Breath alcohol test results are expressed in terms of weight-to-volume: weight of alcohol (in grams) per volume of breath (210 liters). We abbreviate this expression as “G/210L.” We say this expression as “Grams of alcohol per 210 liters of breath.
Results of a breath test are also commonly expressed as Breath Alcohol Concentration, abbreviated as BrAC or BAC. For example, an alcohol test result of .100 means that there are .100 grams of alcohol per 210 liters of breath (G/210L). This is the same as .100 BrAC or .100 BAC.
Scientists chose 210 liters of breath as the unit of measurement for ease of correlating breath alcohol test results with blood alcohol test results. In other words, breath alcohol tests and blood alcohol tests will yield the same numbers.