Uncertainty of breath alcohol measUrement

Although the analysis of ethyl alcohol (ethanol) concentration in exhaled air (breath alcohol content, BrAC) is a method that is widely used for estimation of the alcohol content in the body and assessment of the state of sobriety, the results of measurements are often a source of controversy both during an investigation and during the subsequent trial. One of the most important elements of this controversy is the uncertainty of the measurements and, as a consequence, the reliability of the results. Analysis of data gathered at the Institute of Forensic Research (IFR), which consisted of duplicated measurements performed using both portable and stationary evidential breath analysers (Alcomat V5.4, Alkometr A2.0, Alcometer SD-400P, AlcoQuant 3020, AlcoSensor IV, Alcotest 7110, 7410, 7410+), together with a literature review, allowed us to conclude that the measurement uncertainty connected with duplicated BrAC tests conducted at short intervals for ethanol concentrations up to 0.5 mg/l was equal to 0.03 mg/l. It was also demonstrated that the measurement uncertainty was independent of the analyser, i.e., it was not connected with the type of detector used – electrochemical (fuel cell) or spectrophotometric (infrared detector). However, besides taking into account the measurement uncertainty, the interpretation of a BrAC test result performed in real conditions should be carried out very carefully due to the fact that there are many additional factors which can impact the result obtained.