How a breathalyser works

An individual’s BAC can be measured in two ways — invasively by drawing a blood sample, or non-invasively by breath, saliva or urine samples. Of the various types of non-invasive testing, breath testing is the most common procedure because, according to the experts, it is quick and less expensive.

Breath alcohol content is converted into BAC as air moves from the nose/mouth to the trachea to the bronchi. The bronchi separate into several subdivisions, which end in a large number of tubules. At the end of the tubules are the alveoli and a person’s lung contains millions of alveoli. Surrounding the alveoli is a fine network of capillary blood vessels.

At this level the blood exchanges chemicals with the air in the lungs. Studies show that the alcohol concentration in the lower portion of the lungs is proportionate to the concentration of alcohol in the blood at a ratio of 2100:1.

When a person blows into a breathalyser, alcohol molecules are expelled with the air and the concentration of alcohol in the blood can be measured. This is as accurate as a blood test.

The more a person drinks, the higher their BAC. The higher their BAC, the more physically and mentally impaired they become.

To obtain an accurate correlation between blood and breath alcohol levels, the alcohol tester must be able to capture an air sample from the lower portion of the lungs. A person’s vital capacity — the volume of air that can be expelled without collapsing the lung — can range from one to seven litres, depending on factors such as age, gender, physical condition and disease status. If an alcohol tester requires too long of a breath sample before testing the person’s breath alcohol content, the risk is that the person will not be able to provide a sufficient breath sample. On the other hand, if the alcohol tester captures a person’s breath alcohol content only after the person exhales a minimum amount of air, the correlation between breath alcohol content and BAC will be poor.

Typically, alcohol tester protocols will require the test subject to exhale air for at least five seconds to obtain a high correlation between breath alcohol content and BAC.

Newer, more advanced alcohol testers can track the force of exhalation over time to ensure an adequate lung volume is achieved, Most professional-grade alcohol testers measure the force of the breath sample to prevent “fake-blowing” through the device and inaccurate results.