The Science of Blood Alcohol Content in DUI Defense

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How does a breath alcohol concentration reading from a breathalyzer reflect the concentration of alcohol in your blood?

The scientific theory that prosecutors rely on to show that a breathalyzer reading is a reliable way of testing blood alcohol concentration, is a concept known as “Henry’s Law.”

Henry’s Law essentially explains that a numerical relationship exists between the amount of alcohol in your blood and the amount in your lungs. At a fixed temperature, the numerical relationship between the alcohol in the gas (breath) can be related to the alcohol in the liquid (blood).

The ratio that’s commonly used in breathalyzers is 2100 to 1. This means that 2100 milliliters of breath will contain the same amount of alcohol as 1 milliliter of blood.

Most importantly, the concept of Henry’s Law has two fundamental flaws when it is applied to breathalyzer testing for blood alcohol concentration.

The first flaw is the assumption that breath and body temperature are fixed or constant. When air is exhaled, the alveolar air travels through the upper respiratory tract. A drop in temperature occurs before the air is exhaled, which physiologically prevents the breath temperature from remaining fixed or constant.

The other big flaw is that every person has a different ratio of blood to breath in their body, yet the breathalyzer device converts the reading by using the 2100 to 1 ratio in order to calculate a subject’s blood alcohol concentration. If the subject’s breath to blood ratio is different from 2100 to 1, then the reading is inaccurate.

For more information on the relationship between blood alcohol and DUI, please visit our website.