A breathalyser measures the alcohol in your lungs by generating and measuring a small amount of electricity when the alcohol from the breath sample comes in contact with the sensor.
When you drink alcohol, it is digested in the stomach and passes through the stomach wall into the blood stream. Broadly speaking, neat alcohol (such as a straight whisky, for example) taken on an empty stomach is likely to enter the bloodstream more quickly than, say, a milk-based cocktail drink after a big meal. This does not mean you will become more intoxicated from the straight whisky – it’s just that the effect is likely to be felt more quickly.
Once in the blood stream, the alcohol flows round the body and generates the usual effects on the body and brain. As the blood passes through the liver, it is gradually filtered out from the bloodstream, reducing at each “pass” until there is no longer any residual alcohol in the body.
Alcohol also passes through the alveoli in the lungs. As you breathe, and oxygen passes into the bloodstream, some of the alcohol in your blood ‘evaporates’ into the air in your lungs. It is this alcohol that a breathalyser is designed to measure. This is why it is necessary to measure deep lung air when using a breathalyzer. It’s also why it is important not to drink within 15 minutes of testing – otherwise alcohol that remains in your mouth will be blown directly into the detector, at far higher concentrations than is the case from alcohol that has passed through the stomach, into the bloodstream, and into the air you breathe out. Clearly the concentrations are often very low and the sensors have to be very sensitive to detect the levels involved – hence why it is so important not to smoke or drink before using them and why obtaining an accurate and consistent sample of air is so important.