Distribution describes how alcohol moves from the bloodstream into the body's water-filled tissues after it has been absorbed. In practice, the speed at which the stomach empties its contents into the small intestine largely determines how quickly alcohol enters the blood and becomes available to distribute into body water compartments. A faster gastric emptying rate means alcohol is absorbed more quickly, leading to a quicker distribution into tissues and a steeper rise in BAC. A slower emptying rate slows both absorption and the subsequent distribution phase. The other processes—metabolism in the liver and elimination via the lungs—are separate steps that occur after distribution has begun, so they don’t define how distribution itself occurs.

In a fuel cell breathalyzer, the alcohol in the breath undergoes an electrochemical reaction at the sensor. The amount of alcohol present drives how fast electrons are produced—the current. More alcohol means more electrons per second, so the current increases. The instrument converts that current into a voltage reading (through its circuitry and the sensor’s load), so as breath alcohol concentration rises, the measured voltage also rises. In normal operating ranges the relationship is essentially direct, meaning higher alcohol gives higher voltage. Only at very high concentrations or with sensor saturation might the relationship level off or become nonlinear.

The main idea is that the chamber path length is a fixed dimension the device uses to relate infrared absorption to alcohol concentration. For the Intox EC/IR II, the distance the IR beam travels through the breath sample in the analysis chamber is five inches. This fixed path length is built into the instrument’s design and calibration, so the absorbance readings map consistently to blood alcohol concentration using that exact length. If the path were different, the amount of light absorbed at the same concentration would change, requiring a different calibration and producing inconsistent results. Therefore five inches is the correct chamber path length.

Breath alcohol concentration is reported as parts per million of alcohol in breath. The breath sample is a gas, so the instrument expresses ethanol levels as a very small fraction of ethanol molecules per unit of breath, which is naturally described in ppm. This unit is what the device directly displays and is then used internally (via a standard ratio) to relate the breath result to blood alcohol if needed. Other options describe different contexts—grams per 210 liters of breath is an older forensic reference, percent BAC refers to blood rather than breath, and milligrams per liter is not the unit this device uses.