Every driver should know the legal level for drunkenness in a driver: a BAC (Blood Alcohol Concentration) of .08%, or .08 grams of alcohol per 100 grams of blood.

But an ignition interlock, or car breathalyzer, doesn’t stick a needle in your arm and draw blood to test it. It gets the information it needs from your breath, and then shuts off the ignition if the driver has too much alcohol on his or her breath – usually more than .02 or .025.

How can your breath indicate what’s in your blood? The answer is something called Henry’s Law. In 1803 chemist William Henry discovered that the amount of alcohol that evaporates into the breath is proportional to the amount in the blood*. The greater the amount of alcohol, the greater the concentration of fumes that are emitted through evaporation.

An ignition interlock accurately measures the concentration on the breath, which comes from evaporation, and is then able to calculate the amount in the blood.

The Blood:Breath Ratio and Blood Alcohol

So, if the interlock device detects breath amount X, how does it calculate blood alcohol amount Y? The answer is the blood:breath ratio, which is generally considered 2300 to 1. That means for every 2300 units of alcohol in the blood, there will be one unit in the breath. Using this ratio, the interlock will calculate BAC, and if the number is too high, shut off the ignition.

That’s the science behind an ignition interlock. If you have one in your vehicle, all you need to know is that it works, and you can drive without any problem as long as you don’t drink first.

*Henry’s Law specifically states: “At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.”