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Why is blood alcohol measured in milligrams per millilitre?
By Stuart Jones on February 11, 1998:

Breath tests and blood samples:

Why is blood alcohol measured and reported in milligrams per millilitre?

Why are units of measurement mixed in this way when it would seem possible to report millilitres or microlitres per millilitre ?

Stuart Jones

By David:

Dear Stuart,

I have often wondered this myself!! I will try and find out for you. It is possible that the police might know, so you could try contacting them.

David.

By David:

Dear Stuart,

Well I've asked a few people what they think, and the basic conclusion seems to be that no-one really knows! They all give rather waffly answers with no real content.

Basically, the blood alcohol is a concentration. Although alcohol is a liquid, it still has a mass (as one of my friends so kindly pointed out), and so it's certainly possible to express it in mg / ml, or whatever. If you tried doing it as ml / ml instead, you would end up with a dimensionless quantity ( i.e. something with no units (no pun intended...)), which wouldn't really seem quite right. Also, alcohol isn't really just alcohol in the blood - it mixes with everything else, so it isn't very clear whether you could exactly say how much alcohol there is.

(In fact, by this time, I believe the 'alcohol' has reacted with other things in your body to make a different compound, and it is this which is measured - I don't know how, though.)

Sorry this isn't a very good answer, but it's the best I can do!

David.

By Sujata Krishna on February 20, 1998:

It seems to me one reason might be that mass is easier to measure accurately compared to volume. Volume can be significantly affected by temperature and pressure (though this parameter is not relevant here), whereas the mass remains constant.

Sujata.

By Stephen Moratti on February 23, 1998:

I do not know for sure. However, volume is not a completely accurate way of measuring the amount of a substance, as the density changes with temperature. The mass of a substance is always constant. So 1g of ethanol will always be 1g at any temperature, but the volume will vary from 0.79 ml at room temperature to perhaps 0.85 ml at a higher temperature (this is how thermometers work). So one would also then have to specify what temperature the ethanol volume was measured, which gets complicated, so one gets round this by specifying weight.

The volume measurement for blood I suspect is for ease of measuring, although one could speculate that perhaps they should have used weight for it as well. The precise units (mg/ml versus g/l etc) I suspect are an accident of history.

I could be completely wrong above, but I feel that the answer is probably right.

Yours,

Stephen Moratti

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Dr. S. Moratti,
Melville Polymer Laboratory,
Chemistry Department,
Lensfield Rd, Cambridge CB2 1EW, UK.