Illinois breathalyzer accuracy depends on machine calibration, operator technique, subject physiology, and environmental conditions. Instrument-level factors include certification currency, accuracy check history, and RFI shielding. Operator-level factors include the 20-minute observation, proper mouthpiece use, and correct sample delivery. Subject-level factors include GERD, diabetic ketoacidosis, dentures with alcohol residue, recent oral use of mouthwash or breath spray, and hematocrit. Temperature, atmospheric conditions at the station, and elapsed time since the last drink all affect the result. Under 625 ILCS 5/11-501.2 the state must lay foundation, but individual accuracy remains contestable at trial through experts. A Chicago DUI lawyer builds these variables into cross-examination.
