In a breath test, one must calculate how much ethanol measured from the expired air sample is in the blood. To do this, one must take into account the volume of blood from which the ethanol originated.
First, a few assumptions must be made:
- The ethanol concentration in the expired alveolar air is directly proportional to the ethanol concentration in the blood. This assumption is based on Henry’s Law which states that, at constant temperature, the concentration of gas dissolved in a liquid is proportional to its concentration in the air directly above the liquid.
- The exhaled alveolar air is in equilibrium with the blood. In the case of ethanol, the ratio of concentrations of ethanol in blood to expired alveolar air has achieved a constant value—on average, this is 2100:1.
This relationship is called the blood-to-breath ratio or partition ratio.
The blood-to-breath ratio can vary between 1500:1 and 3000:1. The ratio varies:
- among individuals according to age, gender, genetic makeup, and state of intoxication
- within a given individual at different times
- among measuring devices
The 2100:1 ratio is used as a standard conversion factor in determining BAC from the Breathalyzer™ test. It is actually an average based on comparisons of blood and breath samples collected from many individuals using several types of measuring devices.
The variability in the inter-person and intra-person blood-to-breath ratio could under or overestimate the true BAC as measured directly from the blood. This could be acceptable in court if there is proof of a discrepancy.