DUI breath test machines can only sense the amount of alcohol vapor in your breath—they do not test your blood. But DUI convictions are based on blood alcohol concentration, or BAC. So breath tests have to calculate BAC based on the breath result—and the results can be completely wrong.
To do this calculation, every breath test device must use a ratio known as the “blood-breath partition ratio.” The tests use the same ratio for everyone, even though we know that isn’t always accurate. This leaves room to challenge the breath test results.
What is the blood-breath partition ratio?
The blood-breath partition ratio (BBP ratio) is a number that reflects how much alcohol the lungs absorb from the blood. In California, this ratio is set at 2100:1. In other words, the amount of alcohol in 2,100 milliliters of air from the lungs is the same as the amount of alcohol in 1 milliliter of blood, from the same person at the same time.
This ratio is roughly accurate for most people, most of the time. But it’s not accurate for everybody all the time. In fact, people’s lungs absorb alcohol at very different rates, so the BBP ratio can be wildly wrong in any given case. If that happens, you could get a high breath test result even if you’re not that intoxicated.
What causes the BBP ratio to be “off” for some people?
There are many factors. Every individual’s metabolism is different, so there is enormous variation from one individual to the next. But, even for just one person, the rate at which their lungs absorb alcohol can change under different circumstances.
- If your breath temperature is high for any reason, the test will give a higher BAC reading
- If you have a fever you could get a false high BAC reading
- Dehydration for any reason can cause a higher BAC reading
Can I use the BBP ratio as a defense in my DUI case?
It depends. California charges most DUIs under one of two laws:
- Vehicle Code 23152(a), which says you were impaired by alcohol or drugs, or
- Vehicle Code 23152(b), which says your BAC result was .08% or higher
If you are charged under VC 23152(a), then your DUI lawyer can introduce evidence about the BBP ratio and use it to attempt to discredit your breath test. This is because people who have a lower BAC are less likely to be “impaired” and thus there is doubt about whether you’re guilty at all. If the BBP ratio was off in a way that could have elevated your breath test result, you may not have been impaired at all.
But if you were charged under VC 23152(b), evidence about the BBP ratio will not be considered. This is because simply testing at .08% BAC or higher on an approved testing device is enough to convict you.
How do I prove that the BBP ratio was wrong in my DUI breath test?
The only way to know for sure whether the BBP ratio is accurate would be to give a suspect both a blood test and a breath test at the same time and compare the results. Since our criminal justice system doesn’t do this, there is always room for doubt.
Generally, your lawyer will work to introduce evidence that:
- You were sick or running a fever at the time of your DUI arrest
- You were dehydrated
- Your car, the weather, or the precinct where you were tested was particularly warm
- You were engaged in an activity that may have raised your body temperature and breath temperature
Questioning the BBP ratio is not, on its own, a sure fire win. But it can make a crucial difference, especially in cases where your BAC was close to .08%. In these cases, questions about the BBP ratio mean that you may have been legally sober, and the prosecution is likely to offer you a deal. They may even drop the charges.
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