A breathalyzer is a specialized instrument designed to estimate an individual’s Blood Alcohol Content (BAC) from a sample of their exhaled breath. The device operates on the principle that the concentration of alcohol in deep lung air has a fixed, proportional relationship to the alcohol concentration in the blood. By analyzing the breath, the instrument provides a non-invasive and rapid method for determining the approximate level of systemic alcohol intoxication.
The Physiological Link: Alcohol Transfer from Blood to Breath
When a person consumes an alcoholic beverage, the ethanol is rapidly absorbed primarily through the stomach and small intestine and then enters the bloodstream. The circulatory system distributes this alcohol throughout the body, including the tissues and organs. As the blood travels, it eventually reaches the lungs to participate in the process of gaseous exchange.
In the lungs, blood flows through capillaries surrounding the tiny air sacs known as alveoli. Since alcohol is a volatile substance, a portion of the ethanol dissolved in the blood vaporizes and diffuses across the alveolar membranes into the air within the lung sacs.
This physical process is governed by Henry’s Law, which states that a volatile compound dissolved in a liquid seeks equilibrium with the air above it. For breath testing, the blood acts as the liquid and the alveolar air acts as the gas. The concentration of alcohol vapor in the alveolar air is directly proportional to the concentration of alcohol dissolved in the blood circulating through the lungs.
The breathalyzer is designed to sample this deep lung air, which is the last portion of the breath exhaled. The alcohol content in this alveolar air, known as Breath Alcohol Concentration (BrAC), accurately reflects the alcohol content of the blood. This physiological link allows a chemical measurement of the breath to serve as a reliable proxy for the alcohol level in the bloodstream.
The Core Chemistry: How Ethanol is Oxidized
The detection of ethanol in the breath relies on a chemical reaction that oxidizes the alcohol, a process achieved through two primary technologies: a colorimetric reaction or an electrochemical fuel cell. The earliest breathalyzers utilized the colorimetric method, which involved a distinct chemical change measured optically. These devices contained a solution of potassium dichromate, an oxidizing agent, in sulfuric acid.
When a breath sample containing ethanol was bubbled through this orange solution, the alcohol was oxidized to acetic acid, reducing the potassium dichromate. The hexavalent chromium ion (Cr6+) in the dichromate, which gives the solution its orange color, was reduced to a trivalent chromium ion (Cr3+). This reduction resulted in a measurable change of the solution from orange to green. The extent of this color change was directly proportional to the amount of ethanol present.
Modern breathalyzers rely almost exclusively on the electrochemical fuel cell technology due to its higher specificity and reliability. The fuel cell sensor contains two platinum electrodes separated by an electrolyte material. When the exhaled breath sample is introduced, ethanol molecules contact the platinum catalyst on the anode (positive electrode).
At the anode, ethanol is electrochemically oxidized into acetic acid, releasing electrons and hydrogen ions (H+). The electrons travel through an external circuit, creating an electrical current. Simultaneously, at the cathode (negative electrode), oxygen combines with the hydrogen ions and electrons to form water. This flow of current is the measurable signal the device uses to quantify the alcohol.
The strength of the electrical current produced is directly proportional to the amount of ethanol vapor oxidized. This method is highly specific to ethanol because the platinum catalyst is engineered to react efficiently with alcohol molecules. By measuring the total charge generated, the device precisely determines the concentration of alcohol in the breath sample.
Translating the Signal: Measurement and BAC Calculation
The final stage involves converting the measured chemical or electrical signal into a quantifiable estimation of Blood Alcohol Content (BAC). The device first determines the Breath Alcohol Concentration (BrAC) based on the magnitude of the measured reaction. Older colorimetric devices measured the intensity of the green color change using a photodetector, while modern fuel cell devices directly measure the strength of the electrical current.
To translate the BrAC into the legally significant BAC, the device applies a fixed conversion factor known as the partition ratio. This ratio is a standardized measure representing the fixed relationship between alcohol concentration in the breath and in the blood. The standard partition ratio utilized in most jurisdictions is 2100:1.
This 2100:1 ratio means that 2,100 milliliters of deep lung air are assumed to contain the same amount of alcohol as 1 milliliter of blood. The device takes the measured BrAC value and applies this ratio to calculate the estimated BAC, which is displayed as a percentage. The actual physiological partition ratio can vary among individuals, potentially ranging from 1800:1 to 2400:1 depending on factors like body temperature.
Despite these individual variations, the standardized 2100:1 ratio allows the breathalyzer to provide a consistent, rapid, and legally accepted estimate of blood alcohol concentration. This conversion step transforms the initial chemical reaction into the final, actionable number used to assess a person’s level of intoxication.