Do you smoke?
"In all cases, exposing the sensor to any type of smoke or oxygen ion generator will produce false positive test results and inaccurate readings. This includes residual smoke in the lungs of a smoker and ambient smoke that may be present in the immediate area. Do not use an alcohol breath analyzer near any type of ion generator including popular air cleaners and central hvac electronic filtration systems. Smokers must wait a minimum 8-10 minutes after smoking to use a breathalyzer to insure that all residual smoke is absent from the lungs." Breathalyzer Methods and Facts
Check the web - lots of case studies....