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Detailing the Margin of Error in Breath Testing

When people in Clearwater hear the acronym “BAC,” they may easily assume that it stands for “breath-alcohol content” given that authorities obtain the measurement most associate with drunk driving through a breath test. Yet that tactually measures the concentration of alcohol in one’s blood.

The obvious question that arises from this is how could one’s breath offer an indication of the contents of their blood? The answer comes from understanding how breath testing devices generate measurements.

Assuming a Blood-To-Breath Ratio

Per information shared by the Alcohol Pharmacology Education Partnership, breath provides a measurement of the contents of the blood thanks to Henry’s Law (named for English chemist William Henry). This gas law assumes that the concentration of gas dissolved in a liquid is proportional to the concentration of gas in the air above that liquid. This means that (in theory) the amount of gaseous alcohol in one’s lungs (which comes from ethanol dissolved in the blood that vaporizes upon coming into the contact with oxygen in the lungs) is proportional to the amount remaining in the blood.

Introducing a Margin of Error

Breath testing devices assume that this blood-to-breath ratio of alcohol is 2100:1 (that is, 1 milliliter of blood having 2100 times of the amount of alcohol in it as 1 milliliter of breath). The trouble is that research shows that a person’s actual blood-to-breath ratio can measure anywhere between 1500:1-3000:1 (the 2100:1 assumption comes from a series of test results taken using different devices).

Given this wide range of variability, it may come as little surprise to many to learn that according to the American Motorists Association, study data shows that breath testing devices have a margin of error as high as 50%. For this reason, breath tests are often determined to be inadmissible in court.