In the alveoles of the lung it comes to a transition of the alcohol from the (arterial) blood stream into the inhaled fresh air, whereby when breathing out alcohol is blow out. The breath alcohol concentration (BrAC) corresponds thereby approx. to 1/2100 of the (venous) blood alcohol concentration (BAC), however, this relationship is not constant and changes in temporal dependence on drinking (see diagram), whereby also individual factors - in particular the body temperature - has to be considered.
The alcohol distribution between arterial and venous blood circulation during the absorption phase has likewise strong effects on the balanced distribution blood alcohol/breath alcohol. All efforts to determine from the measured breath alcohol concentration by conversion the accurate blood alcohol concentration failed in principle. For this reason the legislative of many european countries gives its own breath alcohol limit value, which is occupied according to the blood alcohol limit values with sanctions.
As alternative to the BrAC preliminary test exists the possibility of the saliva alcohol determination.
The first measuring instruments, which already emerged in the 30's in the USA, had still chemical reactions with color changes as basis. Also the recently used tubes as preliminary test were still based on a color reaction. As substitution of the tubes handy measuring instruments, which are equipped with an electro-chemical gas sensor and with a digital announcement of the breath alcohol value (or a converted blood alcohol concentration) are in use, usually refered as breathalyser (UK) resp. breathalyzer (USA). Electro-chemical detection is used also for the interlock System to prevent car driving under influence of alcohol.
In order to limit false positive results, a period of at least 10 minutes has to be passed before breath alcohol testing. Otherwise residual alcohol in the mouth will lead to falsely too high results.