Analytical and Physiological Specificity Issues in Breath Alcohol Analysis

Drink-drivers frequently allege that their breath alcohol reading was so high because some non-ethanolic chemical was in their breath at the time of the test. This paper reviews the author’s experiences and methodology in dealing with such matters in the British Courts on behalf of the Police. Conceivable sources of breath contaminants are ingestion (with food, medication or smoking), endogenous production and occupational or environmental exposure. Under British Law the prosecution have only to prove that at the time of the test the subject’s breath alcohol level (BrAC) exceeded the per se limit: it is not required to prove the actual concentration which at that time existed. The forensic scientist should therefore consider the following questions, in this order: (a) is the alleged interfering substance ‘X’ volatile? (b) does the analyser used measure substance ‘X’ at all? (c) what is the analyser’s relative response between ‘X’ and ethanol? (d) what maximum breath level of ‘X’ could the driver have had? (e) could this have elevated the reading by the difference between the per se level (or the Widmark level as calculated from the driver’s admitted consumption) and that recorded by the instrument? In most instances the case is solved well before question ‘e’ is asked. New analytical specificity requirements, such as those of OIML, while physiologically unjustified, should help reduce spurious defence claims at source.