›› Breathalyzer Facts

What is a breathalyzer?

A breathalyzer (or breathalyser) is a machine that estimates blood alcohol content (BAC) from a breath sample. It is used to gauge the likelihood of impairment due to alcohol consumption.

How does a breathalyzer work?

Breathalyzers compute a Partition Ratio that is an estimation measuring 1 part of alcohol in the breath and equating it to 2100 parts per million in the blood. This is the 2100 to 1 ratio. The partition ratio is an estimate based on an idealized average; in reality the actual partition ratio can vary effected by metabolism, diet and medical conditions.

Many breathalyzers don’t measure Alcohol directly. What they measure are Methyl Group compounds of which there are over 2000. Approximately 200 can appear in human breath. This can result from inhaling paint fumes, pumping gas or handling glue or similar compounds as these chemicals may also be absorbed through the skin.

The half life for these compounds in your body means that traces may appear on your breath even 24 hours later. Medical Conditions such as diabetes or strict dieting may also cause a false positive from Acetone. The machines cannot differentiate* between these compounds and give a cumulative reading.

The machine is supposed to measure breath alcohol that is residue filtered from the blood through the lungs. So another potential problem is mouth alcohol. This can cause an elevated reading due to using mouthwash, or burping that’s is why an officer administering a breathalyzer test should do it over a 15 minute period while observing the alleged perpetrator. Many times this rule is not followed.

Duplicate Analysis, most states require some form of duplicate analysis meaning that the officer is required to get two readings within .01% of each other. The actual rules can vary greatly. North Carolina for instance gives the officer 3 shots at this while California requires two consecutive readings but the officer may try up to 15 times. Note: The range of error on these machines can be as much as .04 on a .01 test that equates to an error rate of 40%.

These false positives along with the range of error can be problematic especially when a driver may have been drinking earlier and no longer has an elevated BAC. This is why a good DUI attorney, one who specializes in DUI law and understands the mechanics of the DUI tests can be essential.

*Some machines can filter for Acetone but must be regularly calibrated, something seldom done by cash strapped police agencies.