When Police pull a driver over, they are usually concerned with the drivers Blood Alcohol Content or Concentration.
They usually measure BAC in concentration of alcohol in the bloodstream and is usually measured as mass per volume.
For example, using the English drink drive limit a BAC of 0.8% means the same as 80 milligrams of alcohol for every 100 millilitres of blood.
Or if we use the Scottish drink drive limit, a BAC of 0.05% means the same as 50 milligrams of alcohol for every 100 millilitres of blood.
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