How can a breath sample be used to estimate how much alcohol is in blood? Henry's law answers that question: at a constant temperature, the amount of a dissolved volatile substance in a liquid is proportional to the partial pressure of that substance in the air above the liquid. Put simply, alcohol moves between blood and breath in a predictable way until it reaches equilibrium. That principle is the scientific basis for using deep-lung, or alveolar, breath to estimate blood alcohol concentration.

In DUI and crash cases, Henry's law matters because breath testing assumes a stable relationship between alcohol in blood and alcohol in exhaled air. Instruments convert breath alcohol into an estimated blood value using a partition ratio, commonly 2100:1. That ratio is an accepted average, not a biological constant for every person. Body temperature, breathing pattern, the timing of the last drink, and instrument calibration can affect the reading. Those issues often appear in disputes over chemical testing, probable cause, and impairment.

In Mississippi, Miss. Code Ann. § 63-11-30 (2024) sets the per se DUI limit at 0.08% for most drivers. Henry's law does not set that legal threshold, but it underpins the breath-test number used to prove it. In an injury claim arising from a crash, a challenged breath result can affect liability, comparative fault, and the value of damages.