Detecting environmental interferent in a transdermal alcohol monitor

An improvement in monitoring alcohol levels through transdermal testing is provided by detecting if environmental gasses, referred to as interferents, have been introduced into a transdermal vapor sample. An insensible skin perspiration sample may not be completely controlled and can contain interferents from an environmental source rather than the subject. Before testing a skin perspiration sample, the alcohol sensor is sampled and averaged to establish a baseline value. A sample is drawn and presented to the alcohol sensor. The output of the alcohol sensor is monitored to determine the amount of alcohol in the sample. A maximum alcohol sensor value for the transdermal alcohol monitor is set by determining the baseline value when no alcohol is present in the sample. Subsequent baseline values during a reading above the maximum alcohol sensor value indicate that an environmental interferent is present in the alcohol sensor.