Final answer:
A person's Blood Alcohol Concentration (BAC) is primarily measured by law enforcement through a Breathalyzer test, which assesses the ethanol concentration in the breath.
Step-by-step explanation:
Law enforcement agencies determine a person's Blood Alcohol Concentration (BAC) primarily through a Breathalyzer test. The Breathalyzer is a portable device that measures the ethanol concentration in a person's breath, correlating this with their BAC. When using a Breathalyzer, the suspect's breath is bubbled through a chemical solution that changes from yellow-orange to green when ethanol is present, reflecting the level of alcohol inhaled. The reaction involves the oxidation of ethanol to acetic acid by the dichromate ion. The intensity of the green color indicates the amount of ethanol in the sample, which is used to estimate the BAC.