), "permille" is always used for BAC and rarely elsewhere. An average layperson likely thinks "permille" is specifically the unit of BAC. – ntoskrnl Dec 30 '14 at 13:18
  • @ntoskrnl The misuse of permille in English by a German native speaker is the very origin of my question. – painfulenglish Dec 30 '14 at 14:27
  • Interesting. While basis point is in general usage in the U.S., it is almost exclusively used in financial contexts. The average American would be confounded to see it measuring much beyond mortgage rates or mutual fund fees. – choster Dec 30 '14 at 16:55
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I prosecuted people for DUI in the state of Illinois, so I may be able to offer a perspective of the legal jargon, the police vernacular, and the language of the lay person. To me, the following road sign speaks volumes:

enter image description here

The sign is an official government sign to warn the population. It does not mention units at all. David Garner is right that extremely few people in the US could name the unit involved. I know from working with police officers and from attending training for prosecutors who prosecute DUIs that most police officers and lawyers cannot describe the unit of measurement.

As further evidence of the lack of understanding of the unit of measurement, some people will make statements such as, "Susan had a point oh six and Jack had a point twelve, so he was twice as drunk as she was." Of course, BAC is not a magically precise measure of the degree of impairment, but because most people do not understand what is being measured, they cannot make proper comparisons between different BAC values.

To explicitly address your two examples, in my opinion, a non-professional writer but native speaker of American English would write the following:

According to the police report, his blood alcohol content was .16.
The legal limit for driving a car is .08.

I believe they would say ".16" as "point one six" and ".08" as "point oh eight."

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    As a ratio of like measurements, the BAC is unit-less. Compare f-stop and albedo. – Malvolio Dec 30 '14 at 2:34
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    @Malvolio: Well, sort of, but keep in mind a BAC of ".08" is actually 0.08% -- that is, an alcohol-to-blood ratio of 0.0008, not of 0.08. (And actually even that is a simplification -- the .08 on the sign is actually a weight/volume percentage, not a true dimensionless percentage.) – ruakh Dec 30 '14 at 7:57
  • @ruakh, please see the Wikipedia article cited in the question. It says, "Each is defined as either a mass of alcohol per volume of blood or a mass of alcohol per mass of blood (never a volume per volume)." When it is used as the latter (ratio of mass to mass) then it is would be a dimensionless percentage. – rajah9 Dec 30 '14 at 15:42
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    @rajah9: Yes, I'm aware of that. (Which part of my comment do you think you're disagreeing with?) – ruakh Dec 30 '14 at 16:02
  • When BAC is in mg/dL or mmol/L then I agree: it is not true, dimensionless percentage. I also agree with you that the sign portrayed is using a weight/volume percentage. However, when BAC is a mass of ethanol to a mass of blood, then I disagree: it is a true, dimensionless percentage, as you are dividing mg by mg. (Admittedly, this is not what's used as a legal standard.) – rajah9 Dec 30 '14 at 16:24
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A quick Web-search confirms that the current UK limit is "80 milligrams of alcohol per 100 millilitres of blood", but I don't think we use units 'in everyday English'. We might say, "The police measured his blood-alcohol and it was 160 - twice the legal limit", but if you asked the average Brit, I don't think they'd be able to name the unit.

  • I think the "basis points" used in the UK, to which the OP referred, may relate to the units of maximum recommended consumption, for health purposes. These have nothing to do with the 80mg per 100ml, the legal limit for drivers. – WS2 Jul 12 '18 at 23:47
  • Your example is good - in the UK most newspaper articles talk in multiples of the "legal limit", rather than in any normal units system. – AndyT Jul 13 '18 at 11:01
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According to this Blood alcohol content article,

in North America a BAC of 0.1 means that there are 0.10 g of alcohol for every dL of blood.

where dL is a deciliter (one-tenth of a liter). A percentage of blood alcohol content is the ratio of a volume of alcohol to a volume of blood. The units in the US and Canada are grams per deciliter.

(Deciliters, as a unit of measure, are often used in medical settings.)

The table on the same article has an excellent chart which compares the US/Canadian percent with the European permille with the GB basis point. In it, 1% is 217.4 mmol/L, 1 permille is 21.7 mmol/L, and one basis point is 2.17 mmol/L.

This article about an auto accident with fatalities says

Soto, who has prior convictions including DUI and hit-and-run, was allegedly driving on a revoked license with a blood alcohol content higher than .20 percent when he crashed Sunday night, according to the Los Angeles County District Attorney's Office.

So in the US and Canada, the unit is percent. Police or news reports will often say point 15 or 0.15 (that is, without the units), but the units are assumed.

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    It's not just news reports that omit percent; AmE speakers in ordinary conversation also say point fifteen or point one five or point oh eight. I think in many cases, people may not even be aware that the units are actually percent. – Nate Eldredge Dec 29 '14 at 22:41
  • Point taken, @NateEldredge. Do you think I should change the last phrase "but the units are assumed" to "but the units are omitted"? – rajah9 Dec 30 '14 at 15:31
  • Or perhaps "and most speakers are blissfully unaware that there are units" ? – rajah9 Dec 30 '14 at 15:32
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Drink driving and the legal alcohol limit:

  • In England and Wales, the alcohol limit for drivers is 80 milligrammes of alcohol per 100 millilitres of blood, 35 microgrammes per 100 millilitres of breath or 107 milligrammes per 100 millilitres of urine. In most other European countries, the limit is less, usually 50 milligrammes per 100 millilitres of blood.

  • On December 5th 2014 the alcohol limit for drivers in Scotland reduced from 80 milligrams of alcohol in every 100 millilitres of blood to 50 milligrams of alcohol in every 100 millilitres of blood.The breath alcohol equivalent reduced from 35 micrograms of alcohol per 100 millilitres of breath to 22 micrograms of alcohol per 100 millilitres of breath.

(from www.drinkaware.co.uk)

  • According to the police report, his blood alcohol content was above the legal limit of 80 (milligrammes).

  • The legal limit for driving is 80 (milligrammes).

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    The legal limit is not 80 milligrammes; it is technically 80 milligrammes/decilitre, or to most of us '80 units'. Besides, it is known to all drivers that the principal effect of alcohol in the bloodstream is to increase the number of police cars/mile. – TimLymington Dec 29 '14 at 21:57
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The units are basically national; not language. The shorthand terms (e.g., "point 15") are obviously language, but should be fairly universal within either system. Thus, the North American "weight/volume %" will be different from the European "permille" measurement.

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