Abstract: A breath measurement instrument using a combination of breath pressure, temperature, and humidity measurements to determine whether or not breath samples are human and properly delivered. Valid samples require maintenance of a threshold dynamic pressure of a sample being introduced for a predetermined time, a valid temperature of the sample and a valid humidity of the sample, wherein the range of valid sample temperatures is adjusted according to a measured sample humidity. Also disclosed is a breath alcohol instrument having an alcohol-specific fuel cell and a fuel cell circuit for generating a breath alcohol signal. Further disclosed is an interlock system for inhibiting operation of machinery such as a vehicle, and a method for screening breath samples and determining an alcohol content thereof.