When law enforcement conducts a DUI test, they typically use a breathalyzer to measure alcohol in a driver’s breath. However, courts prosecute DUI cases based on BAC (Blood Alcohol Concentration), which is found in the bloodstream.
So how do officers convert a breathalyzer reading (BRAC) into BAC, and is it always accurate? Understanding this process can help you make informed decisions and recognize potential flaws in DUI testing.
How BRAC and BAC Are Connected
Breath alcohol concentration (BRAC) measures the amount of alcohol in exhaled air. Blood alcohol concentration (BAC), on the other hand, measures the amount of alcohol in the bloodstream.
Because a breathalyzer cannot directly measure BAC, law enforcement uses a conversion ratio called the partition ratio to estimate BAC from BRAC.
The standard partition ratio used in DUI cases is 2100:1, meaning:
- 1 milliliter of blood contains the same amount of alcohol as 2100 milliliters of breath.
- This ratio assumes a direct correlation between BRAC and BAC for all individuals.
However, this assumption is not always accurate because individual metabolism, temperature, and other factors can affect breathalyzer readings.
How Breathalyzer Readings Are Converted to BAC
When you take a breathalyzer test, the device measures the amount of alcohol in your breath and applies the partition ratio to estimate your BAC.
For example:
- If a breathalyzer detects 0.04% BRAC, it would estimate a 0.08% BAC using the 2100:1 ratio.
- If a breathalyzer detects 0.06% BRAC, it would estimate a 0.12% BAC.
While this method provides a quick estimate of impairment, it may not be entirely accurate for every individual.
Factors That Can Affect BRAC-to-BAC Conversion
While the 2100:1 ratio is used as the legal standard, several factors can cause variations in actual BAC levels.
- Individual Physiology
Some people naturally have a higher or lower partition ratio, meaning their BAC might be overestimated or underestimated based on breath tests alone.
- Breath Temperature
Higher body temperatures can lead to higher BRAC readings, making it seem like a person has a higher BAC than they actually do.
- Breathing Patterns
- Hyperventilating before a test can lower BRAC readings.
- Holding your breath before exhaling can increase BRAC readings.
- Mouth Alcohol Contamination
Residual alcohol from recent drinking, mouthwash, or even burping can cause false high readings.
- Medical Conditions
People with acid reflux (GERD), diabetes, or certain metabolic disorders may produce higher BRAC readings without a corresponding BAC increase.
Why BRAC-to-BAC Conversions Matter in DUI Cases
Because breathalyzers rely on an assumed partition ratio, there are potential legal and scientific challenges in DUI cases:
- If a person’s natural partition ratio is higher than 2100:1, their BAC could be overestimated.
- Some states allow DUI defendants to challenge breathalyzer results and request a blood test for accuracy.
- Mouth alcohol or improper testing procedures can lead to inflated BRAC readings.
This is why some DUI cases rely on blood tests as the final measure of BAC rather than breath tests alone.
Final Thoughts: Know How BRAC and BAC Are Calculated
Understanding how breathalyzer readings are converted into BAC can help you:
- Recognize potential flaws in DUI testing.
- Be aware of factors that might lead to inaccurate results.
- Understand your rights if facing a DUI charge.
Since breathalyzers estimate BAC based on assumptions that don’t apply to everyone, the best way to avoid legal trouble is to avoid driving after drinking altogether.
