Which metric is used to quantify alcohol concentration in breath tests?

The metric used to quantify alcohol concentration in breath tests is grams per 210 liters. This measurement reflects the amount of alcohol vapor present in the breath sample collected during the test. The choice of this particular metric is based on the established relationship between the concentration of alcohol in the blood and the concentration of alcohol in breath, allowing law enforcement and forensic professionals to derive an estimate of blood alcohol content (BAC) from the breath sample. This conversion is grounded in the principles of Henry's Law, which describes the equilibrium between a gas and a liquid, and it standardizes breath alcohol measurements to facilitate consistent and reliable assessments of impairment due to alcohol consumption.

The other options, while related to measurements of substances, do not accurately represent the standard method used in breath alcohol testing processes.