A breathalyzer measures a color change after a chemical reaction and uses it to calculate the person's blood alcohol concentration (BAC). Since alcohol is not digested, it is absorbed directly into the bloodstream from the mouth, throat and stomach. As the blood circulates around the lung's air sacs, the alcohol evaporates into the air. Each breath, then, contains an alcohol concentration that is proportional to that of the blood.
When the test subject blows into the breathalyzer, the air sample bubbles into a vial containing a mixture of chemicals. As the alcohol reacts, the solution turns from red-orange to green. Because the presence of the green color correlates to the amount of alcohol that was present, the breathalyzer measures the extent of the color change and uses the result to determine the BAC.
Though many techniques have been developed to trick the breathalyzer, most, like chewing gum or pennies, do not work. Others, on the other hand, actually make the test results worse. For instance, using a mouthwash or breath spray that contains alcohol, or even breath mints that contain sugar alcohol, can increase measured BAC. Additionally, holding a breath before the test allows more alcohol to diffuse into the lungs, increasing its concentration.